Serial HOWTO

Table of Contents

  1. Introduction
     1.1 Copyright, Disclaimer, & Credits
        1.1.1 Copyright
        1.1.2 Disclaimer
        1.1.3 Trademarks.
        1.1.4 Credits
     1.2 New Versions of this Serial-HOWTO
     1.3 New in Recent Versions
     1.4 Related HOWTO's, etc. about the Serial Port
     1.5 Feedback
     1.6 What is a Serial Port?

  2. Quick Help
  3. How the Hardware Transfers Bytes
     3.1 Transmitting
     3.2 Receiving
     3.3 The Large Serial Buffers

  4. Serial Port Basics
     4.1 What is a Serial Port ?
        4.1.1 Intro to Serial
        4.1.2 Pins and Wires
        4.1.3 RS-232  (EIA-232, etc.)
     4.2 IO Address & IRQ
     4.3 Names: ttyS0, ttyS1, etc.
     4.4 Interrupts
     4.5 Data Flow (Speeds)
     4.6 Flow Control
        4.6.1 Example of Flow Control
        4.6.2 Symptoms of No Flow Control
        4.6.3 Hardware vs. Software Flow Control
     4.7 Data Flow Path; Buffers
     4.8 Complex Flow Control Example
     4.9 Serial Driver Module
     4.10 The Serial Port is Now Obsolete on PCs
     4.11 RS-232 Cable Is Low Speed & Short Distance
     4.12 Inefficient PCI Interface to the Computer (in some cases)

  5. Multiport Serial Boards/Cards/Adapters
     5.1 Intro to Multiport Serial
     5.2 Dumb vs. Smart Cards
     5.3 Getting/Enabling a Driver
        5.3.1 Introduction
        5.3.2 Build (compile) support into the kernel?
        5.3.3 Using module support
        5.3.4 Getting info on multiport boards
     5.4 Multiport Devices in the /dev Directory,
     5.5 Making Legacy Multiport Devices in the /dev Directory
     5.6 Standard PC Serial Cards
     5.7 Dumb Multiport Serial Boards (with standard UART chips)
     5.8 Intelligent Multiport Serial Boards
     5.9 Unsupported Multiport Boards

  6. Servers for Serial Ports
  7. Configuring Overview
  8. Locating the Serial Port: IO address, IRQs
     8.1 What Bus is my Serial Port On?
     8.2 IO & IRQ Overview
     8.3 PCI Bus Support
        8.3.1 Introduction
        8.3.2 More info on PCI
     8.4 Common mistakes made re low-level configuring
     8.5 IRQ & IO Address Must be Correct
     8.6 What is the IO Address and IRQ per the driver ?
        8.6.1 Introduction
        8.6.2 I/O Address & IRQ: Boot-time messages
        8.6.3 The /proc directory and setserial
     8.7 What is the IO Address & IRQ of my Serial Port Hardware?
        8.7.1 Introduction
        8.7.2 PCI: What IOs and IRQs have been set?
        8.7.3 PCI: Enabling a disabled port
        8.7.4 ISA PnP ports
        8.7.5 Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)
        8.7.6 Exploring via MS Windows (a last resort)
     8.8 Choosing Serial IRQs
        8.8.1 IRQ 0 is not an IRQ
        8.8.2 Interrupt sharing,  Kernels 2.2+
        8.8.3 What IRQs to choose?
     8.9 Choosing Addresses --Video card conflict with ttyS3
     8.10 Set IO Address & IRQ in the hardware (mostly for PnP)
        8.10.1 Using a PnP BIOS to IO-IRQ Configure
     8.11 Giving the IRQ and IO Address to Setserial

  9. Configuring the Serial Driver (high-level) "stty"
     9.1 Overview
     9.2 Flow Control

  10. Serial Port Devices /dev/ttyS2, etc.
     10.1 Serial Port Names: ttyS4, etc
     10.2 The PCI Bus
     10.3 Serial Port Device Names & Numbers
     10.4 More on Serial Port Names
     10.5 USB (Universal Serial Bus) Serial Ports
     10.6 Link ttySN to /dev/modem
     10.7 Which Connector on the Back of my PC is ttyS1, etc?
        10.7.1 Inspect the connectors
        10.7.2 Send bytes to the port
        10.7.3 Connect a device to the connector
        10.7.4 Missing connectors
     10.8 Creating Devices In the /dev directory

  11. Interesting Programs You Should Know About
     11.1 Serial Monitoring/Diagnostics Programs
     11.2 Changing Interrupt Priority
     11.3 What is Setserial ?
        11.3.1 Setserial problems with linmodems, laptops
        11.3.2 Introduction
        11.3.3 Serial module unload
        11.3.4 Slow baud rates of 1200 or less
        11.3.5 Giving the setserial command
        11.3.6 Configuration file
        11.3.7 Probing
        11.3.8 Boot-time Configuration
        11.3.9 Edit a script (required prior to version 2.15)
        11.3.10 Configuration method using /etc/serial.conf, etc.
        11.3.11 IRQs
        11.3.12 Laptops: PCMCIA
     11.4 Stty
        11.4.1 Introduction
        11.4.2 Flow control options
        11.4.3 Using stty at a "foreign" terminal
        11.4.4 Two interfaces at a terminal
        11.4.5 Where to put the stty command ?
        11.4.6 Obsolete redirection method
     11.5 What is isapnp ?
     11.6 Connecting two PCs together via serial ports
     11.7 Connect the serial port to a fast network: ser2net

  12. Speed (Flow Rate)
     12.1 Very High Speeds
        12.1.1 Speeds over 115.2kbps
        12.1.2 How speed is set in hardware: the divisor and baud_base
        12.1.3 Setting the divisor, speed accounting
        12.1.4 Crystal frequency is higher than baud_base
     12.2 Higher Serial Throughput

  13. Locking Out Others
     13.1 Introduction
     13.2 Lock-Files
     13.3 Change Owners, Groups, and/or Permissions of Device Files

  14. Serial Communications Programs And Utilities
     14.1 List of Software
     14.2 kermit and zmodem

  15. Serial Tips And Miscellany
     15.1 Serial Modules
     15.2 Kernel Configuration
     15.3 Number of Serial Ports Supported
     15.4 Serial Console (console on the serial port)
     15.5 Line Drivers
     15.6 Stopping the Data Flow when Printing, etc.
     15.7 Known IO Address Conflicts
        15.7.1 Avoiding IO Address Conflicts with Certain Video Boards
        15.7.2 IO address conflict with ide2 hard drive
     15.8 Known Defective Hardware
        15.8.1 Problem with AMD Elan SC400 CPU (PC-on-a-chip)

  16. Troubleshooting
     16.1 Serial Electrical Test Equipment
        16.1.1 Breakout Gadgets, etc.
        16.1.2 Measuring voltages
        16.1.3 Taste voltage
     16.2 Serial Monitoring/Diagnostics
     16.3 (The following subsections are in both the Serial and Modem HOWTOs)
     16.4 Serial Port Can't be Found
        16.4.1 Scanning/probing legacy ports
     16.5 Linux Creates an Interrupt Conflict (your PC has an ISA slot)
     16.6 Extremely Slow: Text appears on the screen slowly after long delays
     16.7 Somewhat Slow: I expected it to be a few times faster
     16.8 The Startup Screen Shows Wrong IRQs for the Serial Ports
     16.9 "Cannot open /dev/ttyS?: Device or resource busy
     16.10 "Cannot open /dev/ttyS?: Permission denied"
     16.11 "Cannot open /dev/ttyS?"
     16.12 "Operation not supported by device" for ttyS?
     16.13 "Cannot create lockfile. Sorry"
     16.14 "Device /dev/ttyS? is locked."
     16.15 "/dev/tty? Device or resource busy"
     16.16 "Input/output error" from setserial, stty, pppd, etc.
     16.17 "LSR safety check engaged"
     16.18 Overrun errors on serial port
     16.19 Port gets characters only sporadically
     16.20 Troubleshooting Tools
     16.21 Almost all characters are wrong; Many missing or many extras

  17. Interrupt Problem Details
     17.1 Types of interrupt problems
     17.2 Symptoms of Mis-set or Conflicting Interrupts
     17.3 Mis-set Interrupts
     17.4 Interrupt Conflicts
     17.5 Resolving Interrupt Problems

  18. What Are UARTs?  How Do They Affect Performance?
     18.1 Introduction to UARTS
     18.2 Two Types of UARTs
     18.3 FIFOs
     18.4 Why FIFO Buffers are Small
     18.5 UART Model Numbers
  19. Pinout and Signals
     19.1 Pinout of 9-pin and 25-pin serial connectors
     19.2 Signals May Have No Fixed Meaning
     19.3 Cabling Between Serial Ports
     19.4 RTS/CTS and DTR/DSR Flow Control
        19.4.1 The DTR and DSR Pins
     19.5 Preventing a Port From Opening

  20. Voltage Waveshapes
     20.1 Voltage for a Bit
     20.2 Voltage Sequence for a Byte
     20.3 Parity Explained
     20.4 Forming a Byte (Framing)
     20.5 How "Asynchronous" is Synchronized

  21. Other Serial Devices (not async RS-232)
     21.1 Successors to RS-232
     21.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
     21.3 EIA-485
     21.4 EIA-530
     21.5 EIA-612/613
     21.6 The Universal Serial Bus (USB)
     21.7 Firewire
     21.8 MIDI
     21.9 Synchronization & Synchronous
        21.9.1 Defining Asynchronous vs Synchronous
        21.9.2 Synchronous Communication

  22. Other Sources of Information
     22.1 Books
     22.2 Serial Software
     22.3 Related Linux Documents
     22.4 Serial Mailing List
     22.5 Internet

  23. Appendix A: Very Obsolete Hardware/Software
     23.1 Replacing pre 1990 UARTS
     23.2 Two Ports with the Same IO address
     23.3 Configuring by modifying source code
     23.4 Modems on Multiport Cards Obsolete for Sending at 56k
     23.5 Lock-Files if you used the depreciated devfs
     23.6 Devfs (The deprecated Device File System.  History)


  1.  Introduction

  This HOWTO covers basic info on the serial port which is missing on
  newer personal computers but is being used in recent embedded systems
  as well as for routers, point-of-sale equipment, etc.  It includes
  multiport serial cards.  It was written when the serial port was a
  major port for connecting a PC to modems and printers, etc.  and the
  style of this article reflects this.  If you're having problems with
  the serial port, want to understand how it works, or need a detailed
  introduction to it before studying serial programming, this is one
  place to find out about it.

  This HOWTO is about the original serial port which uses a UART chip
  and is sometimes called a "UART serial port" to differentiate it from
  the newer types of serial devices: Universal Serial Bus or Firewire.
  It's ``slow'' compared to newer serial devices but can send text
  several times faster than you can read it.  Information specific to
  devices which use serial ports: analog modems, text-terminals,
  infrared devices, and a few printers are found in Modem-HOWTO, Text-
  Terminal-HOWTO, Infrared-HOWTO, and Printing-HOWTO.  Info on getty
  (the program that runs the login process or the like) has been also
  moved to other related HOWTOs since mgetty and uugetty are best for
  modems while agetty is best for text-terminals.  If you are dealing
  with a modem, text terminal, infrared device, or printer, then you may
  not need to consult this HOWTO.  But if you are using the serial port
  for some other device, using a multiport serial card, trouble-shooting
  the serial port itself, or want to understand more technical details
  of the serial port, then you may want to use this HOWTO as well as
  some of the other HOWTOs.  (See ``Related HOWTO's'')  This HOWTO lists
  info on various multiport serial cards.  This HOWTO addresses Linux
  running on PCs (ISA and/or PCI buses), although it might be valid for
  other architectures.

  1.1.  Copyright, Disclaimer, & Credits

  1.1.1.  Copyright

  Copyright (c) 1993-1997 by Greg Hankins, (c) 1998-2005 by David S.
  Lawyer  <>

  Please freely copy and distribute (sell or give away) this document in
  any format.  Send any corrections and comments to the document
  maintainer.  You may create a derivative work and distribute it
  provided that you:

  1. If it's not a translation: Email a copy of your derivative work (in
     a format LDP accepts) to the author(s) and maintainer (could be the
     same person).  If you don't get a response then email the LDP
     (Linux Documentation Project):

  2. License the derivative work in the spirit of this license or use
     GPL.  Include a copyright notice and at least a pointer to the
     license used.

  3. Give due credit to previous authors and major contributors.

  If you're considering making a derived work other than a translation,
  it's requested that you discuss your plans with the current

  1.1.2.  Disclaimer

  While I haven't intentionally tried to mislead you, there are likely a
  number of errors in this document.  Please let me know about them.
  Since this is free documentation, it should be obvious that I cannot
  be held legally responsible for any errors.

  1.1.3.  Trademarks.

  Any brand names (starts with a capital letter such as MS Windows)
  should be assumed to be a trademark).  Such trademarks belong to their
  respective owners.

  1.1.4.  Credits

  Most of the original Serial-HOWTO was written by  Greg Hankins.
  <> He also rewrote many contributions by
  others in order to maintain continuity in the writing style and flow.
  He wrote: ``Thanks to everyone who has contributed or commented, the
  list of people has gotten too long to list (somewhere over one
  hundred).  Special thanks to Ted Ts'o for answering questions about
  the serial drivers.''  Approximately half of v2.00 was from Greg
  Hankins HOWTO and the other half were additions by David Lawyer.  Ted
  Ts'o has continued to be helpful.  In Jan. 2006 "Charles Brockman"
  reviewed it for typos which resulted in many typos being fixed.

  1.2.  New Versions of this Serial-HOWTO

  New versions will be issued perhaps every year or two.  They will be
  available to browse and/or download at LDP mirror sites see:
  <>.  Various formats are available.
  If you only want to quickly check the date of the latest version look
  at  <> and compare it to
  this version: v2.27 February 2011 .

  1.3.  New in Recent Versions

  For a full revision history going back to the time I started
  maintaining this HOWTO, see the source file (in linuxdoc format):
  (cvs) Serial-HOWTO.sgml

  В·  TO do: explain use of udev for serial port for setting names and
     permissions.  Fix dead links.

  В·  2.27 Feb. 2011: The serial port is still widely used in embedded
     systems, etc., and is not really obsolete.  Better definition of
     input, output.  Deleted devfs names such as tts.

  В·  2.26 Nov. 2010 Changed EIA-232 to RS-232.  PCI-e bus is serial.  No
     serial port on new PCs.  I incorrectly wrote in this update that
     the serial port was obsolete and that this HOWTO is now mainly of
     historical interest.

  В·  2.25 Jan. 2007 picocom.  devfs is obsolete. ser2net.  Revised parts
     on drivers as modules vs. built into kernel.  Serial Programming

  1.4.  Related HOWTO's, etc. about the Serial Port

  Modems, Text-Terminals, some printers, and other peripherals often
  used the serial port.  Get these HOWTOs from the nearest mirror site
  as explained above.

  В·  Modem-HOWTO is about installing and configuring modems

  В·  Printing-HOWTO has info for serial printers using old lpr command

  В·  LPRng-HOWTO (not a LDP HOWTO, may come with software) has info for
     serial printing for "Next Generation" lpr

  В·  Serial Programming
     is a wiki-book on the Internet

  В·  Serial-Programming-HOWTO helps you write C programs that read and
     write to the serial port and/or check/set its state.  A version
     written by Vern Hoxie but not submitted to LDP is at ``Internet

  В·  Text-Terminal-HOWTO is about how they work, how to install
     configure, and repair them.  It includes a section on "Make a
     Terminal the Console" which is useful for using a remote terminal
     to control a server (via the serial port).

  В·  Remote-Serial-Console-HOWTO is about making a text-terminal be the
     console so it can display boot-time messages, etc.

  1.5.  Feedback

  Please send me any  suggestions, correction or additional material.
  Tell me what you don't understand, or what could be clearer.  You can
  reach me via email at  <>.

  1.6.  What is a Serial Port?

  The conventional serial port (not the newer USB port, or Firewire
  port) is a very old I/O (Input/Output) port.  Until around 2006, most
  new desktop PC's had one, and old PC's from the 1990's sometimes had 2
  of them.  Most laptops gave up them before the desktops did.  Macs
  (Apple Computer) after mid-1998 only had the USB port.  However, it's
  possible, to put a conventional serial port device on the USB bus
  which is on all modern PCs.

  Each serial port has a "file" associated with it in the /dev
  directory.  It isn't really a file but it seems like one.  For
  example, /dev/ttyS0. Other serial ports are /dev/ttyS1, /dev/ttyS2,
  etc.  But ports on the USB bus, multiport cards, etc. have different

  The common specification for the conventional serial port is RS-232
  (or RS-232).  So it's often called a "RS-232 serial port".  The
  connector(s) for the serial port are often seen as one or two 9-pin
  connectors (in some cases 25-pin) on the back of a PC.  But the serial
  port is more than just connectors.  It includes the associated
  electronics which must produce signals conforming to the RS-232
  specification.  See ``Voltage Waveshapes''.  One pin is used to send
  out data bytes and another to receive data bytes.  Another pin is a
  common signal ground.  The other "useful" pins are used mainly for
  signalling purposes with a steady negative voltage meaning "off" and a
  steady positive voltage meaning "on".

  The UART (Universal Asynchronous Receiver-Transmitter) chip does most
  of the work.  Today, the functionality of this chip is usually built
  into another chip.  See ``What Are UARTs?'' These have improved over
  time and old models (prior to say 1994) are usually very obsolete.

  The serial port was originally designed for connecting external modems
  to a PC but it's used to connect many other devices also such as mice,
  text-terminals, some printers, etc. to a computer.  You just plug
  these devices into the serial port using the correct cable.  Many
  internal modem cards have a built-in serial port so when you install
  one inside your PC it's as if you just installed another serial port
  in your PC.

  2.  Quick Help

  This repeats more detailed information found elsewhere.  If your
  computer can't seem to find your serial port and you already know
  something about hardware resources (addresses like 3F8 and IRQs like
  5) then try this:  First, get into the BIOS (often called "setup")
  when the computer is powered on by pressing certain keys.  To find out
  what keys to press, watch the screen as your PC starts up.  If the
  words that flash by on the screen too fast to read, freeze them by
  holding down the "pause" and "shift" keys at the same time.  Then when
  read, hit any key to resume (cease pausing) and hold down the key(s)
  required to enter the BIOS setup.   You may have to try this again
  since there may be more than one screen which you can freeze with the
  "pause" key.  Also, look for messages about the serial ports on these
  frozen screens.

  Once in the BIOS menus, try to find  menus dealing with the serial
  port.  They could be shown in a menu dealing with Resources, Plug-and-
  Play, Peripherals, Ports, etc.  Some old BIOSs setups (before 1995 ?)
  didn't deal with the serial ports.  Make sure the ports you need are
  not disabled and note how they are configured (like 3F8 IRQ 4).  You
  may need to change the configuration to prevent conflicts.  There
  could be a shortage of IRQs if the BIOS has reserved some IRQs that it
  didn't need to reserve.

  For serial ports to be found, either the kernel must have been
  compiled with serial support, or serial support must be provided by a
  module.  To check this look in the file /boot/config-2.6... and search
  for SERIAL.  =m means it's a module and you may check to see the
  modules that are being used by typing: lsmod.

  3.  How the Hardware Transfers Bytes

  Below is an introduction to the topic, but for a more advanced
  treatment of it see ``FIFOs''.

  3.1.  Transmitting

  Transmitting is sending bytes out of the serial port away from the
  computer (output).  Once you understand transmitting, receiving
  (input) is easy to understand since it's similar.  The first
  explanation given here will be grossly oversimplified.  Then more
  detail will be added in later explanations.  When the computer wants
  to send a byte out the serial port (to the external cable) the CPU
  sends the byte on the bus inside the computer to the I/O (Input
  Output) address of the serial port.  I/O is often written as just IO.
  The serial port takes the byte, and sends it out one bit at a time (a
  serial bit-stream) on the transmit pin of the serial cable connector.
  For what a bit (and byte) look like electrically see ``Voltage

  Here's a replay of the above in a little more detail (but still very
  incomplete).  Most of the work at the serial port is done by the UART
  chip (or the like).  To transmit a byte, the serial device driver
  program (running on the CPU) sends a byte to the serial port"s I/O
  address.  This byte gets into a 1-byte "transmit shift register" in
  the serial port.  From this shift register bits are taken from the
  byte one-by-one and sent out bit-by-bit on the serial line.  Then when
  the last bit has been sent and the shift register needs another byte
  to send, it could just ask the CPU to send it another byte.  Thus
  would be simple but it would likely introduce delays since the CPU
  might not be able to get the byte immediately.  After all, the CPU is
  usually doing other things besides just handling the serial port.

  A way to eliminate such delays is to arrange things so that the CPU
  gets the byte before the shift register needs it and stores it in a
  serial port buffer (in hardware).  Then when the shift register has
  sent out its byte and needs a new byte immediately, the serial port
  hardware just transfers the next byte from its own buffer to the shift
  register.  No need to call the CPU to fetch a new byte.

  The size of this serial port buffer was originally only one byte, but
  today it is usually 16 bytes (more in higher priced serial ports).
  Now there is still the problem of keeping this buffer sufficiently
  supplied with bytes so that when the shift register needs a byte to
  transmit it will always find one there (unless there are no more bytes
  to send).  This is done by contacting the CPU using an interrupt.

  First we'll explain the case of the old fashioned one-byte buffer,
  since 16-byte buffers work similarly (but are more complex).  When the
  shift register grabs the byte out of the buffer and the buffer needs
  another byte, it sends an interrupt to the CPU by putting a voltage on
  a dedicated wire on the computer bus.  Unless the CPU is doing
  something very important, the interrupt forces it to stop what it was
  doing and start running a program which will supply another byte to
  the port's buffer.  The purpose of this buffer is to keep an extra
  byte (waiting to be sent) queued in hardware so that there will be no
  gaps in the transmission of bytes out the serial port cable.

  Once the CPU gets the interrupt, it will know who sent the interrupt
  since there is a dedicated interrupt wire for each serial port (unless
  interrupts are shared).  Then the CPU will start running the serial
  device driver which checks registers at I/0 addresses to find out what
  has happened.  It finds out that the serial's transmit buffer is empty
  and waiting for another byte.  So if there are more bytes to send, it
  sends the next byte to the serial port's I/0 address.  This next byte
  should arrive when the previous byte is still in the transmit shift
  register and is still being transmitted bit-by-bit.

  In review, when a byte has been fully transmitted out the transmit
  wire of the serial port and the shift register is now empty the
  following 3 things happen in rapid succession:

  1. The next byte is moved from the transmit buffer into the transmit
     shift register

  2. The transmission of this new byte (bit-by-bit) begins

  3. Another interrupt is issued to tell the device driver to send yet
     another byte to the now empty transmit buffer

  Thus we say that the serial port is interrupt driven.  Each time the
  serial port issues an interrupt, the CPU sends it another byte.  Once
  a byte has been sent to the transmit buffer by the CPU, then the CPU
  is free to pursue some other activities until it gets the next
  interrupt.  The serial port transmits bits at a fixed rate which is
  selected by the user (or an application program).  It's sometimes
  called the baud rate.  The serial port also adds extra bits to each
  byte (start, stop and perhaps parity bits) so there are often 10 bits
  sent per byte.  At a rate (also called speed) of 19,200 bits per
  second (bps), there are thus 1,920 bytes/sec (and also 1,920

  Doing all this is a lot of work for the CPU.  This is true for many
  reasons.  First, just sending one 8-bit byte at a time over a 32-bit
  data bus (or even 64-bit) is not a very efficient use of bus width.
  Also, there is a lot of overhead in handing each interrupt.  When the
  interrupt is received, the device driver only knows that something
  caused an interrupt at the serial port but doesn't know that it's
  because a character has been sent.  The device driver has to make
  various checks to find out what happened.  The same interrupt could
  mean that a character was received, one of the control lines changed
  state, etc.

  A major improvement has been the enlargement of the buffer size of the
  serial port from 1-byte to 16-bytes.  This means that when the CPU
  gets an interrupt it gives the serial port up to 16 new bytes to
  transmit.  This is fewer interrupts to service but data must still be
  transferred one byte at a time over a wide bus.  The 16-byte buffer is
  actually a FIFO (First In First Out) queue and is often called a FIFO.
  See ``FIFOs'' for details about the FIFO along with a repeat of some
  of the above info.

  3.2.  Receiving

  Receiving bytes by a serial port is similar to sending them only it's
  in the opposite direction.  It's also interrupt driven.  For the
  obsolete type of serial port with 1-byte buffers, when a byte is fully
  received from the external cable it goes into the 1-byte receive
  buffer.  Then the port gives the CPU an interrupt to tell it to pick
  up that byte so that the serial port will have room for storing the
  next byte which is currently being received.  For newer serial ports
  with 16-byte buffers, this interrupt (to fetch the bytes) may be sent
  after 14 bytes are in the receive buffer.  The CPU then stops what it
  was doing, runs the interrupt service routine, and picks up 14 to 16
  bytes from the port.  For an interrupt sent when the 14th byte has
  been received, there could be 16 bytes to get if 2 more bytes have
  arrived since the interrupt.  But if 3 more bytes should arrive
  (instead of 2), then the 16-byte buffer will overrun.  It also may
  pick up less than 14 bytes by setting it up that way or due to
  timeouts.  See ``FIFOs'' for more details.

  3.3.  The Large Serial Buffers

  We've talked about small 16-byte serial port hardware buffers but
  there are also much larger buffers in main memory.  When the CPU takes
  some bytes out of the receive buffer of the hardware, it puts them
  into a much larger (say 8k-byte) receive buffer in main memory.  Then
  a program that is getting bytes from the serial port takes the bytes
  it's receiving out of that large buffer (using a "read" statement in
  the program).

  A similar situation exists for bytes that are to be transmitted.  When
  the CPU needs to fetch some bytes to be transmitted it takes them out
  of a large (8k-byte) transmit buffer in main memory and puts them into
  the small 16-byte transmit buffer in the hardware.  And when a program
  wants to send bytes out the serial port, it writes them to this large
  transmit buffer.

  Both of these buffers are managed by the serial driver.  But the
  driver does more than just dealing with these buffers.  It also does
  limited filtering (minor modifications) of data passing thru these
  buffers and listens for control certain characters.  All this is
  configurable using the ``Stty'' program.

  4.  Serial Port Basics

  You don't have to understand the basics to use the serial port But
  understanding it may help to determine what is wrong if you run into
  problems.  This section not only presents new topics but also repeats
  some of what was said in the previous section ``How the Hardware
  Transfers Bytes'' but in greater detail.

  4.1.  What is a Serial Port ?

  4.1.1.  Intro to Serial

  The UART serial port (or just "serial port for short" is an I/O
  (Input/Output) device.

  An I/O device is just a way to get data into and out of a computer.
  There are many types of I/O devices such as the older serial ports and
  parallel ports, network cards, universal serial buses (USB), and
  firewire etc.  Most pre-2007 PC's have a serial port or two (on older
  PC's).  Each has a 9-pin connector (sometimes 25-pin) on the back of
  the computer.  Computer programs can send data (bytes) to the transmit
  pin (output) and receive bytes from the receive pin (input).  The
  other pins are for control purposes and ground.

  The serial port is much more than just a connector.  It converts the
  data from parallel to serial and changes the electrical representation
  of the data.  Inside the computer, data bits flow in parallel (using
  many wires at the same time).  Serial flow is a stream of bits over a
  single wire (such as on the transmit or receive pin of the serial
  connector).  For the serial port to create such a flow, it must
  convert data from parallel (inside the computer) to serial on the
  transmit pin (and conversely).

  Most of the electronics of the serial port is found in a computer chip
  (or a part of a chip) known as a UART.  For more details on UARTs see
  the section ``What Are UARTS?'' But you may want to finish this
  section first so that you will hopefully understand how the UART fits
  into the overall scheme of things.

  4.1.2.  Pins and Wires

  Old PC's used 25 pin connectors but only about 9 pins were actually
  used so later on most connectors were only 9-pin.  Each of the 9 pins
  usually connects to a wire.  Besides the two wires used for
  transmitting and receiving data, another pin (wire) is signal ground.
  The voltage on any wire is measured with respect to this ground.  Thus
  the minimum number of wires to use for 2-way transmission of data is
  3.  Except that it has been known to work with no signal ground wire
  but with degraded performance and sometimes with errors.

  There are still more wires which are for control purposes (signalling)
  only and not for sending bytes.  All of these signals could have been
  shared on a single wire, but instead, there is a separate dedicated
  wire for every type of signal.  Some (or all) of these control wires
  are called "modem control lines".  Modem control wires are either in
  the asserted state (on) of +5 volts or in the negated state (off) of
  -5 volts.  One of these wires is to signal the computer to stop
  sending bytes out the serial port cable.  Conversely, another wire
  signals the device attached to the serial port to stop sending bytes
  to the computer.  If the attached device is a modem, other wires may
  tell the modem to hang up the telephone line or tell the computer that
  a connection has been made or that the telephone line is ringing
  (someone is attempting to call in).  See section ``Pinout and
  Signals'' for more details.

  4.1.3.  RS-232  (EIA-232, etc.)

  The serial port (not the USB) is usually a RS-232-C, EIA-232-D, or
  EIA-232-E.  These three are almost the same thing.  The original RS
  (Recommended Standard) prefix officially became EIA (Electronics
  Industries Association) and later EIA/TIA after EIA merged with TIA
  (Telecommunications Industries Association).  The EIA-232 spec
  provides also for synchronous (sync) communication but the hardware to
  support sync is almost always missing on PC's.  The RS designation is
  was intended to become obsolete but is still widely used and will be
  used in this howto for RS-232.  Other documents may use the full
  EIA/TIA designation, or just EIA or TIA.  For info on other (non-
  RS-232) serial ports see the section ``Other Serial Devices (not async

  4.2.  IO Address & IRQ

  Since the computer needs to communicate with each serial port, the
  operating system must know that each serial port exists and where it
  is (its I/O address).  It also needs to know which wire (IRQ number)
  the serial port must use to request service from the computer's CPU.
  It requests service by sending an interrupt voltage on this wire.
  Thus every serial port device must store in its non-volatile memory
  both its I/O address and its Interrupt ReQuest number: IRQ.  See
  ``Interrupts''.  The PCI bus has its own system of interrupts.  But
  since the PCI-aware BIOS sets up these PCI interrupts to map to IRQs,
  it seemingly behaves just as described above.  Except that sharing of
  PCI interrupts is allowed (2 or more devices may use the same IRQ

  I/O addresses are not the same as memory addresses.  When an I/O
  addresses is put onto the computer's address bus, another wire is
  energized.  This both tells main memory to ignore the address and
  tells all devices which have I/O addresses (such as the serial port)
  to listen to the address sent on the bus to see if it matches the
  device's.  If the address matches, then the I/O device reads the data
  on the data bus.

  The I/O address of a certain device (such as ttyS2) will actually be a
  range of addresses.  The lower address in this range is the base
  address.  "address" usually means just the "base address".

  4.3.  Names: ttyS0, ttyS1, etc.

  The serial ports are named ttyS0, ttyS1, etc. (and usually correspond
  respectively to COM1, COM2, etc. in DOS/Windows).  The /dev directory
  has a special file for each port.  Type "ls /dev/ttyS*" to see them.
  Just because there may be (for example) a ttyS3 file, doesn't
  necessarily mean that there exists a physical serial port there.

  Which one of these names (ttyS0, ttyS1, etc.) refers to which physical
  serial port is determined as follows.  The serial driver (software)
  maintains a table showing which I/O address corresponds to which ttyS.
  This mapping of names (such as ttyS1) to I/O addresses (and IRQ's) may
  be both set and viewed by the "setserial" command.  See ``What is
  Setserial''.  This does not set the I/O address and IRQ in the
  hardware itself (which is set by jumpers or by plug-and-play
  software).  Thus which physical port corresponds to say ttyS1 depends
  both on what the serial driver thinks (per setserial) and what is set
  in the hardware.  If a mistake has been made, the physical port may
  not correspond to any name (such as ttyS2) and thus it can't be used.
  See ``Serial Port Devices /dev/ttyS2, etc.'' for more details.

  4.4.  Interrupts

  When the serial port receives a number of bytes (may be set to 1, 4,
  8, or 14) into its FIFO buffer, it signals the CPU to fetch them by
  sending an electrical signal known as an interrupt on a certain wire
  normally used only by that port.  Thus the FIFO waits until it has
  received a number of bytes and then issues an interrupt.

  However, this interrupt will also be sent if there is an unexpected
  delay while waiting for the next byte to arrive (known as a timeout).
  Thus if the bytes are being received slowly (such as from someone
  typing on a terminal keyboard) there may be an interrupt issued for
  every byte received.  For some UART chips the rule is like this: If 4
  bytes in a row could have been received in an interval of time, but
  none of these 4 show up, then the port gives up waiting for more bytes
  and issues an interrupt to fetch the bytes currently in the FIFO.  Of
  course, if the FIFO is empty, no interrupt will be issued.

  Each interrupt conductor (inside the computer) has a number (IRQ) and
  the serial port must know which conductor to use to signal on.  For
  example, ttyS0 normally uses IRQ number 4 known as IRQ4 (or IRQ 4).  A
  list of them and more will be found in "man setserial" (search for
  "Configuring Serial Ports").  Interrupts are issued whenever the
  serial port needs to get the CPU's attention.  It's important to do
  this in a timely manner since the buffer inside the serial port can
  hold only 16 incoming bytes.  If the CPU fails to remove such received
  bytes promptly, then there will not be any space left for any more
  incoming bytes and the small buffer may overflow (overrun) resulting
  in a loss of data bytes.

  There is no ``Flow Control'' to prevent this.

  Interrupts are also issued when the serial port has just sent out all
  of its bytes from its small transmit FIFO buffer out the external
  cable.  It then has space for 16 more outgoing bytes.  The interrupt
  is to notify the CPU of that fact so that it may put more bytes in the
  small transmit buffer to be transmitted.  Also, when a modem control
  line changes state, an interrupt is issued.

  The buffers mentioned above are all hardware buffers.  The serial port
  also has large buffers in main memory.  This will be explained later

  Interrupts convey a lot of information but only indirectly.  The
  interrupt itself just tells a chip called the interrupt controller
  that a certain serial port needs attention.  The interrupt controller
  then signals the CPU.  The CPU then runs a special program to service
  the serial port.  That program is called an interrupt service routine
  (part of the serial driver software).  It tries to find out what has
  happened at the serial port and then deals with the problem such a
  transferring bytes from (or to) the serial port's hardware buffer.
  This program can easily find out what has happened since the serial
  port has registers at IO addresses known to the serial driver
  software.  These registers contain status information about the serial
  port.  The software reads these registers and by inspecting the
  contents, finds out what has happened and takes appropriate action.

  4.5.  Data Flow (Speeds)

  Data (bytes representing letters, pictures, etc.) flows into and out
  of your serial port.  Flow rates (such as 56k (56000) bits/sec) are
  (incorrectly) called "speed".  But almost everyone says "speed"
  instead of "flow rate".

  It's important to understand that the average speed is often less than
  the specified speed.  Waits (or idle time) result in a lower average
  speed.  These waits may include long waits of perhaps a second due to
  ``Flow Control''.  At the other extreme there may be very short waits
  (idle time) of several micro-seconds between bytes.  If the device on
  the serial port (such as a modem) can't accept the full serial port
  speed, then the average speed must be reduced.

  4.6.  Flow Control

  Flow control means the ability to slow down the flow of bytes in a
  wire.  For serial ports this means the ability to stop and then
  restart the flow without any loss of bytes.  Flow control is needed
  for modems and other hardware to allow a jump in instantaneous flow

  4.6.1.  Example of Flow Control

  For example, consider the case where you connect a 33.6k external
  modem via a short cable to your serial port.  The modem sends and
  receives bytes over the phone line at  33.6k bits per second (bps).
  Assume it's not doing any data compression or error correction.  You
  have set the serial port speed to 115,200 bits/sec (bps), and you are
  sending data from your computer to the phone line.  Then the flow from
  the your computer to your modem over the short cable is at 115.2k bps.
  However the flow from your modem out the phone line is only 33.6k bps.
  Since a faster flow (115.2k) is going into your modem than is coming
  out of it, the modem is storing the excess flow (115.2k -33.6k = 81.6k
  bps) in one of its buffers.  This buffer would soon overrun (run out
  of free storage space) unless the high 115.2k flow is stopped.

  But now flow control comes to the rescue.  When the modem's buffer is
  almost full, the modem sends a stop signal to the serial port.  The
  serial port passes on the stop signal on to the device driver and the
  115.2k bps flow is halted.  Then the modem continues to send out data
  at 33.6k bps drawing on the data it previous accumulated in its
  buffer.  Since nothing is coming into this buffer, the number of bytes
  in it starts to drop.  When almost no bytes are left in the buffer,
  the modem sends a start signal to the serial port and the 115.2k flow
  from the computer to the modem resumes.  In effect, flow control
  creates an average flow rate in the short cable (in this case 33.6k)
  which is significantly less than the "on" flow rate of 115.2k bps.
  This is "start-stop" flow control.

  In the above simple example it was assumed that the modem did no data
  compression.  This could happen when the modem is sending a file which
  is already compressed and can't be compressed further.  Now let's
  consider the opposite extreme where the modem is compressing the data
  with a high compression ratio.  In such a case the modem might need an
  input flow rate of say 115.2k bps to provide an output (to the phone
  line) of 33.6k bps (compressed data).  This compression ratio is 3.43
  (115.2/33.6).   In this case the modem is able to compress the 115.2
  bps PC-to-modem flow and send the same data (in compressed form) out
  the phone line at 33.6bps.  There's no need for flow control here so
  long as the compression ratio remains higher than 3.43.  But the
  compression ratio varies from second to second and if it should drop
  below 3.43, flow control will be needed

  In the above example, the modem was an external modem.  But the same
  situation exists (as of early 2003) for most internal modems.  There
  is still a speed limit on the PC-to-modem speed even though this flow
  doesn't take place over an external cable.  This makes the internal
  modems compatible with the external modems.

  In the above example of flow control, the flow was from the computer
  to a modem.  But there is also flow control which is used for the
  opposite direction of flow: from a modem (or other device) to a
  computer.  Each direction of flow involves 3 buffers: 1. in the modem
  2. in the UART chip (called FIFOs) and 3. in main memory managed by
  the serial driver.  Flow control protects all buffers (except the
  FIFOs) from overflowing.

  Under Linux, the small UART FIFO buffers are not protected by flow
  control but instead rely on a fast response to the interrupts they
  issue.  Some UART chips can be set to do hardware flow control to
  protect their FIFOs but Linux (as of early 2003) doesn't seem to
  support it.  FIFO stand for "First In, First Out" which is the way it
  handles bytes in a queue.  All the 3 buffers use the FIFO rule but
  only the one in the UART is named "FIFO".   This is the essence of
  flow control but there are still some more details.

  4.6.2.  Symptoms of No Flow Control

  Understanding flow-control theory can be of practical use.  The
  symptom of no flow control is that chunks of data missing from files
  sent without the benefit of flow control.  When overflow happens,
  often hundreds or even thousands of bytes get lost, and all in
  contiguous chunks.

  4.6.3.  Hardware vs. Software Flow Control

  If feasible, it's best to use "hardware" flow control that uses two
  dedicated "modem control" wires to send the "stop" and "start"
  signals.  Hardware flow control at the serial port works like this:
  The two pins, RTS (Request to send) and CTS (Clear to send) are used.
  When the computer is ready to receive data it asserts RTS by putting a
  positive voltage on the RTS pin (meaning "Request To Send to me").
  When the computer is not able to receive any more bytes, it negates
  RTS by putting a negative voltage on the pin saying: "stop sending to
  me".  The RTS pin is connected by the serial cable to another pin on
  the modem, printer, terminal, etc.  This other pin's only function is
  to receive this signal.

  For the case of a modem, this "other" pin will be the modem's RTS pin.
  But for a printer, another PC, or a non-modem device, it's usually a
  CTS pin so a "crossover" or "null modem" cable is required.  This
  cable connects the CTS pin at one end with the RTS pin at the other
  end (two wires since each end of the cable has a CTS pin).  For a
  modem, a straight-thru cable is used.

  For the opposite direction of flow a similar scheme is used.  For a
  modem, the CTS pin is used to send the flow control signal to the CTS
  pin on the PC.  For a non-modem, the RTS pin sends the signal.  Thus
  modems and non-modems have the roles of their RTS and CTS pins
  interchanged.  Some non-modems such as dumb terminals may use other
  pins for flow control such as the DTR pin instead of RTS.

  Software flow control uses the main receive and transmit data wires to
  send the start and stop signals.  It uses the ASCII control characters
  DC1 (start) and DC3 (stop) for this purpose.  They are just inserted
  into the regular stream of data.  Software flow control is not only
  slower in reacting but also does not allow the sending of binary data
  unless special precautions are taken.  Since binary data will likely
  contain DC1 and DC3 characters, special means must be taken to
  distinguish between a DC3 that means a flow control stop and a DC3
  that is part of the binary code.  Likewise for DC1.

  4.7.  Data Flow Path; Buffers

  It's been mentioned that there are 3 buffers for each direction of
  flow (3 pairs altogether): 16-byte FIFO buffers (in the UART), a pair
  of larger buffers inside a device connected to the serial port (such
  as a modem), and a pair of buffers (say 8k) in main memory.  When an
  application program sends bytes to the serial port they first get
  stashed in the transmit serial port buffer in main memory.  The other
  member of this pair consists of a receive buffer for the opposite
  direction of byte-flow.  Here's an example diagram for the case of
  browsing the Internet with a browser.  Transmit data flow is left to
  right while receive flow is right to left.  There is a separate buffer
  for each direction of flow.

  application     8k-byte         16-byte        1k-byte        tele-
  BROWSER ------- MEMORY -------- FIFO --------- MODEM -------- phone
  program         buffer          buffer         buffer         line

  For the transmit case, the serial device driver takes out say 15 bytes
  from this transmit buffer (in main memory), one byte at a time and
  puts them into the 16-byte transmit buffer in the serial UART for
  transmission.  Once in that transmit buffer, there is no way to stop
  them from being transmitted.  They are then transmitted to the modem
  or (other device connected to the serial port) which also has a fair
  sized (say 1k) buffer.  When the device driver (on orders from flow
  control sent from the modem) stops the flow of outgoing bytes from the
  computer, what it actually stops is the flow of outgoing bytes from
  the large transmit buffer in main memory.  Even after this has
  happened and the flow to the modem has stopped, an application program
  may keep sending bytes to the 8k transmit buffer until it becomes
  full.  At the same time, the bytes stored in the FIFO continue to be
  sent out.  Bytes stored in the modem will continue to be sent out the
  phone line unless the modem has gotten a modem-to-modem flow control
  stop from the modem at the other end of the phone line.

  When the memory buffer gets full, the application program can't send
  any more bytes to it (a "write" statement in a C-program blocks) and
  the application program temporarily stops running and waits until some
  buffer space becomes available.  Thus a flow control "stop" is
  ultimately able to stop the program that is sending the bytes.  Even
  though this program stops, the computer does not necessarily stop
  computing since it may switch to running other processes while it's
  waiting at a flow control stop.

  The above was a little oversimplified in three ways.  First, some
  UARTs can do automatic hardware flow control which can stop the
  transmission out of the FIFO buffers if needed (not yet supported by
  Linux).  Second, while an application process is waiting to write to
  the transmit buffer, it could possibly perform other tasks.  Third,
  the serial driver (located between the memory buffer and the FIFO) has
  it's own small buffer (in main memory) used to process characters.

  4.8.  Complex Flow Control Example

  For many situations, there is a transmit path involving several links,
  each with its own flow control.  For example, I type at a text-
  terminal connected to a PC and the PC (under my control) dials out to
  another computer using a modem.  Today, a "text-terminal" is likely to
  be just another PC emulating a text-terminal.  The main (server) PC,
  in addition to serving my text-terminal, could also have someone else
  sitting at it doing something else.  Note that calling this PC a
  "server" is not technically correct but it does serve the terminal.

  The text-terminal uses a command-line interface with no graphical
  display.  Every letter I type at the text-terminal goes over the
  serial cable to my main PC and then over the phone line to the
  computer that I've dialed out to.  To dial out, I've used the
  communication software: "minicom" which runs on my PC.

  This sounds like a simple data path.  I hit a key and the byte that
  key generates flows over just two cables (besides the keyboard cable):
  1. the cable from my text-terminal to my PC and 2. the telephone line
  cable to some other computer.  Of course, the telephone cable is
  actually a number of telephone system cables and includes switches and
  electronics so that a single physical cable can transmit many phone
  calls.  But I can think of it like one cable (or one link).

  Now, let's count the number and type of electronic devices each
  keystroke-byte has to pass thru.  The terminal-to-PC cable has a
  serial port at each end.  The telephone cable has both a serial port
  and a modem at each end.  This adds up to 4 serial ports and 2 modems.
  Since each serial port has 2 buffers, and each modem one buffer, that
  adds up to 10 buffers.  And that's just for one direction of flow.
  Each byte also must pass thru the minicom software as well.

  While there's just 2 cables in the above scenario, if external modems
  were used there would be an additional cable between each modem and
  it's serial port.  This makes 4 cables in all.  Even with internal
  modems it's like there is a "virtual cable" between the modem and its
  serial port.  On all these 4 links (or cables), flow control takes

  Now lets consider an example of the operation of flow control.
  Consider the flow of bytes from the remote computer at the other end
  of the phone line to the screen on the text-terminal that I'm sitting
  at.  A real text-terminal has a limit to the speed at which bytes can
  be displayed on its screen and issues a flow control "stop" from time
  to time to slow down the flow.  This "stop" propagates in a direction
  opposite to the flow of bytes it controls.  What happens when such a
  "stop" is issued?  Let's consider a case where the "stop" waits long
  enough before canceling it with a "start", so that it gets thru to the
  remote computer at the other end of the phone line.  When it gets
  there it will stop the program at the remote computer which is sending
  out the bytes.

  Let's trace out the flow of this "stop" (which may be "hardware" on
  some links and "software" on others).  First, suppose I'm "capturing"
  a long file from the remote computer which is being sent
  simultaneously to both my text-terminal and to a file on my hard-disk.
  The bytes are coming in faster than the terminal can handle them so it
  sends a "stop" out its serial port to a serial port on my PC.  The
  device driver detects it and stops sending bytes from the 8k PC serial
  buffer (in main memory) to the terminal.  But minicom still keeps
  sending out bytes for the terminal into this 8k buffer.

  When this 8k transmit buffer (on the first serial port) is full,
  minicom must stop writing to it.  Minicom stops and waits.  But this
  also causes minicom to stop reading from the 8k receive buffer on the
  2nd serial port connected to the modem.  Flow from the modem continues
  until this 8k buffer too fills up and sends a different "stop" to the
  modem.  Now the modem's buffer ceases to send to the serial port and
  also fills up.  The modem (assuming error correction is enabled) sends
  a "stop signal" to the other modem at the remote computer.  This modem
  stops sending bytes out of its buffer and when its buffer gets full,
  another stop signal is sent to the serial port of the remote computer.
  At the remote computer, the 8-k (or whatever) buffer fills up and the
  program at the remote computer can't write to it anymore and thus
  temporarily halts.

  Thus a stop signal from a text terminal has halted a program on a
  remote computer computer.  What a long sequence of events!  Note that
  the stop signal passed thru 4 serial ports, 2 modems, and one
  application program (minicom).  Each serial port has 2 buffers (in one
  direction of flow): the 8k one and the hardware 16-byte one.  The
  application program may have a buffer in its C_code.  This adds up to
  11 different buffers the data is passing thru.  Note that the small
  serial hardware buffers do not participate directly in flow control.
  Also note that the two buffers associated with the text-terminal's
  serial port are going to be dumping their contents to the screen
  during this flow control halt.  This leaves 9 other buffers that may
  be getting filled up during the flow control halt.

  If the terminal speed limitation is the bottleneck in the flow from
  the remote computer to the terminal, then its flow control "stop" is
  actually stopping the program that is sending from the remote computer
  as explained above.  But you may ask: How can a "stop" last so long
  that 9 buffers (some of them large) all get filled up?   It seldom
  happens, but it can actually happen this way if all the buffers were
  near their upper limits when the terminal sent out the "stop".

  But if you were to run a simulation on this you would discover that
  it's usually more complicated than this.  At an instant of time some
  links are flowing and others are stopped (due to flow control).  A
  "stop" from the terminal seldom propagates back to the remote computer
  neatly as described above.  It may take a few "stops" from the
  terminal to result in one "stop" at the remote computer, etc.  To
  understand what is going on you really need to observe a simulation
  which can be done for a simple case with coins on a table.  Use only a
  few buffers and set the upper level for each buffer at only a few

  Does one really need to understand all this?  Well, understanding this
  explained to me why capturing text from a remote computer was loosing
  text.  The situation was exactly the above example but modem-to-modem
  flow control was disabled.  Chunks of captured text that were supposed
  to also get to my hard-disk never got there because of an overflow at
  my modem buffer due to flow control "stops" from the terminal.  Even
  though the remote computer had a flow path to the hard-disk without
  bottlenecks, the same flow also went to a terminal which issued flow
  control "stops" with disastrous results for the branch of the flow
  going to a file on the hard-disk.  The flow to the hard-disk passed
  thru my modem and since the overflow happened at the modem, bytes
  intended for the hard-disk were lost.

  4.9.  Serial Driver Module

  The device driver for the serial port is the software that operates
  the serial port.  It is now provided as a serial module.  From kernel
  2.2 on, this module will normally get loaded automatically if it's
  needed.  In earlier kernels, you had to have kerneld running in order
  to do auto-load modules on demand.  Otherwise the serial module needed
  to be explicitly listed in /etc/modules.  Before modules became
  popular with Linux, the serial driver was usually built into the
  kernel (and sometimes still is).  If it's built-in don't let the
  serial module load or else you will have two serial drivers running at
  the same time.  With 2 drivers there are all sorts of errors including
  a possible "I/O error" when attempting to open a serial port.  Use
  "lsmod" to see if the module is loaded.

  When the serial module is loaded it displays a message on the screen
  about the existing serial ports (often showing a wrong IRQ).  But once
  the module is used by setserial to tell the device driver the
  (hopefully) correct IRQ then you should see a second display similar
  to the first but with the correct IRQ, etc.  See ``Serial Module'' See
  ``What is Setserial'' for more info on setserial.

  4.10.  The Serial Port is Now Obsolete on PCs

  The serial port is today (2011) sort of obsolete on PCs (and often
  called a "legacy" device) but it is still in use on some older
  computers and is used in imbedded systems, for communication with
  routers and point-of-sale equipment, etc.  The serial port is ``slow''
  and after about 2005 most new PC's no longer had them.  Most laptops
  and Macs discontinued them even earlier.  During the era when some new
  PC's came with serial ports and others didn't, the PC's that didn't
  have serial ports were euphemistically called "legacy-free".  However,
  while PC's today no longer have serial ports, some do have them built
  into a "legacy" modem (which plugs into a telephone line).  Such a
  serial port only works with the modem and can't be used for any other
  device.  The reason they have such a "built in" serial port is that
  analog modems are designed to only work thru a serial port.

  The physical serial port on the back of a PC (including the chip its
  connected to inside the PC), must pass data between the computer and
  an external cable.  Thus it has two interfaces: the serial-port-to
  cable and the serial-port-to-computer-bus.  Both of these interfaces
  are slow.  First we'll consider the interface via external cable to
  the outside world.

  4.11.  RS-232 Cable Is Low Speed & Short Distance

  The conventional RS-232 serial port is inherently low speed and is
  severely limited in distance.  Ads often read "high speed" but it can
  only work at "high speed" over very short distances such as to a modem
  located right next to the computer.  Compared to a network card, even
  this "high speed" is actually low speed.  All of the RS-232 serial
  cable wires use a common ground return wire so that twisted-pair
  technology (needed for high speeds) can't be used without additional
  hardware.  More modern interfaces for serial ports exist but they are
  not standard on PC's like the RS-232 is.  See ``Successors to
  RS-232''.  Some multiport serial cards support them.

  It is somewhat tragic that the RS-232 standard from 1969 did not use
  twisted pair technology which could operate about a hundred times
  faster.  Twisted pairs have been used in telephone cables since the
  late 1800's.  In 1888 (over 120 years ago) the "Cable Conference"
  reported its support of twisted-pair (for telephone systems) and
  pointed out its advantages.  But over 80 years after this approval by
  the "Cable Conference", RS-232 failed to utilize it.   Since RS-232
  was originally designed for connecting a terminal to a low speed modem
  located nearby, the need for high speed and longer distance
  transmission was apparently not recognized.  The result was that since
  the serial port couldn't handle high speeds, new types of serial
  interfaces were devised that could: Ethernet, USB, Firewire, etc.  The
  final outcome was the demise of the serial port which due to it's
  ancient technology became obsolete for high speed uses.

  4.12.  Inefficient PCI Interface to the Computer (in some cases)

  The serial port communicates with the computer via the PCI bus, the
  LPC bus, X-bus, or ISA bus.  The PCI bus is now 32 or 64 bits wide,
  but the serial port only sends a byte at a time (8 bits wide) which is
  a waste of PCI bus bandwidth.  Not so for the LPC bus which has only a
  4-bit wide bus and thus provides an efficient interface.  The ISA bus
  is usually 16-bits wide and the efficiency is intermediate as compared
  to efficient LPC and inefficient PCI.

  5.  Multiport Serial Boards/Cards/Adapters

  5.1.  Intro to Multiport Serial

  Multiport serial cards install in slots in a PC on the ISA or PCI bus.
  They are also called "...  adapters" or "... boards".  Each such card
  provides you with many serial ports.  Today they are commonly used for
  the control of external devices (including automation for both
  industry and the home).  They can connect to computer servers for the
  purpose of monitoring/controlling the server from a remote location.
  They were once mainly used for connecting up many dumb terminals
  and/or modems to serial ports.  Today, use of dumb terminals has
  declined, and several modems (or digital modems) can now be built into
  an internal card.  So multiport serial cards are not as significant as
  they once were.

  Each multiport card has a number of external connecters (DB-25 or
  RJ45) so that one may connect up a number of devices (modems,
  terminals, etc.).  Each such physical device would then be connected
  to its own serial port.  Since the space on the external-facing part
  of the card is limited there is often not enough room for all the
  serial port connectors.  To solve this problem, the connectors may be
  on the ends of cables which come out (externally) from the card
  (octopus cable).  Or they may be on an external box (possibly rack
  mountable) which is connected by a cable to a multiport card.

  5.2.  Dumb vs. Smart Cards

  Dumb multiport cards are not too much different than ordinary serial
  ports.  They are interrupt driven and the CPU of the computer does
  most all the work servicing them.  They usually have a system of
  sharing a single interrupt for all the ports.  This doesn't decrease
  the load on the CPU since the single interrupt will be sent to the CPU
  each time any one port needs servicing.  Such devices usually require
  special drivers that you must either compile into the kernel or use as
  a module.

  Smart boards may use ordinary UARTs but handle most interrupts from
  the UARTs internally within the board.  This frees the CPU from the
  burden of handling  all these interrupts.  The board may save up bytes
  in its large internal FIFOs and transfer perhaps 1k bytes at a time to
  the serial buffer in main memory.  It may use the full bus width of 32
  bits for making data transfers to main memory (instead of transferring
  only 8-bit bytes like dumb serial cards do).  Not all "smart" boards
  are equally efficient.  Many boards today are Plug-and-Play.

  5.3.  Getting/Enabling a Driver

  5.3.1.  Introduction

  For a multiport board to work, a special driver for it must be used.
  This driver may either be built into the kernel source code or
  supplied as a module.  For the 2.6 kernels on, most drivers are
  supplied both ways: as a module or it can be built into the kernel.
  Take care not to both build support into the kernel and force the
  module to load for a certain serial card.  For older kernels, there
  were often no modules for dumb serial multiport boards so support was
  built into the kernel.

  5.3.2.  Build (compile) support into the kernel?

  A pre-compiled kernel may not have a driver for your multiport card
  built in.  So then you must either compile the kernel yourself and
  build in the right driver, or insure that the module is available and
  loads.  Of course if the driver doesn't come both ways (as a compile-
  time option and as a module) you have no such choice.

  If you want to see what has already been compiled into an existing
  working kernel, go the the /boot directory (or wherever the compiled
  kernel(s) reside) and look in the config... file.

  In the 2.6 kernel there are many options to select from in the
  configuration file for compiling.  Adding support for certain
  multiport cards is listed under the headings "Character devices" or
  "Serial drivers".   Old multiport cards had support as part of the
  serial driver and are found under "Serial Drivers".  More advanced
  cards have their own driver found under "Character devices"

  For compiling kernel 2.6 you should select
  2.4).  Then you will be asked more questions about your serial ports
  with more options to select.  If the resulting configuration is not
  quite right, then you may need to edit the kernel configuration file

  5.3.3.  Using module support

  A pre-compiled kernel may come with a pre-compiled module for the
  board so that you don't need to recompile the kernel.  This module
  must be loaded in order to use it and if there is installation
  software for the driver, it should also set up Linux to load the
  module (probably at boottime).  Some of the modules to load at
  boottime are listed in /etc/modules or /etc/modules.conf Also certain
  parameters may need to be passed to the driver via entries in these
  files or via lilo's "append" command or via grub's "kernel" command.
  For kernel 2.6 (and 2.4) the (unloaded) modules should be found in

  5.3.4.  Getting info on multiport boards

  The board's manufacturer should have info on their website.
  Unfortunately, info for old boards is sometimes not there but might be
  found somewhere else on the Internet (including discussion groups).
  You might also want to look at the kernel documentation in
  /usr/share/doc/linux-doc... (formerly kernel-doc in pre 2.6 kernels).
  For configuring the kernel or modules prior to compiling see: and search for "serial", etc.  There are also kernel
  documentation files for certain boards including computone, hayes-esp,
  moxa-smartio, riscom8, specialix, stallion, and sx (specialix).

  5.4.  Multiport Devices in the /dev Directory,

  The serial ports your multiport board uses depends on what kind of
  board you have.  Some have their own device names like /dev/ttyE27
  (Stallion) or /dev/ttyD2 (Digiboard), etc.  For various other brands,
  see see devices.txt in the kernel documentation.  Some use the
  standard names like /dev/ttyS14 and may be found in configuration
  files that used as arguments to setserial.  Such files may be included
  in a setserial or serial package.

  5.5.  Making Legacy Multiport Devices in the /dev Directory

  An installation script may do this for you.  But if not, here's some
  examples of how to create a device name in the /dev directory.  If you
  use udev, MAKEDEV will not create devices in the device directory
  since this directory is only in memory and will be lost when you turn
  off the computer.  Instead it will create the device indev/.static/dev

  For the names and numbers of other types of serial ports other than
  ttyS.. See devices.txt in the kernel documentation.  Either use the
  mknod command, or the MAKEDEV script.  Typing "man makedev" may show
  instructions on using it.

  Using the MAKEDEV script, you would first become the superuser (root)
  and type (for example) either:

       linux# MAKEDEV ttyS17

  Or if the above doesn't work cd to /dev before giving the above
  command>.  Substitute whatever your port is for ttyS17.

  Using mknod is a more complicated option since you need to know the
  major and minor device numbers.  These numbers are in the "devices"
  file in the kernel documentation.  For ttyS serial ports the minor
  number is: 64 + port number (=81 for the example below).  Note the
  "major" number is always 4 for ttyS devices (and 5 for the obsolete
  cua devices).  So, if you wanted to create a device for ttyS17 using
  mknod, you would type:

       linux# mknod -m 666 /dev/ttyS17 c 4 81

  5.6.  Standard PC Serial Cards

  In olden days, PCs came with a serial card installed.  Later on, the
  serial function was put on the hard-drive interface card.  In the
  1990s and early 2000s one or two serial ports were usually built into
  the motherboard (on-board). Most of them (as of 2002) use a 16550 but
  some use 16650 (32-byte FIFOs).  But one may still buy the individual
  PC serial cards if they need more serial ports.  They can be used to
  connect external serial devices (modems, serial mice, etc...).  Only a
  tiny percentage of retail computer stores carry such cards.  But one
  can purchase them on the Internet.  Before getting one for the PCI
  bus, make sure Linux supports it.

  Here's a list of a few popular brands:

  В·  Byte Runner (may order directly, shows prices)

  В·  SIIG  <>

  В·  Dolphin  <>

  Note: due to address conflicts, you may not be able to use /dev/ttyS3
  with a IBM8514 video card (and some others) simultaneously.  See
  ``Avoiding IO Address Conflicts with Certain Video Boards''

  5.7.  Dumb Multiport Serial Boards (with standard UART chips)

  They are also called "serial adapters".  Each port has its own
  address.  They often have a special method of sharing interrupts which
  requires that you compile support for them into the kernel.

  * =>  The file that ran setserial in Debian shows some details of
  # => See note below for this board

  В·  AST FourPort and clones (4 ports) * #

  В·  Accent Async-4 (4 ports) *

  В·  Arnet Multiport-8 (8 ports)

  В·  Bell Technologies HUB6 (6 ports)

  В·  Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports; See
     the Boca mini-howto revised in 2001) * #

  В·  Boca IOAT66 or? ATIO66 (6 ports, Linux doesn't support its IRQ
     sharing ??  Uses odd-ball 10-cond RJ45-like connectors)

  В·  Boca 2by4 (4 serial ports, 2 parallel ports)

  В·  Byte Runner  <>

  В·  Computone ValuePort V4-ISA (AST FourPort compatible) *

  В·  Digi PC/8 (8 ports) #

  В·  Dolphin  <>

  В·  Globetek  <>

  В·  GTEK BBS-550 (8 ports; See the mini-howto)

  В·  Hayes ESP (after kernel 2.1.15)

  В·  HUB-6 See Bell Technologies.

  В·  Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible) *

  В·  Moxa C104, Moxa C104+ (AST FourPort compatible) *

     by National Instruments

  В·  NetBus (2 ports) <> using patch from

  В·  PC-COMM (4 ports)

  В·  Sealevel Systems <> COMM-2 (2 ports), COMM-4
     (4 ports) and COMM-8 (8 ports)

  В·  SIIG I/O Expander 2S IO1812 (4 ports) #

  В·  STB-4COM (4 ports)

  В·  Twincom ACI/550

  В·  Usenet Serial Board II (4 ports) *

  В·  VScom (uses same driver as ByteRunner)

  In general, Linux will support any serial board which uses a 8250,
  16450, 16550, 16550A, 16650, 16650V2, 16654, 16750, 16850, 16950, and
  16954.  UART.  See the latest man page for "setserial" for a more
  complete list.


  AST Fourport: You might need to specify skip_test in rc.serial.

  BB-1004 and BB-1008 do not support DCD and RI lines, and thus are not
  usable for dialin modems.  They will work fine for all other purposes.

  Digi PC/8 Interrupt Status Register is at 0x140.

  SIIG IO1812 manual for the listing for COM5-COM8 is wrong.  They
  should be COM5=0x250, COM6=0x258, COM7=0x260, and COM8=0x268.

  5.8.  Intelligent Multiport Serial Boards

  Make sure that a Linux-compatible driver is available and read the
  information that comes with it.  These boards use special devices (in
  the /dev directory), and not the standard ttyS ones.  This information
  varies depending on your hardware.  If you have updated info which
  should be shown here please email it to me.

  Names of Linux driver modules are *.ko (*.o prior to kernel 2.6) but
  these may not work for all models shown.  See ``Modules (mostly for
  smart boards)'' The needed module may have been supplied with your
  Linux distribution.  Also, parameters (such as the io and irq often
  need to be given to the module so you need to find instructions on
  this (possibly in the source code tree).

  There are many different brands, each of which often offers many
  different cards.  No attempt is currently being made to list all the
  cards here (and many listed are obsolete and have bad internet links
  to them which need to be fixed).  But all major brands and websites
  should be shown here so it something is missing let me know.  Go to
  the webpage shown for more information.  These websites often also
  have info (ads) on related hardware such as modem pools, remote access
  servers (RASs), and terminal servers.  Where there is no webpage, the
  cards are likely obsolete.  If you would like to put together a better
  list, let me know.

  В·  Chase Research, now Perle Systems Ltd (UK based, ISA/PCI cards)
     webpage:  <>
     driver status: included in kernel 2.4+ for PCI only; otherwise
     supported by Perle
     driver and manual location:
  В·  Comtrol RocketPort (36MHz ASIC; 4, 8, 16, 32, up to 128 ports)
     driver status: supported by Comtrol.  rocket.o
     driver location:

  В·  Computone IntelliPort II (ISA, PCI and EISA busses up to 64 ports)
     webpage:  <>
     driver location: old patch at
     mailing list:  <> with
     "subscribe linux-computone" in body
     note: Old ATvantage and Intelliport cards are not supported by

  В·  Connecttech
     website:  <>
     driver location:  <>

  В·  Cyclades
     Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),
     Cyclom-Z (MIPS R3000; 8 - 64 ports)
     website:  <>
     driver status: supported by Cyclades
     driver location: and included
     in Linux kernel since version 1.1.75: cyclades.o

  В·  Decision PCCOM (2-8 ports; ISA and PCI; aka PC COM)
     contact:  <>
     driver location: (dead link)
     drivers:  <>
     driver status: Support in serial driver 5.03.  For an earlier
     driver, there exists a patch for kernel 2.2.16 at
     <> and for kernels 2.2.14-2.2.17 at

  В·  Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),
     PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),
     PC/Xr (16MHz IDT3041; 4 or 8 ports),
     PC/Xem (20MHz IDT3051; 8 - 64 ports)
     website:  <>
     driver status: supported by Digi
     driver location: and included in
     Linux kernel since version 2.0. epca.o

  В·  Digi COM/Xi (10MHz 80188; 4 or 8 ports)
     contact: Simon Park,
     driver status: ?
     note: Simon is often away from email for months at a time due to
     his job.  Mark Hatle,  <> has
     graciously volunteered to make the driver available if you need it.
     Mark is not maintaining or supporting the driver.

  В·  Equinox SuperSerial Technology (30MHz ASIC; 2 - 128 ports)
     driver status: supported by Equinox
     driver location:

  В·  Globetek
     website:  <>
     driver location:

  В·  GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),
     SmartCard (24MHz Dallas DS80C320; 8 ports),
     BlackBoard-8A (16C654 UARTs; 8 ports),
     PCSS (15/24MHz 8032; 8 ports)
     driver status: supported by GTEK
     driver location:

  В·  Hayes ESP (COM-bic; 1 - 8 ports)
     driver status: Supported by Linux kernel (1998) since v. 2.1.15.
     esp.o.  Setserial 2.15+ supports. Also supported by author
     driver location:

  В·  Intelligent Serial Interface by Multi-Tech Systems
     PCI: 4 or 8 port.  ISA 8 port. DTE speed 460.8k.  Discontinued
     webpage:  <>

  В·  Maxpeed SS (Toshiba; 4, 8 and 16 ports)
     driver status: supported by Maxpeed
     driver location:

  В·  Microgate SyncLink ISA and PCI high speed multiprotocol serial.
     Intended for synchronous HDLC.
     website:  <http://ww/>
     driver status: supported by Microgate: synclink.o

  В·  Moxa C218 (12MHz 80286; 8 ports),
     Moxa C320 (40MHz TMS320; 8 - 32 ports)
     driver status: supported by Moxa
     driver locations:
      <> (also from Taiwan at where ... is the same as above)

  В·  SDL RISCom/8 (Cirrus Logic CD180; 8 ports)
     driver status: supported by SDL
     driver location:

  В·  Specialix SX (25MHz T225; 8? - 32 ports),
     SIO/XIO (20 MHz Zilog Z280; 4 - 32 ports)
     webpage: Old link is broken.  Out of business?
     driver status: Was supported by Specialix
     driver location:  <>
     old driver location:

  В·  Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and
     EasyConnection (8 - 32 ports) - each with Cirrus Logic CD1400
     Stallion (8MHz 80186 CPU; 8 or 16 ports),
     Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),
     ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),
     EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)
     contact: or
     driver status: supported by Stallion
     driver location: and
     included in linux kernel since 1.3.27.  Moved: it's now at ?.

  В·  System Base website:  <>

  A review of Comtrol, Cyclades, Digi, and Stallion products was printed
  in the June 1995 issue of the Linux Journal.  The article is available
  at Review: Intelligent Multiport Serial Boards
  <> Besides the listing of
  various brands of multiports found above in this HOWTO there is Gary's
  Encyclopedia - Serial Cards <>.
  It's not as complete, but may have some different links.

  5.9.  Unsupported Multiport Boards

  The following brands that formerly made boards for with Linux support
  don't mention any Linux support as of 1 Jan.  2000.  Let me know if
  this changes.

  В·  Aurora (PCI only) <>

  6.  Servers for Serial Ports

  A computer that has many serial ports (with many serial cables
  connected to it) is often called a server.  Of course, most servers
  serve other functions besides just serving serial ports, and many do
  not serve serial ports at all (although they likely have a serial port
  on them).  For example, a "serial server" may have serial cables, each
  of which runs to a different (non-serial) server.  The serial server
  (perhaps called a "console server") controls, via a console, all the
  other servers.  The console may be physically located remote from the
  serial server, communicating with the server over a network.

  There are two basic types of serial servers.  One type is just an
  ordinary computer (perhaps rack mounted) that uses multiport cards on
  a PCI bus (or the like).  The other type is a proprietary server that
  is a dedicated computer that serves a special purpose.  Servers of
  both types may be called: serial servers, console servers, print
  servers, or terminal servers.  They are not the same.

  The terminal server was originally designed to provide many serial
  ports, each connected to a dumb text-terminal.  Today, a terminal
  server often connects to graphic terminals over a fast network and
  doesn't use serial ports since they are too slow.  One network cable
  takes the place of many serial cables and each graphic terminal uses
  far more bandwidth than the text-terminals did.  However, graphic
  terminals may be run in text mode to reduce the bandwidth required.  A
  more detailed discussion of terminal servers (serial port) is in Text-
  Terminal-HOWTO.  For networked terminal servers (not serial port) see
  Linux Terminal Server Project (LTSP) <>

  (To-do: Discuss other types of serial servers, but the author knows
  little about them.)

  7.  Configuring Overview

  In most cases configuring is automatically taken care of and you have
  nothing to do.  Linux should detect your serial ports, and load driver
  modules if needed.  Then the driver should insure that IRQ and address
  space resources have been allocated.  Finally, the application program
  which uses the serial port should set the port speed, etc.

  For any of this to work, serial support must either be compiled into
  the kernel (by you or by whoever compiled your kernel) or provided by
  modules that are loaded into the kernel when you start to use the
  serial port.  In most cases, if it hasn't been compiled into the
  kernel, a module(s) will do the job and Linux should hopefully
  automatically find and load the correct modules.

  But if you have more than 4 (or perhaps 2) serial ports, then the
  kernel must be told this as it doesn't seem to do it automatically.
  See ``Number of Serial Ports Supported'', ``Kernel Configuration''
  and. ``Serial Modules''.

  Once the proper support is in your kernel and modules, then The rest
  of the configuring of the serial port should happen automatically.
  This is done by the serial driver software often with help from your
  application software.  But sometimes it doesn't get configured right
  and then you need to do it yourself.  Or perhaps you need to configure
  it in a special way, etc.  This HOWTO only covers configuration of the
  serial port itself and not the configuring of any devices attached to
  the port (such as a modem or printer).

  Resource allocation (locating the hardware or low-level configuring)
  is assigning each port an IO address, IRQ, and name (such as ttyS2).
  This IO-IRQ pair must be set in both the hardware and become known to
  the serial driver.  We might just call this "io-irq" configuring for
  short.  The "setserial" program is sometimes used to tell the driver
  io-irq info that an administrator has put in a configuration file or
  given as parameters to the setserial command.  PnP methods, jumpers,
  etc, are used to set the IO and IRQ in the hardware.  Details will be
  supplied later.  If you need to configure but don't understand certain
  details it's easy to get into trouble.  See ``Locating the Serial
  Port: IO address IRQs'' ``What is Setserial''

  The second part (high-level configuring) is assigning it a speed (such
  as 115.2k bits/sec), selecting flow control, etc.  This is often
  initiated by communication programs such as wvdial, PPP, minicom,
  picocom or by getty (which you may run on the port so that others may
  log into your computer from an old-fashioned dumb terminal connected
  to the port).  However you will need to tell these programs what speed
  you want, etc.  by using a menu or a configuration file.  This high-
  level configuring may also be done manually with the stty program.
  stty is also useful to view the current status if you're having
  problems.  See the section ``Stty''

  8.  Locating the Serial Port: IO address, IRQs

  8.1.  What Bus is my Serial Port On?

  If you need to find a serial port it often helps if you know what bus
  it's on.  If the serial port is on a card, you may know what bus the
  card inserts into such as a PCI slot.  But if the serial port is built
  into the motherboard it may not be clear what bus it's on.  For old
  motherboards that have ISA bus slots, it's likely on the ISA bus and
  may not even be Plug-and-Play.  But even if all your slots are PCI,
  the serial port is likely to be on either an ISA bus or LPC bus (also
  called a "LPC interface").  LPC is common on laptop computers.  Type
  "lspci" to see if it shows "LPC".  Unfortunately, the LPC bus has no
  standard Plug-and-Play method for low-level configuring devices on it.
  But according to the specs, the BIOS is supposed to do such
  configuring (using ACPI ??).  To see if you have a LPC bus, type
  "lspci" and look for a LPC bridge or the like.

  8.2.  IO & IRQ Overview

  For a serial port to work properly it first must be given both an IO
  address and an IRQ.  For old hardware (of mid 1990s), jumpers on a
  card or a saved BIOS setting does it.  For newer hardware the BIOS or
  Linux must set them at boot-time, and the new hardware doesn't
  remember how it was set once it's powered down.  Enabling the hardware
  (by using software) gives it both an IRQ and an IO address.  Without
  an IO address, it can't be used.  Without an IRQ it will need to use
  inefficient polling methods for which one must set the IRQ to 0 in the
  serial driver.  Using digital signals sent to the hardware by the BIOS
  or Linux, it all should get configured automatically (provided the
  BIOS has not been previously set up to disable it).  So you only need
  to read this if you're having problems or if you want to understand
  how it works.

  The driver must of course know both the IO address and IRQ so that it
  can talk to the serial port chip.   Modern serial port drivers (kernel
  2.4) try to determine this by PnP methods so one doesn't normally need
  to tell the driver (by using "setserial").   A driver may also set an
  IO address or IRQ in the hardware.  But unfortunately, there is some
  PCI serial port hardware that the driver doesn't recognize so you
  might need to enable the port yourself.  See ``PCI: Enabling a
  disabled port''

  For the old ISA bus, the driver also probes likely serial port
  addresses to see if there are any serial ports there.  This works for
  the case of jumpers and sometimes works for a ISA PnP port when the
  driver doesn't do ISA PnP (prior to kernel 2.4).

  Locating the serial port by giving it an IRQ and IO address is low-
  level configuring.  It's often automatically done by the serial driver
  but sometimes you have to do it yourself.  What follows repeats what
  was said above but in more detail.

  The low-level configuring consists of assigning an IO address, IRQ,
  and names (such as ttyS2) .  This IO-IRQ pair must be set in both the
  hardware and told to the serial driver.  And the driver needs to call
  this pair a name (such as ttyS2).  We could call this "io-irq"
  configuring for short.  The modern way to do this is for the driver to
  use PnP methods to detect/set the IO/IRQ and then remember what it
  did.  An easy way for a driver to do this is for the driver to ask the
  kernel to enable the device and then the kernel tells the driver what
  IO/IRQ it has used.  The old way is using the "setserial" program to
  tell the driver.  For jumpers, there is no PnP but the driver might
  detect the port if the jumpers are set to the usual IO/IRQ.  If you
  need to configure but don't understand certain details it's easy to
  get into trouble.

  When Linux starts, an effort is made to detect and configure (low-
  level) the serial ports.  Exactly what happens depends on your BIOS,
  hardware, Linux distribution, kernel version, etc.  If the serial
  ports work OK, there may be no need for you to do any more low-level

  If you're having problems with the serial ports, then you may need to
  do low-level configuring.  If you have kernel 2.2 or lower, then you
  need to do it if you:

  В·  Plan to use more than 2 ISA serial ports

  В·  Are installing a new serial port (such as an internal modem)

  В·  One or more of your serial ports have non-standard IRQs or IO

  Starting with kernel 2.2 you may be able to use more than 2 serial
  ports without doing any low-level configuring by sharing interrupts.
  All PCI ports should support this but for ISA it only works for some
  hardware.  It may be just as easy to give each port a unique interrupt
  if they are available.  See ``Interrupt sharing and Kernels 2.2+''

  The low-level configuring (setting the IRQ and IO address) seems to
  cause people more trouble than the high-level stuff, although for many
  it's fully automatic and there is no configuring to be done.  Until
  the port is enabled and the serial driver knows the correct IRQ and IO
  address, the port will not usually not work at all.

  A port may be disabled, either by the BIOS or by failure of Linux to
  find and enable the port.  For modern ports (provided the BIOS hasn't
  disabled them) manual PnP tools such as lspci should be able to find
  them.  Applications, and utilities such as "setserial" and "scanport"
  (Debian only ??) only probe IO addresses, don't use PnP tools, and
  thus can't detect disabled ports.

  Even if an ISA port can be found by the probing done by the serial
  driver it may work extremely slow if the IRQ is wrong.  See
  ``Extremely Slow: Text appears on the screen slowly after long
  delays''.  PCI ports are less likely to get the IRQ wrong.

  IO address, IRQs, etc. are called "resources" and we are thus
  configuring certain resources.  But there are many other types of
  "resources" so the term has many other meanings.  In summary, the low-
  level configuring consists of enabling the device, giving it a name
  (ttyS2 for example) and putting two values (an IRQ number and IO
  address) into two places:

  1. The device driver (done by PnP or "setserial")

  2. Configuration registers of the serial port hardware itself, done by
     PnP software (or jumpers on legacy hardware).

  You may watch the start-up (= boot-time) messages.  They are usually
  correct.  But if you're having problems, your serial port may not show
  up at all or if you do see a message from  "setserial" it may not show
  the true configuration of the hardware (and it is not necessarily
  supposed to).  See ``I/O Address & IRQ: Boot-time messages''.

  8.3.  PCI Bus Support

  8.3.1.  Introduction

  If you have kernel 2.4 or better, then there should be support for PnP
  for both the PCI and ISA buses (either built-in or by modules).  Some
  PCI serial ports can be automatically detected and low-level
  configured by the serial driver.  Others may not be.

  While kernel 2.2 supported PCI in general, it had no support for PCI
  serial ports (although some people got them working anyway).  Starting
  with kernel 2.4, the serial driver will read the id number digitally
  stored in the serial hardware to determine how to support it (if it
  knows how).  It should assign an I/O address to it, determine its IRQ,
  etc.  So you don't need to use "setserial" for it.

  There is a possible problem if you don't use the device filesystem.
  The driver may assign the port to say tt/ttyS4 per a boot-time message
  (use dmesg to see it).  But if you don't have a "file" /dev/ttyS4 then
  the port will not work.  So you will then need to create it, using
  cd /dev; ./MAKEDEV ttyS4

  8.3.2.  More info on PCI

  PCI ports are not well standardized.  Some use main memory for
  communication with the PC.  Some require special enabling of the IRQ.
  The output of "lspci -vv" can help determine if one can be supported.
  If you see a 4-digit IO port, the port might work by just telling
  "setserial" the IO port and the IRQ.  For example, if lspci shows IRQ
  10, I/O at 0xecb8 and you decide to name it ttyS2 then the command is:

  setserial /dev/ttyS2 irq 10 port 0xecb8 autoconfig

  Note that the boot-time message "Probing PCI hardware" means reading
  the PnP configuration registers in the PCI devices which detects info
  about all PCI cards and on-board PCI devices.  This is different than
  the probing of IO addresses by the serial driver which means reading
  certain IO addresses to see if what's read looks like there's a serial
  port at that address.

  8.4.  Common mistakes made re low-level configuring

  Here are some common mistakes people make:

  В·  setserial command: They run it (without the "autoconfig" and
     auto_irq options) and think it has checked the hardware to see if
     what it shows is correct (it hasn't).

  В·  setserial messages:  They see them displayed on the screen at boot-
     time (or by giving the setserial command) and erroneously think
     that the result always shows how their hardware is actually

  В·  /proc/interrupts: When their serial device isn't in use they don't
     see its interrupt there, and erroneously conclude that their serial
     port can't be found (or doesn't have an interrupt set).

  В·  /proc/ioports and /proc/tty/driver/serial: People think this shows
     the actual hardware configuration when it only shows about the same
     info (possibly erroneous) as setserial.

  8.5.  IRQ & IO Address Must be Correct

  There are really two answers to the question "What is my IO and IRQ?"
  1. What the device driver thinks has been set (This is what setserial
  usually sets and shows.).  2. What is actually set in the hardware.
  Both 1. and 2. above should be the same.  If they're not it spells
  trouble since the driver has incorrect info on the physical serial
  port.  In some cases the hardware is disabled so it has no IO address
  or IRQ.

  If the driver has the wrong IO address it will try to send data to a
  non-existing serial port --or even worse, to some other device.  If it
  has the wrong IRQ the driver will not get interrupt service requests
  from the serial port, resulting in a very slow or no response.  See
  ``Extremely Slow: Text appears on the screen slowly after long
  delays''.  If it has the wrong model of UART there is also apt to be
  trouble.  To determine if both IO-IRQ pairs are identical you must
  find out how they are set in both the driver and the hardware.

  8.6.  What is the IO Address and IRQ per the driver ?

  8.6.1.  Introduction

  What the driver thinks is not necessarily how the hardware is actually
  set.  If everything works OK then what the driver thinks is likely
  correct (set in the hardware) and you don't need to investigate
  (unless you're curious or want to become a guru).  Ways to determine
  what the driver thinks include: boot-time messages ``I/O Address &
  IRQ: Boot-time messages'', the /proc directory "files" ``The /proc
  directory and setserial'', and the "setserial" command.

  8.6.2.  I/O Address & IRQ: Boot-time messages

  In many cases your ports will automatically get low-level configured
  at boot-time (but not always correctly).  To see what is happening,
  look at the start-up messages on the screen.  Don't neglect to check
  the messages from the BIOS before Linux is loaded (no examples shown
  here).  These BIOS messages may be frozen by pressing the Pause key
  (while holding down shift).  It's often tricky to freeze them and you
  may need to hit Ctrl-Alt-Del while Linux is booting to start rebooting
  and try again.  What these messages display may change as booting
  progresses and it's often tricky to freeze it at exactly the right

  Use Shift-PageUp to scroll back to the messages after they have
  flashed by.  Shift-PageDown will scroll in the opposite direction.
  The dmesg command (or looking at logs in /var/log) will show only the
  first of these two messages.  Here's an example of the start-up
  messages (as of 2004, almost the same as for 1999).  Note that in
  older versions ttyS1 was displayed in these messages as ttyS01, etc.

  At first you see what was detected (but the irq is only a wild guess):

       Serial driver version 4.27 with no serial options enabled
       ttyS0 at 0x03f8 (irq = 4) is a 16550A
       ttyS1 at 0x02f8 (irq = 3) is a 16550A
       ttyS2 at 0x03e8 (irq = 4) is a 16550A
       ttyS4 at port 0xeff0 (irq = 10) is a 16550A

  Note that ttyS0-ttyS2 were detected by probing the standard addresses
  while ttyS4 is a PCI port detected by probing the PCI configuration.
  Later setserial shows you  what was saved in a configuration file
  (which you may edit), but it's not necessarily correct either:

       Loading the saved-state of the serial devices...
       /dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
       /dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A

  Note that the configuration file only had ttyS1-2 in it so that ttyS0
  and ttyS4 were not affected by it.  There is also a slight
  discrepancy: The first message shows ttyS2 at irq=4 while the second
  shows it at irq=5.  In most cases the second message is the correct
  one.  But if you're having trouble, it may be misleading.  Before
  reading the explanation of all of this complexity in the rest of this
  section, you might just try using your serial port and see if it works
  OK.  If so it may not be essential to read further.

  The second message is from the setserial program being run at boot-
  time from a script in the /etc directory tree.  It shows what the
  device driver thinks is the correct configuration.  But this too could
  be wrong.  For example, the irq could actually be set to irq=8 in the
  hardware (both messages wrong).  The irq=5 could be there because the
  configuration file is incorrect.
  With old jumper-set serial ports Linux sometimes gets IRQs wrong
  because it doesn't by default probe for IRQs.  It just assumes the
  "standard" ones (first message) or accepts what is in a configuration
  file (second message).  Neither of these is necessarily correct.  If
  the serial driver has the wrong IRQ, the serial port is very slow or
  doesn't seem to work at all.

  The first message is a result of Linux probing the ISA serial port
  addresses but it doesn't probe for IRQs.  If a port shows up here it
  exists but the IRQ may be wrong.  Linux doesn't check IRQs because
  doing so is not foolproof.  It just assumes the IRQs are as shown
  because they are the "standard" values.  You may check them manually
  with setserial using the autoconfig and auto_irq options but this
  isn't guaranteed to be correct either.

  The data shown by the BIOS messages (which you see at first before
  Linux is booted) are what is initially set in the hardware.  If your
  serial port is Plug-and-Play (PnP) then it's possible that "isapnp" or
  "setpci" will run and change these settings.  Look for messages about
  this after Linux starts.  The last serial port message shown in the
  example above should agree with the BIOS messages (as possibly
  modified by isapnp or setpci).  If they don't agree then you either
  need to change the setting in the port hardware or use setserial to
  tell the driver what is actually set in the hardware.

  Also, if you have Plug-and-Play (PnP) serial ports, they can only be
  found by PnP software unless the IRQ and IO has been set inside the
  hardware by Plug-and-Play software.  Prior to kernel 2.4 this was a
  common reason why the start-up messages did not show a serial port
  that physically exists.  A PnP BIOS may automatically low-level
  configure them.  PnP configuring will be explained later.

  8.6.3.  The /proc directory and setserial

  Type "setserial -g /dev/ttyS*".   There are some other ways to find
  this info by looking at "files" in the /proc directory.  Be warned
  that there is no guarantee that the same is set in the hardware.

  /proc/ioports will show the IO addresses that the drivers are using.
  /proc/interrupts shows the IRQs that are used by drivers of currently
  running processes (that have devices open).  It shows how many
  interrupts have actually be issued.  /proc/tty/driver/serial shows
  much of the above, plus the number of bytes that have been received
  and sent (even if the device is not now open).

  Note that for the IO addresses and IRQ assignments, you are only
  seeing what the driver thinks and not necessarily what is actually set
  in the hardware.  The data on the actual number of interrupts issued
  and bytes processed is real however.  If you see a large number of
  interrupts and/or bytes then it probably means that the device is (or
  was) working.  But the interrupts might be from another device.  If
  there are no bytes received (rx:0) but bytes were transmitted (tx:3749
  for example), then only one direction of flow is working (or being

  Sometimes a showing of just a few interrupts doesn't mean that the
  interrupt is actually being physically generated by any serial port.
  Thus if you see almost no interrupts for a port that you're trying to
  use, that interrupt might not be set in the hardware.  To view
  /proc/interrupts to check on a program that you're currently running
  (such as "minicom") you need to keep the program running while you
  view it.

  8.7.  What is the IO Address & IRQ of my Serial Port Hardware?

  8.7.1.  Introduction

  If it's PCI or ISA PnP then what's set in the hardware has been done
  by PnP methods.  Even if nothing has been set or the port disabled,
  PnP ports may still be found by using "lspci -v" or "isapnp
  --dumpregs".  Ports disabled by jumpers (or hardware failures) are
  completely lost.  See ``ISA PnP ports'', ``PCI: What IOs and IRQs have
  been set?'', ``PCI: Enabling a disabled port''

  PnP ports don't store their configuration in the hardware when the
  power is turned off.  This is in contrast to Jumpers (non-PnP) which
  remain the same with the power off.  That's why a PnP port is more
  likely to be found in a disabled state than an old non-PnP one.

  8.7.2.  PCI: What IOs and IRQs have been set?

  For PCI, the BIOS almost always sets the IRQ and may set the IO
  address as well.  To see how it's set use "lspci -vv" (best) or look
  in /proc/bus/pci (or for kernels <2.2 /proc/pci).  The modem's serial
  port is often called a "Communication controller".  Look for this.  If
  lspci shows "I/O ports at ... [disabled]" then the serial port is
  disabled and the hardware has no IO address so it's lost and can't be
  used.  See ``PCI: Enabling a disabled port'' for how to enable it.

  If more than one IO address is shown, the first one is more likely to
  be it.  You can't change the IRQ (at least not with "setpci")   This
  is because if one writes an IRQ it it's hardware register no action is
  taken on it.  It's the BIOS that should actually set up the IRQs and
  then write the correct value to this register for lspci to view.  If
  you must, change the IO address with "setpci" by changing the
  BASE_ADDRESS_0 or the like.

  8.7.3.  PCI: Enabling a disabled port

  If the port communicates via an IO address then "lspci -vv" should
  show "Control: I/O+ ..." with + meaning that the IO address is
  enabled.  If it shows "I/O-" (and "I/O ports at ... [disabled]") then
  you may need to use the setpci command to enable it.  For example
  "setpci -d 151f:000 command=101".  151f is the vendor id, and 000 is
  the device id both obtained from "lspci -n -v" or from /proc/bus/pci
  or from "scanpci -v".  The "command=101" means that 101 is put into
  the command register which is the same as the "Control" register
  displayed by "lspci".  The 101h sets two bits: the 1 sets I/O to + and
  the 100 part keeps SERR# set to +.   In this case only the SERR# bit
  of the Control register was initially observed to be + when the lspci
  command was run.  So we kept it enabled to + by setting bit 8 (where
  bit 0 is I/O) to 1 by the first 1 in 101.  Some serial cards don't use
  SERR# so if you see SERR#- then there's no need to enable it so then
  use: command=1.  Then you'll need to set up "setserial" to tell the
  driver the IO and IRQ.

  Bit 8 is actually the 9th bit since we started counting bits from 0.
  Don't be alarmed that lspci shows a lot of - signs showing that the
  card doesn't have many features available (or enabled).  Serial ports
  are relatively slow and don't need these features.

  Another way to enable it is to let the BIOS do it by telling the BIOS
  that you don't have a plug-and-play operating system.  Then the BIOS
  should enable it when you start your PC. If you have MS Windows9x on
  the same PC then doing this might cause problems with Windows (see

  8.7.4.  ISA PnP ports

  For an ISA Plug-and-Play (PnP) port one may try the pnpdump program
  (part of isapnptools).  If you use the --dumpregs option then it
  should tell you the actual IO address and IRQ set in the port.  It
  should also find an ISA PnP port that is disabled.  The address it
  "trys" is not the device's IO address, but a special address used for
  communicating with PnP cards.

  8.7.5.  Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)

  Perhaps the BIOS messages will tell you some info before Linux starts
  booting.  Use the shift-PageUp key to step back thru the boot-time
  messages and look at the very first ones which are from the BIOS.
  This is how it was before Linux started.  Setserial can't change it
  but isapnp or setpci can.  Starting with kernel 2.4, the serial driver
  can make such changes for many (but not all) serial ports.

  Using "scanport" (Debian only ??) will probe all I/O ports and will
  indicate what it thinks may be serial port.  After this you could try
  probing with setserial using the "autoconfig" option.  You'll need to
  guess the addresses to probe at (using clues from "scanport").  See
  ``What is Setserial''.

  For a port set with jumpers, the IO ports and IRQs are set per the
  jumpers. If the port is not Plug-and-Play (PnP) but has been setup by
  using a DOS program, then it's set at whatever the person who ran that
  program set it to.

  8.7.6.  Exploring via MS Windows (a last resort)

  For PnP ports, checking on how it's configured under DOS/Windows may
  (or may not) imply how it's under Linux.  MS Windows stores its
  configuration info in its Registry which is not used by Linux so they
  are not necessarily configured the same.  If you let a PnP BIOS
  automatically do the configuring when you start Linux (and have told
  the BIOS that you don't have a PnP operating system when starting
  Linux) then Linux should use whatever configuration is in the BIOS's
  non-volatile memory.  Windows also makes use of the same non-volatile
  memory but doesn't necessarily configure it that way.

  8.8.  Choosing Serial IRQs

  If you have Plug-and-Play ports then either a PnP BIOS or a serial
  driver may configure all your devices for you so then you may not need
  to choose any IRQs.  PnP software determines what it thinks is best
  and assigns them (but it's not always best).  But if you directly use
  isapnp (ISA bus) or jumpers then you have to choose.  If you already
  know what IRQ you want to use you could skip this section except that
  you may want to know that IRQ 0 has a special use (see the following

  8.8.1.  IRQ 0 is not an IRQ

  While IRQ 0 is actually the timer (in hardware) it has a special
  meaning for setting a serial port with setserial.  It tells the driver
  that there is no interrupt for the port and the driver then will use
  polling methods.  Such polling puts more load on the CPU but can be
  tried if there is an interrupt conflict or mis-set interrupt.  The
  advantage of assigning IRQ 0 is that you don't need to know what
  interrupt is set in the hardware.  It should be used only as a
  temporary expedient until you are able to find a real interrupt to

  8.8.2.  Interrupt sharing,  Kernels 2.2+

  Sharing of IRQs is where two devices use the same IRQ.  As a general
  rule, this wasn't allowed for the ISA bus.   The PCI bus may share
  IRQs but one can't share the same IRQ between the ISA and the PCI bus.
  Most multi-port boards may share IRQs.  Sharing is not as efficient
  since every time a shared interrupt is given a check must be made to
  determine where it came from.  Thus if it's feasible, it's nicer to
  allocate every device its own interrupt.

  Prior to kernel 2.2, serial IRQs could not be shared with each other
  except for most multiport boards.  Starting with kernel 2.2 serial
  IRQs may be sometimes shared between serial ports.  In order for
  sharing to work in 2.2 the kernel must have been compiled with
  CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support
  sharing (so that if two serial cards put different voltages on the
  same interrupt wire, only the voltage that means "this is an
  interrupt" will prevail).  Since the PCI bus specs permit sharing, any
  PCI card should allow sharing.

  8.8.3.  What IRQs to choose?

  The serial hardware often has only a limited number of IRQs.  Also you
  don't want IRQ conflicts.  So there may not be much of a choice.  Your
  PC may normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1 and
  ttyS3 at IRQ 3.  Looking at /proc/interrupts will show which IRQs are
  being used by programs currently running.  You likely don't want to
  use one of these.  Before IRQ 5 was used for sound cards, it was often
  used for a serial port.

  Here is how Greg (original author of Serial-HOWTO) set his up in
  /etc/rc.d/rc.serial.  rc.serial is a file (shell script) which runs at
  start-up (it may have a different name or location).  For versions of
  "setserial" after 2.15 it's not always done this way anymore but this
  example does show the choice of IRQs.

       /sbin/setserial /dev/ttyS0 irq 3        # my serial mouse
       /sbin/setserial /dev/ttyS1 irq 4        # my Wyse dumb terminal
       /sbin/setserial /dev/ttyS2 irq 5        # my Zoom modem
       /sbin/setserial /dev/ttyS3 irq 9        # my USR modem

  Standard IRQ assignments:

          IRQ  0    Timer channel 0 (May mean "no interrupt".  See below.)
          IRQ  1    Keyboard
          IRQ  2    Cascade for controller 2
          IRQ  3    Serial port 2
          IRQ  4    Serial port 1
          IRQ  5    Parallel port 2, Sound card
          IRQ  6    Floppy diskette
          IRQ  7    Parallel port 1
          IRQ  8    Real-time clock
          IRQ  9    Redirected to IRQ2
          IRQ 10    not assigned
          IRQ 11    not assigned
          IRQ 12    not assigned
          IRQ 13    Math co-processor
          IRQ 14    Hard disk controller 1
          IRQ 15    Hard disk controller 2

  There is really no Right Thing to do when choosing interrupts.  Try to
  find one that isn't being used by the motherboard, or any other
  boards.  2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices.  Note
  that IRQ 2 is the same as IRQ 9.  You can call it either 2 or 9, the
  serial driver is very understanding.  If you have a very old serial
  board it may not be able to use IRQs 8 and above.

  Make sure you don't use IRQs 1, 6, 8, 13 or 14!  These are used by
  your motherboard.  You will make her very unhappy by taking her IRQs.
  When you are done you might want to double-check /proc/interrupts when
  programs that use interrupts are being run and make sure there are no

  8.9.  Choosing Addresses --Video card conflict with ttyS3

  Here's a problem with some old serial cards.  The IO address of the
  IBM 8514 video board (and others like it) is allegedly 0x?2e8 where ?
  is 2, 4, 8, or 9.  This may conflict with the IO address of ttyS3 at
  0x02e8.  Your may think that this shouldn't happen since the addresses
  are different in the high order digit (the leading 0 in 02e8).  You're
  right, but a poorly designed serial port may ignore the high order
  digit and respond to any address that ends in 2e8.  That is bad news
  if you try to use ttyS3 (ISA bus) at this IO address.

  For the ISA bus you should try to use the default addresses shown
  below.  PCI cards use different addresses so as not to conflict with
  ISA addresses.  The addresses shown below represent the first address
  of an 8-byte range.  For example 3f8 is really the range 3f8-3ff.
  Each serial device (as well as other types of devices that use IO
  addresses) needs its own unique address range.  There should be no
  overlaps (conflicts).  Here are the default addresses for commonly
  used serial ports on the ISA bus:

       ttyS0 address 0x3f8
       ttyS1 address 0x2f8
       ttyS2 address 0x3e8
       ttyS3 address 0x2e8

  Suppose there is an address conflict (as reported by setserial -g
  /dev/ttyS*) between a real serial port and another port which does not
  physically exist (and shows UART: unknown).  Such a conflict shouldn't
  cause problems but it sometimes does in older kernels.  To avoid this
  problem don't permit such address conflicts or delete /dev/ttySx if it
  doesn't physically exist.

  8.10.  Set IO Address & IRQ in the hardware (mostly for PnP)

  After it's set in the hardware don't forget to insure that it also
  gets set in the driver by using setserial.  For non-PnP serial ports
  they are either set in hardware by jumpers or by running a DOS program
  ("jumperless") to set them (it may disable PnP).  The rest of this
  subsection is only for PnP serial ports.  Here's a list of the
  possible methods of configuring PnP serial ports:

  В·  Using a PnP BIOS CMOS setup menu (usually only for external devices
     on ttyS0 (Com1) and ttyS1 (Com2))

  В·  Letting a PnP BIOS automatically configure a PnP serial port See
     ``Using a PnP BIOS to IO-IRQ Configure''

  В·  Doing nothing if the serial driver recognized your card OK

  В·  Using isapnp for a PnP serial port (non-PCI)

  В·  Using setpci (pciutils or pcitools) for the PCI bus

  The IO address and IRQ must be set (by PnP) in their registers each
  time the system is powered on since PnP hardware doesn't remember how
  it was set when the power is shut off.  A simple way to do this is to
  let a PnP BIOS know that you don't have a PnP OS and the BIOS will
  automatically do this each time you start.  This might cause problems
  in Windows (which is a PnP OS) if you start Windows with the BIOS
  thinking that Windows is not a PnP OS.  See Plug-and-Play-HOWTO.

  Plug-and-Play (PnP) was designed to automate this io-irq configuring,
  but for Linux it initially made life much more complicated.  In modern
  Linux (2.4 kernels --partially in 2.2 kernels), each device driver has
  to do its own PnP (using supplied software which it may utilize).
  There is unfortunately no centralized planning for assigning IO
  addresses and IRQs as there is in MS Windows.   But it usually works
  out OK in Linux anyway.

  8.10.1.  Using a PnP BIOS to IO-IRQ Configure

  While the explanation of how to use setpci or isapnp for io-irq
  configuring should come with such software, this is not the case if
  you want to let a PnP BIOS do such configuring.  Not all PnP BIOS can
  do this.  The BIOS usually has a CMOS menu for setting up the first
  two serial ports.  This menu may be hard to find.  For an "Award" BIOS
  it was found under "chipset features setup"  There is often little to
  choose from.  For ISA serial ports, the first two ports normally get
  set at the standard IO addresses and IRQs.  See ``More on Serial Port

  Whether you like it or not, when you start up a PC, a PnP BIOS starts
  to do PnP (io-irq) configuring of hardware devices.  It may do the job
  partially and turn the rest over to a PnP OS (which Linux is in some
  sense) or if thinks you don't have a PnP OS it may fully configure all
  the PnP devices but not configure the device drivers.

  If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS
  should do the configuring of all PnP serial ports --not just the first
  two.  An indirect way to control what the BIOS does (if you have
  Windows 9x on the same PC) is to "force" a configuration under
  Windows.  See Plug-and-Play-HOWTO and search for "forced".  It's
  easier to use the CMOS BIOS menu which may override what you "forced"
  under Windows.  There could be a BIOS option that can set or disable
  this "override" capability.

  If you add a new PnP device, the BIOS should PnP configure it.  It
  could even change the io-irq of existing devices if required to avoid
  any conflicts.  For this purpose, it keeps a list of non-PnP devices
  provided that you have told the BIOS how these non-PnP devices are io-
  irq configured.  One way to tell the BIOS this is by running a program
  called ICU under DOS/Windows.

  But how do you find out what the BIOS has done so that you set up the
  device drivers with this info?  The BIOS itself may provide some info,
  either in its setup menus of via messages on the screen when you turn
  on your computer.  See ``What is set in my serial port hardware?''.
  Other ways of finding out is to use lspci for the PCI bus or isapnp
  --dumpregs for the ISA bus.  The cryptic results it shows you may not
  be clear to a novice.

  8.11.  Giving the IRQ and IO Address to Setserial

  Once you've set the IRQ and IO address in the hardware (or arranged
  for it to be done by PnP) you also need to insure that the "setserial"
  command is run each time you start Linux.  See the subsection ``Boot-
  time Configuration''

  9.  Configuring the Serial Driver (high-level) "stty"

  9.1.  Overview

  See the section ``Stty''.  The "stty" command sets many things such as
  flow control, speed, and parity.  The only one discussed in this
  section is flow control.

  9.2.  Flow Control

  Configuring Flow Control: Hardware Flow Control is Usually Best See
  ``Flow Control'' for an explanation of it.  It's usually better to use
  hardware flow control rather than software flow control using
  Xon/Xoff.  To use full hardware flow control you must normally have
  two dedicated wires for it in the cable between the serial port and
  the device.  If the device is on a card or the motherboard, then it
  should always be possible to use hardware flow control.

  Many applications (and the getty program) give you an option regarding
  flow control and will set it as you specify or it might enable
  hardware flow control by default if you don't set it.  It must be set
  both in the serial driver and in the hardware connected to the serial
  port.  How it's set into this hardware is hardware dependent.
  Sometimes there is a certain "init string" you send to the hardware
  device via the serial port from your PC.  For a modem, the
  communication program should set it in both places.

  If a program you use doesn't set flow control in the serial driver,
  then you may do it yourself using the stty command.  Since the driver
  doesn't remember the setting after you stop Linux, you could put the
  stty command in a file that runs at start-up or when you login (such
  as /etc/profile for the bash shell).  Here's what you would add for
  hardware flow control for port ttyS2:

       stty -F /dev/ttyS2 crtscts

  or for old stty versions < 1.17:

       stty crtscts < /dev/ttyS2

  crtscts stands for a Control setting to use the RTS and CTS pins of
  the serial port for hardware flow control.  Note that RTS+CTS almost
  spells: crtscts and the initial "c" means "control".

  10.  Serial Port Devices /dev/ttyS2, etc.

  10.1.  Serial Port Names: ttyS4, etc

  Common serial port names are /dev/ttyS0, /dev/ttyS1, etc.  Then around
  the year 2000 came the USB bus with names like /dev/ttyUSB0 and
  /dev/ttyACM1 (for the ACM modem on the USB bus).  Multiport serial
  card used somewhat differnt names (depending on the brand) such as

  10.2.  The PCI Bus

  Since DOS provided for 4 serial ports on the old ISA bus: COM1-COM4,
  or ttyS0-ttyS3 in Linux, most serial ports on the newer PCI bus used
  higher numbers such as ttyS4 or ttyS14 (prior to kernel 2.6.13).  But
  since most PCs only came with one or two serial ports, ttyS0 and
  possibly ttyS1 (for the second port) the PCI bus can now use ttyS2
  (kernel 2.6.15 on).  All this permits one to have both ISA serial
  ports and PCI serial ports on the same PC with no name conflicts.  0-1
  (or 0-3) are reserved for the old ISA bus (or the newer LPC bus) and
  2-upward (or 4-upward or 14-upward) are used for PCI, where older
  schemes are shown in parentheses .  It's not required to be this way
  but it often is.

  If you're using udev (which puts only the device you have on your
  computer into the /dev directory at boottime) then there's an easy way
  to change the device names by editing files in /etc/udev/.  For
  example, to change the name of what the kernel detects as ttyS3 to
  what you want to name it: ttyS14, add a line similar to this to

  On-board serial ports on motherboards which have both PCI and ISA
  slots are likely to still be ISA ports.  Even for all-PCI-slot
  motherboards, the serial ports are often not PCI.  Instead, they are
  either ISA, on an internal ISA bus or on a LPC bus which is intended
  for slow legacy I/O devices: serial/parallel ports and floppy drives.

  10.3.  Serial Port Device Names & Numbers

  Devices in Linux have major and minor numbers.  The serial port ttySx
  (x=0,1,2, etc.) is major number 4.  You can see this (and the minor
  numbers too) by typing: "ls -l ttyS*" in the /dev directory.  To find
  the device names for various devices, see the "devices" file in the
  kernel documentation.
  There formerly was a "cua" name for each serial port and it behaved
  just a little differently.  For example, ttyS2 would correspond to
  cua2.  It was mainly used for modems.  The cua major number was 5 and
  minor numbers started at 64.  You may still have the cua devices in
  your /dev directory but they are now deprecated.  For details see
  Modem-HOWTO, section: cua Device Obsolete.

  For creating the old devices in the device directory see:

  ``Creating Devices In the /dev directory''

  10.4.  More on Serial Port Names

  Dos/Windows use the COM name while the messages from the serial driver
  use ttyS00, ttyS01, etc.  Older serial drivers (2001 ?) used just
  tty00, tty01, etc.

  The tables below shows some examples of serial device names.  The IO
  addresses are the default addresses for the old ISA bus (not for the
  newer PCI and USB buses).

       dos     common                  IO       USB-BUS ( ACM => acm modem )
       name     name     major minor address || common name      common name
       COM1   /dev/ttyS0  4,  64;   3F8      || /dev/ttyUSB0  |  /dev/ttyACM0
       COM2   /dev/ttyS1  4,  65;   2F8      || /dev/ttyUSB1  |  /dev/ttyACM1
       COM3   /dev/ttyS2  4,  66;   3E8      || /dev/ttyUSB2  |  /dev/ttyACM2
       COM4   /dev/ttyS3  4,  67;   2E8      || /dev/ttyUSB3  |  /dev/ttyACM3
        -     /dev/ttyS4  4,  68;   various

  10.5.  USB (Universal Serial Bus) Serial Ports

  For more info see the usb subdirectory in the kernel documentation
  directory for files: usb-serial, acm, etc.

  10.6.  Link ttySN to /dev/modem

  On some installations, two extra devices will be created, /dev/modem
  for your modem and /dev/mouse for a mouse.  Both of these are symbolic
  links to the appropriate device in /dev.

  Historical note: Formerly (in the 1990s) the use of /dev/modem (as a
  link to the modem's serial port) was discouraged since lock files
  might not realize that it was really say /dev/ttyS2.  The newer lock
  file system doesn't fall into this trap so it's now OK to use such

  10.7.  Which Connector on the Back of my PC is ttyS1, etc?

  10.7.1.  Inspect the connectors

  Inspecting the connectors may give some clues but is often not
  definitive.  The serial connectors on the back side of a PC are
  usually DB connectors with male pins.  9-pin is the most common but
  some are 25-pin (especially older PCs like 486s).  There may be one
  9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??).  For two
  9-pin ones the top one might be ttyS0.

  If you only have one serial port connector on the back of your PC,
  this may be easy.  If you also have an internal modem, a program like
  wvdial may be able to tell you what port it's on (unless it's a PnP
  that hasn't been enabled yet).  A report from setserial (at boot-time
  or run by you from the command line) should help you identify the non-
  modem ports.

  If you have two serial ports it may be more difficult.  You could have
  only one serial connector but actually have 2 ports, one of which
  isn't used (but it's still there electronically).  First check manuals
  (if any) for your computer.  Look at the connectors for meaningful
  labels.  You might even want to take off the PC's cover and see if
  there are any meaningful labels on the card where the internal ribbon
  serial cables plug in.  Labels (if any) are likely to say something
  like "serial 1", "serial 2" or A, B.  Which com port it actually is
  will depend on jumper or PnP settings (sometimes shown in a BIOS setup
  menu).  But 1 or A are more likely to be ttyS0 with 2 or B ttyS1.

  10.7.2.  Send bytes to the port

  Labels are not apt to be definitive so here's another method.  If the
  serial ports have been configured correctly per setserial, then you
  may send some bytes out a port and try to detect which connector (if
  any) they are coming out of.  One way to send such a signal is to copy
  a long text file to the port using a command like: cp my_file_name
  /dev/ttyS1.  A voltmeter connected to the DTR pin (see Serial-HOWTO
  for Pinout) will display a positive voltage as soon as you give the
  copy command.

  The transmit pin should go from several volts negative to a voltage
  fluctuating around zero after you start sending the bytes.  If it
  doesn't (but the DTR went positive) then you've got the right port but
  it's blocked from sending.  This may be due to a wrong IRQ, -clocal
  being set, etc.  The command "stty -F /dev/ttyS1 -a" should show
  clocal (and not -clocal).  If not, change it to clocal.

  Another test is to jumper the transmit and receive pins (pins 2 and 3
  of either the 25-pin or 9-pin connector) of a test serial port.  Then
  send something to each port (from the PCs keyboard) and see if it gets
  sent back.  If it does it's likely the port with the jumper on it.
  Then remove the jumper and verify that nothing gets sent back.  Note
  that if "echo" is set (per stty) then a jumper creates an infinite
  loop.  Bytes that pass thru the jumper go into the port and come right
  back out of the other pin back to the jumper.  Then they go back in
  and out again and again.  Whatever you send to the port repeats itself
  forever (until you interrupt it by removing the jumper, etc.).  This
  may be a good way to test it as the repeating test messages halt when
  the jumper is removed.

  As a jumper you could use a mini (or micro) jumper cable (sold in some
  electronic parts stores) with mini alligator clips.  A small scrap of
  paper may be used to prevent the mini clips from making electrical
  contact where it shouldn't.  Metal paper clips can sometimes be bent
  to use as jumpers.  Whatever you use as a jumper take care not to bend
  or excessively scratch the pins.  To receive something from a port,
  you can go to a virtual terminal (for example Alt-F2 and login) and
  type something like "cp /dev/ttyS2 /dev/tty".  Then at another virtual
  terminal you may send something to ttyS2 (or whatever) by "echo
  test_message > /dev/ttyS2".  Then go back to the receive virtual
  terminal and look for the test_message.  See ``Serial Electrical Test
  Equipment'' for more info.

  10.7.3.  Connect a device to the connector

  Another way to try to identify a serial port is to connect some
  physical serial device to it and see if it works.  But a problem here
  is that it might not work because it's not configured right.  A serial
  mouse might get detected at boot-time if connected.

  You may put a device, such as a serial mouse (use 1200 baud), on a
  port and then use minicom or picocom to communicate with that port.
  Then by clicking on the mouse, or otherwise sending characters with
  the device, see if they get displayed.  It not you may have told
  picocom the wrong port (such as ttyS0 instead of ttyS1) so try again.

  10.7.4.  Missing connectors

  If the software shows that you have more serial ports than you have
  connectors for (including an internal modem which counts as a serial
  port) then you may have a serial port that has no connector.  Some
  motherboards come with a serial port with no cable or external serial
  DB connector.  Someone may build a PC from this and decide not to use
  this serial port.  There may be a "serial" connector and label on the
  motherboard but no ribbon cable connects to its pins.  To use this
  port you must get a ribbon cable and connector.  I've seen different
  wiring arrangements for such ribbon cables so beware.

  10.8.  Creating Devices In the /dev directory

  If you don't use devfs (which automatically creates such devices) and
  don't have a device "file" that you need, you will have to create it.
  Use the mknod command or with the MAKEDEV shell script.  Example,
  suppose you needed to create ttyS0:

       linux# mknod -m 666 /dev/ttyS0 c 4 64

  The MAKEDEV script is easier to use.  See the man page for it.  For
  example, if you needed to make the device for ttyS0 you would just

       linux# MAKEDEV ttyS0

  If the above command doesn't work (and you are the root user), look
  for the MAKEDEV script in the /dev directory and run it.

  This handles the devices creation and should set the correct
  permissions.  For making multiport devices see ``Making multiport
  devices in the /dev directory''.

  11.  Interesting Programs You Should Know About

  Most info on getty has been moved to Modem-HOWTO with a little info on
  the use of getty with directly connected terminals now found in Text-

  11.1.  Serial Monitoring/Diagnostics Programs

  A few Linux programs (and one "file") will monitor various modem
  control lines and indicate if they are positive (1 or green) or
  negative (0 or red).

  В·  serlook <> can snoop
     on serial line traffic (via a wiretap) and also send/receive on a
     serial line.

  В·  The "file": /proc/tty/driver/serial lists those that are asserted
     (positive voltage)

  В·  modemstat (Only works correctly on Linux PC consoles.)  Status
     monitored in a tiny window.  Color-coded and compact.  Must kill it
     (a process) to quit.

  В·  statserial (Info displayed on entire screen)

  В·  serialmon (Doesn't monitor RTS, CTS, DSR but logs other functions)

     As of June 1998, I know of no diagnostic program in Linux for the
     serial port.

  11.2.  Changing Interrupt Priority

  В·  irqtune will give serial port interrupts higher priority to improve

  В·  hdparm for hard-disk tuning may help some more.

  11.3.  What is Setserial ?

  This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal.  There are
  some minor differences, depending on which HOWTO it appears in.

  11.3.1.  Setserial problems with linmodems, laptops

  If you have a Laptop (PCMCIA) don't use setserial until you read
  ``Laptops: PCMCIA''.

  11.3.2.  Introduction

  setserial is a program used for the user to communicate with the
  serial device driver.  You normally never need to use it, provided
  that you only use the one or two serial ports that come as standard
  equipment with a PC.  Even in other cases, most extra serial ports
  should be auto-detected by modern kernels.  Except you'll need to use
  setserial if you have an old ISA serial port set by jumpers on the
  physical hardware or if your kernel (such as 2.2 or older) doesn't
  both detect and set your add-on PCI serial ports.

  setserial allows you (or a shell script) to talk to the serial
  software.  But there's also another program, tt/stty/, that also deals
  with the serial port and is used for setting the port speed, etc.

  setserial deals with the lower-level configuring of the serial port,
  such as dealing with IRQs (such as 5), port addresses (such as 3f8),
  and the like.  A major problem with it is that it can't set or
  configure the serial port hardware: It can't set the IRQ or port
  addresses into the hardware.  Furthermore, when it seemingly reports
  the configuration of the hardware, it's sometimes wrong since it
  doesn't actually probe the hardware unless you specifically tell it
  to.  Even then, it doesn't do the modern type of bus probing and some
  hardware may never be found by it.  Still, what it shows is right most
  all the time but if you're having trouble getting a serial port to
  work, then there's a fair chance it's wrong.

  In olden days, when the IRQ and port address was set by jumpers on the
  serial card, one would use setserial to tell the driver how these
  jumpers were set.  Today, when plug-and-play methods detect how the
  jumperless serial port is set, setserial is not really needed anymore
  unless you're having problems or using old hardware.  Furthermore, if
  the configuration file used by setserial is wrong, then there's
  trouble.  In this case, if you use setserial to try to find out how
  the port is configured, it may just repeat the incorrect information
  in the configuration file.

  setserial can sometimes be of help to find a serial port.  But it's
  only of use if you know the port address and use the right options.
  For modern ports, there's usually better ways to look for them by
  plug-and-play methods.

  Thus the name setserial is somewhat of a misnomer since it doesn't set
  the I/O address nor IRQ in the hardware, it just "sets" them in the
  driver software.  And the driver naively believes that what setserial
  tells it, even if it conflicts with what the driver has found by using
  plug-and-play methods.  Too bad that it fails to at least issue a
  warning message for such a conflict.  Since the device driver is
  considered to be part of the kernel, the word "kernel" is often used
  in other documentation with no mention made of any "serial driver".

  Some distributions (and versions) set things up so that setserial is
  run at boot-time by an initialization shell script (in the /etc
  directory tree).   But the configuration file which this script uses
  may be either in the /etc tree or the /var tree.  In some cases, if
  you want setserial to run at boot-time, you may have to take some
  action.  setserialwill not work without either serial support built
  into the kernel or loaded as a module.  The module may get loaded
  automatically if you (or a script) attempt to use setserial.

  While setserial can be made to probe the hardware IO port addresses to
  try to determine the UART type and IRQ, this has severe limitations.
  See ``Probing''.  It can't set the IRQ or the port address in the
  hardware of PnP or PCI serial ports (but the plug-and-play features of
  the serial driver may do this).  It also can't directly read the PnP
  data stored in configuration registers in the hardware.  But since the
  device driver can read these registers and setserial tells you what
  the device driver thinks, it might be correct.  Or it could be telling
  you what setserial had previously (and perhaps erroneously) told the
  driver.  There's no way to know for sure without doing some other

  The serial driver (for Linux Kernel 2.4+) looks for a few "standard"
  legacy serial ports, for PnP ports on the ISA bus, and for all
  supported port hardware on the PCI bus.  If it finds your ports
  correctly, then there's no need to use setserial.  The driver doesn't
  probe for the IRQs of old ISA serial ports set with jumpers on the
  card and may get these wrong.

  Besides the man page for setserial, check out info in
  /usr/doc/setserial.../ or /usr/share/doc/setserial.  This should tell
  you how setserial is handled for your distribution of Linux.  While
  setserial behaves the same in all distributions, the scripts for
  running it, how to configure such scripts (including automatic
  configuration), and the names and locations of the script files, etc.,
  are all distribution-dependent.

  11.3.3.  Serial module unload

  If a serial module gets unloaded, the changes previously made by
  setserial will be forgotten by the driver.  But while the driver
  forgets it, a script provided by the distribution may save it in a
  file somewhere so that it can the restored if the module is reloaded.

  11.3.4.  Slow baud rates of 1200 or less

  There once was a problem with slow serial printers (especially the old
  ones of the 1980s).  The printing program would close the serial port
  at the "end" of printing well before all the characters from the large
  serial buffer (in main memory) were sent to the printer.  The result
  was a truncated print job that didn't print the last paragraph or last
  page, etc.

  But the newer lprng print program (and possibly other printing
  programs) keeps the port open until printing is finished so "problem
  solved", even if you're using an antique printer.  Setserial can
  modify the time that the port will keep operating after it's closed
  (in order to output any characters still in its buffer in main RAM).
  This is done by the "closing_wait" option per the setserial man page.
  For "bad" software that closes the port too soon, it might also be
  needed at speeds above 1200 if there are a lot of "flow control"

  11.3.5.  Giving the setserial  command

  Remember, that setserial can't set any I/O addresses or IRQs in the
  hardware.  That's done either by plug-and-play software (run by the
  driver) or by jumpers for legacy serial ports.  Even if you give an
  I/O address or IRQ to the driver via setserial it will not set such
  values and assumes that they have already been set.  If you give it
  wrong values, the serial port will not work right (if at all).

  For legacy ports, if you know the I/O address but don't know the IRQ
  you may command setserial to attempt to determine the IRQ.

  You can see a list of possible commands by just typing setserial with
  no arguments.  This fails to show you the one-letter options such as
  -v for verbose which you should normally use when troubleshooting.
  Note that setserial calls an IO address a "port".  If you type:

       setserial -g /dev/ttyS*

  You'll see some info about how the device driver is configured for
  your ports.  In many cases you'll see some ports displayed with what
  appears at first glance to be erroneous IRQs and addresses.  But if
  you also see: "UART: unknown" just ignore the entire line since no
  serial port exists at that address.

  If you add -a to the option -g you will see more info although few
  people need to deal with (or understand) this additional info since
  the default settings you see usually work fine.  In normal cases the
  hardware is set up the same way as "setserial" reports.  But if you
  are having problems there is a good chance that setserial has it
  wrong.  In fact, you can run "setserial" and assign a purely
  fictitious I/O port address, any IRQ, and whatever uart type you would
  like to have.  Then the next time you type "setserial ..." it will
  display these bogus values you've supplied to the driver.  They will
  also be officially registered with the kernel as displayed (at the top
  of the screen) by the "scanport" command (Debian).  Of course the
  serial port driver will not work correctly (if at all) if you attempt
  to use such a port.  Thus, when giving parameters to setserial,
  "anything goes".  Well almost.  If you assign one port a base address
  that is already assigned (such as 3e8) it may not accept it.  But if
  you use 3e9 it will accept it.  Unfortunately 3e9 is actually assigned
  since it is within the range starting at base address 3e8.  Thus the
  moral of the story is to make sure your data is correct before
  assigning resources with setserial.

  11.3.6.  Configuration file

  While assignments made by setserial are lost when the PC is powered
  off, a configuration file may restore them when the PC is started up
  again.  In newer versions, what you change by setserial might get
  automatically saved to a configuration file.  When setserial runs it
  uses the info from the configuration file.

  Where this configuration file resides depends on your distribution.
  Look at the start-up scripts somewhere in the /etc/ tree (such as
  /etc/init.d/ or /etc/rc.d/) and read the startup script for "serial"
  or "setserial" or the like.  It should show where the configuration
  file(s) reside.  In Debian there are 4 options for use of this
  configuration file:

  1. Don't use this file at all.  At each boot, the serial driver alone
     detects the ports and setserial doesn't ever run.  ("kernel"

  2. Save what setserial reports when the system is first shutdown and
     put it in the configuration file.  After that, don't ever make any
     changes to the configuration file, even if someone has made changes
     by running the setserial command on the command line and then shuts
     down the system. ("autosave-once" option)

  3. At every shutdown, save whatever setserial detects to the
     configuration file.  ("autosave" option)

  4. Manually edit the configuration file to set the configuration.
     Don't ever do any automatic saves to it. ("manual" option)

  In olden days (perhaps before 2000), there wasn't any configuration
  file and the configuration was manually set (hard coded) inside the
  shell script that ran setserial.  See ``Edit a script (prior to
  version 2.15)''.

  11.3.7.  Probing

  You probe for a port with setserial only when you suspect that it has
  been enabled (by PnP methods, the BIOS, jumpers, etc.).  Otherwise
  setserial probing will never find it since its address doesn't exist.
  A problem is where the software looks for a port at specified I/O
  addresses.  Prior to probing with "setserial", one may run the
  "scanport" (Debian) command to check all possible ports in one scan.
  It makes crude guesses as to what is on some ports but doesn't
  determine the IRQ.  It's a fast first start.  It may hang your PC but
  so far it's worked fine for me.  Note that non-Debian distributions
  don't seem to supply "scanport".  Is there another scan program?

  With appropriate options, setserial can probe (at a given I/O address)
  for a serial port but you must guess the I/O address.  If you ask it
  to probe for /dev/ttyS2 for example, it will only probe at the address
  it thinks ttyS2 is at (2F8).  If you tell setserial that ttyS2 is at a
  different address, then it will probe at that address, etc.  See

  The purpose of such probing is to see if there is a uart there, and if
  so, what its IRQ is.  Use setserial mainly as a last resort as there
  are faster ways to attempt it such as wvdialconf to detect modems,
  looking at very early boot-time messages, or using pnpdump --dumpregs,
  or lspci -vv.  But if you want to detect hardware with setserial use
  for example :
  /dev/ttyS2 -v autoconfig
  If the resulting message shows a uart type such as 16550A, then you're
  OK.  If instead it shows "unknown" for the uart type, then there is
  supposedly no serial port at all at that I/O address.  Some cheap
  serial ports don't identify themselves correctly so if you see
  "unknown" you still might have a serial port there.

  Besides auto-probing for a uart type, setserial can auto-probe for
  IRQ's but this doesn't always work right either.  In one case it first
  gave the wrong irq but when the command was repeated it found the
  correct irq.  In versions of setserial >= 2.15, the results of your
  last probe test could be automatically saved and put into a
  distribution-specific configuration file such as /etc/serial.conf or
  /etc/sysconfig/serial or /var/lib/setserial/autoserial.conf for
  Debian.  This will be used next time you start Linux.

  It may be that two serial ports both have the same IO address set in
  the hardware.  Of course this is not normally permitted for the ISA
  bus but it sometimes happens anyway.  Probing detects one serial port
  when actually there are two.  However if they have different IRQs,
  then the probe for IRQs may show IRQ = 0.  For me, it only did this if
  I first used setserial to give the IRQ a fictitious value.

  11.3.8.  Boot-time Configuration

  While setserial may run via an initialization script, something akin
  to setserial also runs earlier when the serial module is loaded (or
  when the kernel starts the built-in serial driver if it was compiled
  into the kernel).  Thus when you watch the start-up messages on the
  screen it may look like it ran twice, and in fact it has.

  If the first message is for a legacy port, the IRQs shown may be wrong
  since it didn't probe for IRQs.  If there is a second report of serial
  ports, it may the result of a script such as /etc/init.d/setserial.
  It usually does no probing and thus could be wrong about how the
  hardware is actually set.  It only shows configuration data that got
  saved in a configuration files.  The old method, prior to setserial
  2.15, was to manually write such data directly into the script.

  When the kernel loads the serial module (or if the "module equivalent"
  is built into the kernel) then all supported PnP ports are detected.
  For legacy (non-PnP) ports, only ttyS{0-3} are auto-detected and the
  driver is set to use only IRQs 4 and 3 (regardless of what IRQs are
  actually set in the hardware).  No probing is done for IRQs but it's
  possible to do this manually.  You see this as a boot-time message
  just as if setserial had been run.

  To correct possible errors in IRQs (or for other reasons) there may be
  a script file somewhere that runs setserial.  Unfortunately, if this
  file has some IRQs wrong, the kernel will still have incorrect info
  about the IRQs.  This file is usually part of the initialization done
  at boot-time.  Whether it runs or not depends on how you (and/or your
  distribution) have set things up.  It may also depends on the

  Before modifying a configuration file, you can test out a "proposed"
  setserial command by just typing it on the command line.  In some
  cases the results of this use of setserial will automatically get
  saved somewhere such as /etc/serial.conf (or autoserial.conf or
  serial) when you shutdown.  So if it worked OK (and solved your
  problem) then there's no need to modify any configuration file.  See
  ``Configuration method using /etc/serial.conf, etc.''.

  11.3.9.  Edit a script (required prior to version 2.15)

  This is how it was done prior to setserial 2.15 (1999) The objective
  was to modify (or create) a script file in the /etc tree that runs
  setserial at boot-time.  Most distributions provided such a file (but
  it may not have initially resided in the /etc tree).

  So prior to version 2.15 (1999) it was simpler.  All you did was edit
  a script.  There was no /etc/serial.conf file (or the like) to
  configure setserial.   Thus you needed to find the file that runs
  "setserial" at boot time and edit it.  If it didn't exist, you needed
  to create one (or place the commands in a file that ran early at boot-
  time).  If such a file was currently being used it's likely was
  somewhere in the /etc directory-tree.  But Redhat <6.0 has supplied it
  in /usr/doc/setserial/ but you need to move it to the /etc tree before
  using it.

  The script /etc/rc.d/rc.serial was commonly used in the past.  The
  Debian distribution used /etc/rc.boot/0setserial.  Another file once
  used was /etc/rc.d/rc.local but it's may not have run early enough.
  It was reported that other processes may try to open the serial port
  before rc.local ran resulting in serial communication failure.  Later
  on it most likely was found in /etc/init.d/ but wasn't normally
  intended to be edited.

  If such a file was supplied, it likely contained a number of
  commented-out examples.  By uncommenting some of these and/or
  modifying them, you could set things up correctly.  It was important
  use a valid path for setserial, and a valid device name.  You could do
  a test by executing this file manually (just type its name as the
  super-user) to see if it works right.  Testing like this was a lot
  faster than doing repeated reboots to get it right.

  For versions >= 2.15 (provided your distribution implemented the
  change, Redhat didn't at first) it may be more tricky to do since the
  file that runs setserial on startup, /etc/init.d/setserial or the like
  was not intended to be edited by the user.  See ``Configuration method
  using /etc/serial.conf, etc.''.

  An example line in such a script was:

       /sbin/setserial /dev/ttyS3 irq 5 uart 16550A  skip_test

  or, if you wanted setserial to automatically determine the uart and
  the IRQ for ttyS3 you would have used something like this:

       /sbin/setserial  /dev/ttyS3 auto_irq skip_test autoconfig

  This was done for every serial port you wanted to auto configure,
  using a device name that really does exist on your machine.  In some
  cases it didn't work right due to the hardware.

  11.3.10.  Configuration method using /etc/serial.conf, etc.

  Prior to setserial version 2.15 (1999), the way to configure setserial
  was to manually edit the shell-script that ran setserial at boot-time.
  See ``Edit a script (before version 2.15)''.  This was simple, but the
  simple and clear method has been changed to something that is
  unnecessarily complex.  Today the script and configuration file are
  two different files instead of one.  This shell-script is not edited
  but gets its data from a configuration file such as /etc/serial.conf
  (or /var/lib/setserial/autoserial.conf).

  Furthermore you may not even need to edit serial.conf (or the like)
  because using the "setserial" command on the command line may
  automatically cause serial.conf to be edited appropriately.  This was
  done so that you may not need to edit any file in order to set up (or
  change) what setserial does each time that Linux is booted.

  What often happens is this:  When you shut down your PC the script
  that ran "setserial" at boot-time is run again, but this time it only
  does what the part for the "stop" case says to do:  It uses
  "setserial" to find out what the current state of "setserial" is, and
  it puts that info into the serial configuration file such as
  serial.conf.  Thus when you run "setserial" to change the serial.conf
  file, it doesn't get changed immediately but only when and if you shut
  down normally.

  Now you can perhaps guess what problems might occur.  Suppose you
  don't shut down normally (someone turns the power off, etc.) and the
  changes don't get saved.  Suppose you experiment with "setserial" and
  forget to run it a final time to restore the original state (or make a
  mistake in restoring the original state).  Then your "experimental"
  settings are saved.  And worst of all, unless you know which options
  were set in the configuration file, you don't know what will happen.
  One option in Debian (and likely other distributions) is known as
  "AUTOSAVE-ONCE" which saves changes only for the first time you make
  them with the setserial command.

  If the option "###AUTOSAVE###" is set and you manually edit
  serial.conf, then your editing is destroyed when you shut down because
  it gets changed back to the state of setserial at shutdown.  There is
  a way to disable the changing of serial.conf at shutdown and that is
  to remove "###AUTOSAVE###" or the like from first line of serial.conf.
  In the Debian distribution, the removal of "###AUTOSAVE###" from the
  first line was once automatically done after the first time you
  shutdown just after installation.  To retain this effect the
  "AUTOSAVE-ONCE" option was created which only does a save when time
  the system is shut down for the first time (just after you install or
  update the setserial program).

  The file most commonly used to run setserial at boot-time (in
  conformance with the configuration file) is now /etc/init.d/setserial
  (Debian) or /etc/init.d/serial (Redhat), or etc.,  but it should not
  normally be edited.  For 2.15, Redhat 6.0 just had a file
  /usr/doc/setserial-2.15/rc.serial which you have to move to
  /etc/init.d/ if you want setserial to run at boot-time.

  To disable a port, use setserial to set it to "uart none".  This will
  not be saved.  The format of /etc/serial.conf appears to be just like
  that of the parameters placed after "setserial" on the command line
  with one line for each port.  If you don't use autosave, you may edit
  /etc/serial.conf manually.
  In order to force the current settings set by setserial to be saved to
  the configuration file (serial.conf) without shutting down, do what
  normally happens when you shutdown: Run the shell-script
  /etc/init.d/{set}serial stop.  The "stop" command will save the
  current configuration but the serial ports still keep working OK.

  In some cases you may wind up with both the old and new configuration
  methods installed but hopefully only one of them runs at boot-time.
  Debian labeled obsolete files with "...pre-2.15".

  11.3.11.  IRQs

  By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
  ttyS3 share IRQ 3.  But while sharing serial interrupts (using them in
  running programs) is OK for the PCI bus, it's not permitted for the
  ISA bus unless you: 1. have kernel 2.2 or better, and 2. you've
  compiled in support for this, and 3. your serial hardware supports it.

  ``Interrupt sharing and Kernels 2.2+''

  If you only have two serial ports, ttyS0 and ttyS1, you're still OK
  since IRQ sharing conflicts don't exist for non-existent devices.

  If you add a legacy internal modem (without plug-and-play) and retain
  ttyS0 and ttyS1, then you should attempt to find an unused IRQ and set
  it in your serial port (or modem card) and then use setserial to
  assign it to your device driver.  If IRQ 5 is not being used for a
  sound card, this could be used for a modem.

  11.3.12.  Laptops: PCMCIA

  If you have a Laptop, read PCMCIA-HOWTO for info on the serial
  configuration.  For serial ports on the motherboard, setserial is used
  just like it is for a desktop.  But for PCMCIA cards (such as a modem)
  it's a different story.  The configuring of the PCMCIA system should
  automatically run setserial so you shouldn't need to run it.  If you
  do run it (by a script file or by /etc/serial.conf) it might be
  different and cause trouble.  The autosave feature for serial.conf
  shouldn't save anything for PCMCIA cards (but Debian did until
  2.15-7).  Of course, it's always OK to use setserial to find out how
  the driver is configured for PCMCIA cards.

  11.4.  Stty

  11.4.1.  Introduction

  stty does much of the configuration of the serial port but since
  application programs (and the getty program) often handle this, you
  may not need to use it much.  It's handy if you're having problems or
  want to see how the port is set up.  Try typing ``stty -a'' at your
  terminal/console to see how it's now set.  Also try typing it without
  the -a (all) for a short listing which shows how it's set different
  than "normal" which is how it's set using the command "stty sane".
  Don't try to learn all the setting unless you want to become a serial
  historian since many of the settings are only for slow antique dumb
  terminals of the 1970's.  Most of the defaults should work OK.

  stty is documented in the man pages with a more detailed account in
  the info pages.  Type "man stty" or "info stty".

  Many of the stty options start with an "o" (output) or an "i" (input).
  For example: onlcr.  Output is the flow of bytes out of the computer
  while input is the flow of bytes into the computer.  The "point of
  view" is the computer, not the serial port or the device connected to
  the serial port.  For example, received input data comes in on a cable
  and goes to the serial port chip.  This chip, after converting the
  bits from the serial to parallel representation, then sends it (via a
  program read) to the large serial port buffer in main computer memory.
  Thus the chip has both input and output but since it's input data to
  the computer, its output is considered to be input.  The situation is
  similar for output flowing thru this chip.  The "input" and "output"
  refer to the direction of flow with respect to the computer and not
  the serial port hardware (the chip).

  Whereas setserial only deals with actual serial ports, stty is used
  both for serial ports and for virtual terminals such as the standard
  Linux text interface on a PC monitor.  For the PC monitor, many of the
  stty settings are meaningless.  Changing the baud rate, etc. doesn't
  appear to actually do anything.

  Here are some of the items stty configures: speed (bits/sec), parity,
  bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
  control, break signal, end-of-line markers, change case, padding, beep
  if buffer overrun?, echo what you type to the screen, allow background
  tasks to write to terminal?, define special (control) characters (such
  as what key to press for interrupt).  See the stty man or info page
  for more details.  Also see the man page: termios which covers the
  same options set by stty but (as of mid 1999) covers features which
  the stty man page fails to mention.

  With some implementations of getty (getty_ps package), the commands
  that one would normally give to stty are typed into a getty
  configuration file: /etc/gettydefs.  Even without this configuration
  file, the getty command line may be sufficient to set things up so
  that you don't need stty.

  One may write C programs which change the stty configuration, etc.
  Looking at some of the documentation for this may help one better
  understand the use of the stty command (and its many possible
  arguments).  Serial-Programming-HOWTO may be useful but it's outdated.
  The manual page: termios contains a description of the C-language
  structure (of type termios) which stores the stty configuration in
  computer memory.  Many of the flag names in this C-structure are
  almost the same (and do the same thing) as the arguments to the stty

  11.4.2.  Flow control options

  To set hardware flow control use "crtscts".  For software flow control
  there are 3 settings: ixon, ixoff, and ixany.

  ixany: Mainly for terminals.  Hitting any key will restart the flow
  after a flow-control stop.  If you stop scrolling with the "stop
  scroll" key (or the like) then hitting any key will resume scrolling.
  It's seldom needed since hitting the "scroll lock" key again will do
  the same thing.

  ixon: Enables the port to listen for Xoff and to stop transmitting
  when it gets an Xoff.  Likewise, it will resume transmitting if it
  gets an Xon.

  ixoff: enables the port to send the Xoff signal out the transmit line
  when its buffers in main memory are nearly full.  It protects the
  device where the port is located from being overrun.
  For a slow dumb terminal (or other slow device) connected to a fast
  PC, it's unlikely that the PC's port will be overrun.  So you seldom
  actually need to enable ixoff.  But it's often enabled "just in case".

  11.4.3.  Using stty at a "foreign" terminal

  How do you use stty to view or set a terminal other than the terminal
  you are currently using?  It's usually impossible to do it if the
  foreign terminal is in use and has a shell running on it.  In other
  cases for dealing with say ttyS2 while typing at another terminal
  (such as tty1) use  stty -F /dev/ttyS2 ... (or --file instead of F).
  If ... is -a it displays all the stty settings (-a means all).

  But if the foreign terminal (ttyS2 in this example) has a shell
  running on it, then what you see will likely be deceptive and trying
  to set it will not work.  This problem exists for virtual terminals
  also such as dealing with tty3 from tty1, etc.  See ``Two interfaces
  at a terminal'' to understand it.

  11.4.4.  Two interfaces at a terminal

  When using a shell (such as bash) with command-line-editing enabled
  there are two different terminal interfaces (what you see when you
  type stty -a).  When you type in modern shells at the command line you
  have a temporary "raw" interface (or raw mode) where each character is
  read by the command-line-editor as you type it.  Once you hit the
  <return> key, the command-line-editor is exited and the terminal
  interface is changed to the nominal "cooked" interface (cooked mode)
  for the terminal.  This cooked mode lasts until the next prompt is
  sent to the terminal (which is only a small fraction of a second).
  Note that one never gets to type any command in this cooked mode but
  what was typed in raw mode on the command line gets read by the shell
  while in cooked mode.

  When a prompt is sent to the terminal, the terminal goes from "cooked"
  to "raw" mode (just like it does when you start an editor such as vim.
  The prompt signals starting the command-line editor.  The settings for
  the "raw" mode are based only on the basic stty settings taken from
  the "cooked" mode.  Raw mode keeps these setting but changes several
  other settings in order to change the mode to "raw".  It is not at all
  based on the settings used in the previous "raw" mode.  Thus if one
  uses stty to change settings for the raw mode, such settings will be
  permanently lost as soon as one hits the <return> key at the terminal
  that has supposedly been "set".

  Now when one types stty to look at the terminal interface, one may
  either get a view of the cooked mode or the raw mode.  You need to
  figure out which one you're looking at.  It you use stty from a
  foreign terminal (other than the terminal you are currently typing at)
  then you will see the raw mode settings.  Any changes made will only
  be made to the raw mode and will be lost when someone presses <return>
  at the foreign terminal you tried to "set".  But if you type a stty
  command to view/change the configuration of the terminal you are
  using, and then hit <return> it's a different story.  The <return>
  puts the terminal in cooked mode.  Your changes are saved and will
  still be there when the terminal goes back into raw mode (unless of
  course it's a setting not allowed in raw mode).

  This situation can create problems.  For example, suppose you corrupt
  your terminal interface.  To restore it you go to another terminal and
  "stty -F dev/ttyS1 sane" (or the like).  It will not work!  Of course
  you can try to type "stty sane ..." at the terminal that is corrupted
  but you can't see what you typed.  All the above not only applies to
  dumb terminals but to virtual terminals used on a PC Monitor as well
  as to the terminal windows in X.  In other words, it applies to almost
  everyone who uses Linux.

  Luckily, when you start up Linux, any file that runs stty at boot-time
  will likely deal with a terminal (or serial port with no terminal)
  that has no shell running on it so there's no problem for this special

  11.4.5.  Where to put the stty command ?

  Should you need to have stty set up the serial interface each time the
  computer starts up then you need to put the stty command in a file
  that will be executed each time the computer is started up (Linux
  boots).  It should be run before the serial port is used (including
  running getty on the port).  There are many possible places to put it.
  If it gets put in more than one place and you only know about (or
  remember) one of those places, then a conflict is likely.  So make
  sure to document what you do.

  One place to put it would be in the same file that runs setserial when
  the system is booted.  The location is distribution and version
  dependent.  It would seem best to put it after the setserial command
  so that the low level stuff is done first.  If you have directories in
  the /etc tree where every file in them is executed at boot-time
  (System V Init) then you could create a file named "stty" for this

  11.4.6.  Obsolete redirection method

  Prior to about 2000 you needed to use the redirection operator "<" if
  you wanted to use stty on a foreign terminal.  For example to use stty
  on ttyS2 sitting at tty1 you would type: stty .... < /dev/ttyS2.
  After 2000 (provided your version of setserial is >= 1.17 and stty >=
  2.0) a better method was created using the -F option: stty -F
  /dev/ttyS2.  This will work when the old redirection method fails.

  The old redirection example above makes ttyS2 the standard input to
  stty.  This gives the stty program a link to the "file" ttyS2 so that
  it may "read" it.  But instead of reading the bytes sent to ttyS2 as
  one might expect, it uses the link to find the configuration settings
  of the port so that it may read or change them.  In olden days, some
  people tried to use ``stty ... > /dev/ttyS2'' to set the terminal.
  This didn't work.  Instead, it takes the message normal displayed by
  the stty command for the terminal you are on (say tty1) and sends this
  message to ttyS2.  But it doesn't change any settings for ttyS2.

  Here's a problem with the old redirection operator (which doesn't
  happen if you use the newer -F option instead).  Sometimes when trying
  to use stty, the command hangs and nothing happens (you don't get a
  prompt for a next command even after hitting <return>).  This is
  likely due to the port being stuck because it's waiting for one of the
  modem control lines to be asserted.  For example, unless you've set
  "clocal" to ignore modem control lines, then if no CD signal is
  asserted the port will not open and stty will not work for it (unless
  you use the newer -F option).  A similar situation seems to exist for
  hardware flow control.  If the cable for the port doesn't even have a
  conductor for the pin that needs to be asserted then there is no easy
  way to stop the hang.

  One way to try to get out of the above hang is to use the newer -F
  option and set "clocal" and/or "crtscts" as needed.  If you don't have
  the -F option then you may try to run some program (such as minicom)
  on the port that will force it to operate even if the control lines
  say not to.  Then hopefully this program might set the port so it
  doesn't need the control signal in the future in order to open: clocal
  or -crtscts.  To use "minicom" to do this you likely will have to
  reconfigure minicom and then exit it and restart it.  Instead of all
  this bother, it may be simpler to just reboot the PC or via using a
  virtual terminal kill the process using "top" (or "ps" to get the
  process number and then "kill" to kill that process.

  The obsolete redirection method (which still works in later versions)
  is to type ``stty ... < /dev/ttyS2''.   If the new method using -F
  works but the obsolete one hangs, it implies that the port is hung due
  to a modem control line not being asserted.  Thus the obsolete
  redirection method might still useful for troubleshooting.

  11.5.  What is isapnp ?

  isapnp is a program to configure Plug-and-Play (PnP) devices on the
  ISA bus including internal modems.  It comes in a package called
  "isapnptools" and includes another program, "pnpdump" which finds all
  your ISA PnP devices and shows you options for configuring them in a
  format which may be added to the PnP configuration file:
  /etc/isapnp.conf.  The isapnp command may be put into a startup file
  so that it runs each time you start the computer and thus will
  configure ISA PnP devices.  It is able to do this even if your BIOS
  doesn't support PnP.  See Plug-and-Play-HOWTO.

  11.6.  Connecting two PCs together via serial ports

  This is where you run a serial cable (crossover type = null-modem
  type) between the serial ports of two PCs.  Then how do you use this
  line?  One way is for one PC to run login on the serial line and for
  the other PC to run say minicom or picocom to emulate a terminal.  See
  Text-Terminal-HOWTO.  There is no network protocol used in this case
  and no error detection.

  The other method is to run a network protocol on the line.  For
  example, to use PPP in combination with TCP/IP see Serial Laplink
  HOWTO.  Although this HOWTO doesn't mention the old program "slattach"
  (serial line attach) it can put the serial line into a networking mode
  using the protocol you select.  Protocols for slattach include PPP or
  SLIP (an older protocol widely used prior to PPP).

  The Debian package, net-tools, contains slattach.  SLIP is provided as
  a kernel module or can be built into the kernel (2.2, 2.4, or 2.6).

  11.7.  Connect the serial port to a fast network: ser2net

  ser2net is a Linux program which will connect a network to the serial
  port.  For example, someone connects to your PC via an ethernet port
  or fast modem using say telnet.  Then (without ser2net) they could
  remotely login to your PC and then run programs on your PC that
  utilize a serial port on the PC.  However, it might be better if they
  didn't need to login and use your software, but instead could
  immediately connect to the serial port.  ser2net running on your PC
  can make this happen.

  It could be like a bridge between the ethernet cable and the serial
  cable.  The ethernet cable would have TCP/IP protocol on it but the
  serial port would just have the raw data taken out of the TCP/IP
  packets.  Optionally, you could use TCP/IP packets on the serial line
  too.  Since an ethernet port has high bandwidth, it could communicate
  with several serial ports at the same time and also have data flowing
  elsewhere as well.
  To set up ser2net, you must specify which network ports (on the
  ethernet) will connect to which serial ports.  Then when network
  packets arrive at your PC over ethernet which are addressed to a
  network port you've tied to a serial port, the data in those packets
  flows to the serial port.  And conversely.  Of course the network
  doesn't have to be ethernet.  It could be a cable modem or DSL line,

  12.  Speed (Flow Rate)

  By "speed" we really mean the "data flow rate" but almost everybody
  incorrectly calls it speed.  The speed is measured in bits/sec (or
  baud).  Speed is set using the "stty" command or by a program which
  uses the serial port.  See ``Stty''

  12.1.  Very High Speeds

  12.1.1.  Speeds over 115.2kbps

  The top speed of 115.2k has been standard since the mid 1990's.  But
  by the year 2000, most new serial ports supported higher speeds of
  230.4k and 460.8k.  Some also support 921.6k.  Unfortunately Linux
  seldom uses these speeds due to lack of drivers.  Thus such ports
  behave just like 115.2k ports unless the higher speeds are enabled by
  special software.  To get these speeds you need to compile the kernel
  with special patches or use modules until support is built into the
  kernel's serial driver.

  Unfortunately serial port manufacturers never got together on a
  standard way to support high speeds, so the serial driver needs to
  support a variety of hardware.  Once high speed is enabled, a standard
  way to choose it is to set baud_base to the highest speed with
  setserial (unless the serial driver does this for you).  The software
  will then use a divisor of 1 to set the highest speed.  All this will
  hopefully be supported by the Linux kernel sometime in 2003.

  A driver for the w83627hf chip (used on many motherboards such as the
  Tyan S2460) is at  <>

  A non-standard way that some manufacturers have implemented high speed
  is to use a very large number for the divisor to get the high speed.
  This number isn't really a divisor at all since it doesn't divide
  anything.  It's just serves as a code number to tell the hardware what
  speed to use.  In such cases you need to compile the kernel with
  special patches.

  One patch to support this second type of high-speed hardware is called
  shsmod (Super High Speed Mode).  There are both Windows and Linux
  versions of this patch.  See  <>.  There
  is also a module for the VIA VT82C686 chip
  <>.  Using it may result in buffer overflow.

  For internal modems, only a minority of them advertise that they
  support speeds of over 115.2k for their built-in serial ports.  Does
  shsmod support these ??

  12.1.2.  How speed is set in hardware: the divisor and baud_base

  Speed is set by having the serial port's clock change frequency.  But
  this change happens not by actually changing the frequency of the
  oscillator driving the clock but by "dividing" the clock's frequency.
  For example, to divide by two, just ignore every other clock tick.
  This cuts the speed in half.  Dividing by 3 makes the clock run at 1/3
  frequency, etc.  So to slow the clock down (meaning set speed), we
  just send the clock a divisor.  It's sent by the serial driver to a
  register in the port.  Thus speed is set by a divisor.

  If the clock runs at a top speed of 115,000 bps (common), then here
  are the divisors for various speeds (assuming a maximum speed of
  115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24
  (4.8k), 48 (2.4k), 96 (1.2k), etc.  The serial driver sets the speed
  in the hardware by sending the hardware only a "divisor" (a positive
  integer).  This "divisor" divides the "maximum speed" of the hardware
  resulting in a slower speed (except a divisor of 1 obviously tells the
  hardware to run at maximum speed).

  There are exceptions to the above since for certain serial port
  hardware, speeds above 115.2k are set by using a very high divisor.
  Keep that exception in mind as you read the rest of this section.
  Normally, if you specify a speed of 115.2k (in your communication
  program or by stty) then the serial driver sets the port hardware to
  divisor 1 which sets the highest speed.

  Besides using a very high divisor to set high speed, the conventional
  way to do it is as follows: If you happen to have hardware with a
  maximum speed of say 230.4k (and the 230.4k speed has been enabled in
  the hardware), then specifying 115.2k will result in divisor 1.  For
  some hardware this will actually give you 230.4k.  This is double the
  speed that you set.  In fact, for any speed you set, the actual speed
  will be double.  If you had hardware that could run at 460.8k then the
  actual speed would be quadruple what you set.  All the above assumes
  that you don't use "setserial" to modify things.

  12.1.3.  Setting the divisor, speed accounting

  To correct this accounting (but not always fix the problem) you may
  use "setserial" to change the baud_base to the actual maximal speed of
  your port such as 230.4k.  Then if you set the speed (by your
  application or by stty) to 230.4k, a divisor of 1 will be used and
  you'll get the same speed as you set.

  If you have very old software which will not allow you to tell it such
  a high speed (but your hardware has it enabled) then you might want to
  look into using the "spd_cust" parameter.  This allows you to tell the
  application that the speed is 38,400 but the actual speed for this
  case is determined by the value of "divisor" which has also been set
  in setserial.  I think it best to try to avoid using this kludge.

  There are some brands of UARTs that uses a very high divisor to set
  high speeds.  There isn't any satisfactory way to use "setserial" (say
  set "divisor 32770") to get such a speed since then setserial would
  then think that the speed is very low and disable the FIFO in the

  12.1.4.  Crystal frequency is higher than baud_base

  Note that the baud_base setting is usually much lower than the
  frequency of the crystal oscillator since the crystal frequency of say
  1.8432 MHz is divided by 16 in the hardware to get the actual top
  speed of 115.2k.  The reason the crystal frequency needs to be higher
  is so that this high crystal speed can generate clock ticks to take a
  number of samples of each  bit to determine if it's a 1 or a 0.

  Actually, the 1.8432 MHz "crystal frequency" may be obtained from a
  18.432 MHz crystal oscillator by dividing by 10 before being fed to
  the UART.  Other schemes are also possible as long as the UART
  performs properly.
  12.2.  Higher Serial Throughput

  If you are seeing slow throughput and serial port overruns on a system
  with (E)IDE disk drives, you can get hdparm.  This is a utility that
  can modify (E)IDE parameters, including unmasking other IRQs during a
  disk IRQ.  This will improve responsiveness and will help eliminate
  overruns.  Be sure to read the man page very carefully, since some
  drive/controller combinations don't like this and may corrupt the

  Also have a look at a utility called irqtune that will change the IRQ
  priority of a device, for example the serial port that your modem is
  on.  This may improve the serial throughput on your system.  The
  irqtune FAQ is at

  13.  Locking Out Others

  13.1.  Introduction

  When you are using a serial port, you may want to prevent others from
  using it at the same time.  However there may be cases where you do
  want others to use it, such as sending you an important message if you
  are using a text-terminal.

  There are various ways of preventing others (or other processes) from
  using your serial port when you are using it (locking).  This should
  all happen automatically but it's important to know about this if it
  gives you trouble.  If a program is abnormally exited or the PC is
  abruptly turned off (by pulling the plug, etc.) your serial port might
  wind up locked.  Even if the lock remains, it's usually automatically
  removed when you want to use the serial port again.  But in rare cases
  it isn't.  That's when you need to understand what happened.

  One way to implement locking is to design the kernel to handle it but
  Linux thus far has shunned this solution (with an exception involving
  the cua device which is now obsolete).  Two solutions used by Linux is

  1. create lock-files

  2. modify the permissions and/or owners of devices such as /dev/ttyS2

  13.2.  Lock-Files

  If you use the new device-filesystem (devfs) then see the next
  section.  A lock-file is simply a file created to mean that a
  particular device is in use.  They are kept in /var/lock.  Formerly
  they were in /usr/spool/uucp.  Linux lock-files are usually named, where name may be a device name, a process id number, a
  device's major and minor numbers, or a UUCP site name.  Most processes
  (an exception is getty) create these locks so that they can have
  exclusive access to devices.  For instance if you dial out on your
  modem, some lockfiles will appear to tell other processes that someone
  else is using the modem.  In older versions (in the 1990s) there was
  usually only one lockfile per process.  Lock files contain the PID of
  the process that has locked the device.  Note that if a process
  insists on using a device that is locked, it may ignore the lockfile
  and use the device anyway.  This is useful in sending a message to a
  text-terminal, etc.

  When a program wants to use a serial port but finds it locked with
  lock-files it should check to see if the lock-file's PID is still in
  use.  If it's not it means that the lock is stale and it's OK to go
  ahead and use the port anyway (after removing the stale lock-files).
  Unfortunately, there may be some programs that don't do this and give
  up by telling you that a device is already in use when it really

  If the lockfile only uses device names, the following problem could
  arise:  If the same device has two different names then two different
  processes could each use a different name for the same device.  This
  results in lockfiles with different names that actually are the same
  device.  Formerly each physical serial port was known by two different
  device names: ttyS0 and cua0.  To solve this lockfile alias problem, 3
  methods have been used.  It may be overkill since any one of these
  methods would have fixed the problem.

  1. The lock checking software was made aware of ttyS vs. cua.

  2. The device cua was deprecated

  3. Additional locks were created which use unique device numbers
     instead of names.

  Using alternate names such as /dev/modem for /dev/ttyS2 may cause
  problems if one program opens /dev/ttyS2 while another program opens
  /dev/modem.  This problem was supposedly fixed around 2000, but is
  still present in 2005.  For dumb terminals, lockfiles are not used
  since this would not permit someone else to send a message to your
  terminal using the write or talk program.

  13.3.  Change Owners, Groups, and/or Permissions of Device Files

  In order to use a device, you (or the program you run if you have "set
  user id") needs to have permission to read and write the device "file"
  in the /dev directory.  So a logical way to prevent others from using
  a device is to make yourself the temporary owner of the device and set
  permissions so that no one else can use it.  A program may do this for
  you.  A similar method can be used with the group of the device file.

  While lock files prevent other process from using the device, changing
  device file owners/permissions restricts other users (or the group)
  from using it.  One case is where the group is permitted to write to
  the port, but not to read from it.  Writing to the port might just
  mean a message sent to a text-terminal while reading means destructive
  reading.  The original process that needs to read the data may find
  data missing if another process has already read that data.  Thus a
  read can do more harm that a write since a read causes loss of data
  while a write only adds extra data.  That's a reason to allow writes
  but not reads.  This is exactly the opposite of the case for ordinary
  files where you allow others to read the file but not write (modify)
  it.  Use of a port normally requires both read and write permissions.

  A program that changes the device file attributes should undo these
  changes when it exits.  But if the exit is abnormal, then a device
  file may be left in such a condition that it gives the error
  "permission denied" when one attempts to use it again.

  14.  Serial Communications Programs And Utilities

  14.1.  List of Software

  Here is a list of some communication software you can choose from,
  available via FTP, if they didn't come with your Linux distribution.

  В·  ecu - a communications program

  В·  C-Kermit <> - portable, scriptable,
     serial and TCP/IP communications including file transfer,
     character-set translation, and zmodem support

  В·  gkermit Tiny GPLed kermit run only from the command line.  Can't
     connect to another computer

  В·  gtkterm - a simple gtk terminal, X-based

  В·  minicom - telix-like communications program

  В·  picocom - like a small minicom but no automatic phone dialing

  В·  pppd - establishes a ppp connection on the serial line

  В·  seyon - X based communication program

  В·  xc - xcomm communication package

  В·  term and SLiRP offer TCP/IP functionality using a shell account.

  В·  screen is another multi-session program.  This one behaves like the
     virtual consoles.

  В·  callback is where you dial out to a remote modem and then that
     modem hangs up and calls you back (to save on phone bills).

  В·  mgetty+fax handles FAX stuff, and provides an alternate ps_getty.

  В·  ZyXEL is a control program for ZyXEL U-1496 modems.  It handles
     dialin, dialout, dial back security, FAXing, and voice mailbox

  В·  SLIP and PPP software (if not in your Linux distribution) can be
     found at

  14.2.  kermit and zmodem

  For use of kermit with modems see the Modem-HOWTO.  One can run zmodem
  within the kermit program.  To do this (for ttyS3), add the following
  to your .kermrc file:

       define rz !rz < /dev/ttyS3 > /dev/ttyS3
       define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3

  Be sure to put in the correct port your modem is on.  Then, to use it,
  just type rz or sz <filename> at the kermit prompt.

  15.  Serial Tips And Miscellany

  15.1.  Serial Modules

  Often the serial driver is provided as a module(s) such as
  generic_serial.ko.  Drivers for USB serial ports and multiport cards
  are often provided as modules.  Linux should automatically load any
  needed module, so in most cases you have nothing to do.

  But sometimes you need to configure Linux to load certain modules or
  gives parameters to the module or to the kernel.

  Such parameters may be supplied to certain modules on the command line
  for the kernel or in /etc/modules, /etc/modules.conf or
  /etc/modprobe.conf.  Since kernel 2.2 you don't edit the modprobe.conf
  file but use the program update-modules to change it.  The info that
  is used to update modules.conf is put in /etc/modutils/.

  The Debian/GNU Linux has a file named /etc/modutils/setserial which
  runs the serial script in /etc/init.d/ every time the serial module is
  loaded or unloaded.  When the serial module is unloaded this script
  will save the state of the module in /var/run/setserial.conf.  Then if
  the module loads again this saved state is restored.  When the serial
  module first loads at boot-time, there's nothing in
  /var/run/setserial.conf so the state is obtained from
  /etc/serial.conf.  So there are two files that save the state.  Other
  distributions may do something similar.

  Serial modules are found in subdirectories of
  /lib/modules/.../kernel/drivers/.  For multiport cards, look in the
  serial subdirectory and/or char.  For USB serial, look in the
  usb/serial subdirectory.  The module, parport_serial is for PCI cards
  that contain both serial and parallel ports.

  As a last resort, one may modify the serial driver by editing the
  source code.  Much of the serial driver is found in the file serial.c.
  For info regarding writing of programs for the serial port see Serial-
  Programming-HOWTO.  It was revised in 1999 by Vern Hoxie but that
  revision is not at LDP.

  15.2.  Kernel Configuration

  15.3.  Number of Serial Ports Supported

  If you have more than 4 (or possibly 2) serial ports, then you must
  insure that the kernel knows this.  It can be done by configuring the
  kernel when compiling or by a parameter given to the kernel when it
  starts (boot-prompt or kernel command line).

  The kernel configuration parameters:
  set the maximum number of ordinary serial ports (UARTs) equal to 4.
  If you have more than 4 ordinary serial ports, then you need to change
  the 4 to whatever.  But you may override this via the kernel command
  line for example: nr_uarts=16 (if serial support built into the
  kernel) or 8250.nr_uarts=16 (if serial support is via a module).  The
  boot loader such as lilo or grub can be told to do this.

  15.4.  Serial Console (console on the serial port)

  See the kernel documentation in: Documentation/serial-console.txt.
  Kernel 2.4+ has better documentation.  See also "Serial Console" in

  15.5.  Line Drivers

  For a text terminal, the RS-232 speeds are fast enough but the usable
  cable length is often too short.  Balanced technology could fix this.
  The common method of obtaining balanced communication with a text
  terminal is to install 2 line drivers in the serial line to convert
  unbalanced to balanced (and conversely).  They are a specialty item
  and are expensive if purchased new.
  15.6.  Stopping the Data Flow when Printing, etc.

  Normally flow control and/or application programs stop the flow of
  bytes when its needed.  But sometimes they don't.  The problem is that
  output to the serial port first passes thru the large serial buffer in
  the PC's main memory.  So if you want to abort printing, whatever is
  in this buffer should be removed.  When you tell an application
  program to stop printing, it may not empty this buffer so printing
  continues until it's empty.  In addition, your printer has it's own
  buffer which needs to be cleared.  So telling the PC to stop printing
  may not work due to these two buffers that continue to supply bytes
  for the printer.  It's a problem with printer software not knowing
  about the serial port and that modem control lines need to be dropped
  to stop the printer.

  One way to insure that printing stops is to just turn off the printer.
  With newer serial drivers, this works OK.  The buffers are cleared and
  printing doesn't resume.  With older serial drivers, the PC's serial
  buffer didn't clear and it would sometimes continue to print when the
  printer was turned back on.  To avoid this, you must wait a time
  specified by setserial's closing_wait before turning the printer back
  on again.  You may also need to remove the print job from the print
  queue so it won't try to resume.

  15.7.  Known IO Address Conflicts

  15.7.1.  Avoiding IO Address Conflicts with Certain Video Boards

  The IO address of the IBM 8514 video board (and others) is allegedly
  0x?2e8 where ? is 2, 4, 8, or 9.  This may conflict (but shouldn't if
  the serial port is well designed) with the IO address of ttyS3 at
  0x02e8 if the serial port ignores the leading 0 hex digit when it
  decodes the address (many do).  That is bad news if you try to use
  ttyS3 at this IO address.  Another story is that Linux will not detect
  your internal modem on ttyS3 but that you can use setserial to put
  ttyS3 at this address and the modem will work fine.

  15.7.2.  IO address conflict with ide2 hard drive

  The address of ttyS2 is 3e8-3ef while hard drive ide2 uses 3ee which
  is in this range.  So when booting Linux you may see a report of this
  conflict.  Most people don't use ide2 (the 3rd hard drive cable) and
  may ignore this conflict message.  You may have 2 hard drives on ide0
  and two more on ide1 so most people don't need ide2.

  15.8.  Known Defective Hardware

  15.8.1.  Problem with AMD Elan SC400 CPU (PC-on-a-chip)

  This has a race condition between an interrupt and a status register
  of the UART.  An interrupt is issued when the UART transmitter
  finishes the transmission of a byte and the UART transmit buffer
  becomes empty (waiting for the next byte).  But a status register of
  the UART doesn't get updated fast enough to reflect this.  As a
  result, the interrupt service routine rapidly checks and determines
  (erroneously) that nothing has happened.  Thus no byte is sent to the
  port to be transmitted and the UART transmitter waits in vain for a
  byte that never arrives.  If the interrupt service routine had waited
  just a bit longer before checking the status register, then it would
  have been updated to reflect the true state and all would be OK.

  There is a proposal to fix this by patching the serial driver.  But
  Should linux be patched to accommodate defective hardware, especially
  if this patch may impair performance of good hardware?
  16.  Troubleshooting

  See Modem-HOWTO for troubleshooting related to modems or getty for
  modems.  For a Text-Terminal much of the info here will be of value as
  well as the troubleshooting info in Text-Terminal-HOWTO.

  16.1.  Serial Electrical Test Equipment

  16.1.1.  Breakout Gadgets, etc.

  While a multimeter (used as a voltmeter) may be all that you need for
  just a few serial ports, simple special test equipment has been made
  for testing serial port lines.  Some are called "breakout ... " where
  breakout means to break out conductors from a cable.  These gadgets
  have a couple of connectors which connect to serial port connectors
  (either at the ends of serial cables or at the back of a PC).  Some
  have test points for connecting a voltmeter.  Others have LED lamps
  which light when certain modem control lines are asserted (turned on).
  The color of the light may indicate the polarity of the signal
  (positive or negative voltage).  Still others have jumpers so that you
  can connect any wire to any wire.  Some have switches.

  Radio Shack sells (in 2002) a "RS-232 Troubleshooter" (formerly called
  "RS-232 Line Tester") Cat. #276-1401.  It checks TD, RD, CD, RTS, CTS,
  DTR, and DSR.  A green light means on (+12 v) while red means off (-12
  v).  They also sell a "RS-232 Serial Jumper Box" Cat.  #276-1403.
  This permits connecting the pins anyway you choose.  Both these items
  are under the heading of "Peripheral hookup helpers".  Unfortunately,
  they are not listed in the index to the printed catalog.  They are on
  the same page as the D type connecters so look in the index under
  "Connectors, Computer, D-Sub".  A store chain named "Active
  Components" may have them.

  16.1.2.  Measuring voltages

  Any voltmeter or multimeter, even the cheapest that sells for about
  $10, should work fine.  Trying to use other methods for checking
  voltage is tricky.  Don't use a LED unless it has a series resistor to
  reduce the voltage across the LED.  A 470 ohm resistor is used for a
  20 ma LED (but not all LED's are 20 ma).  The LED will only light for
  a certain polarity so you may test for + or - voltages.  Does anyone
  make such a gadget for automotive circuit testing??  Logic probes may
  be damaged if you try to use them since the TTL voltages for which
  they are designed are only 5 volts.  Trying to use a 12 V incandescent
  light bulb is not a good idea.  It won't show polarity and due to
  limited output current of the UART it probably will not even light up.

  To measure voltage on a female connector you may plug in a bent paper
  clip into the desired opening.  The paper clip's diameter should be no
  larger than the pins so that it doesn't damage the contact.  Clip an
  alligator clip (or the like) to the paper clip to connect up.  Take
  care not to touch two pins at the same time with any metal object.

  16.1.3.  Taste voltage

  As a last resort, if you have no test equipment and are willing to
  risk getting shocked (or even electrocuted) you can always taste the
  voltage.  Before touching one of the test leads with your tongue, test
  them to make sure that there is no high voltage on them.  Touch both
  leads (at the same time) to one hand to see if they shock you.  Then
  if no shock, wet the skin contact points by licking and repeat.  If
  this test gives you a shock, you certainly don't want to use your
  For the test for 12 V, Lick a finger and hold one test lead in it.
  Put the other test lead on your tongue.  If the lead on your tongue is
  positive, there will be a noticeable taste.  You might try this with
  flashlight batteries first so you will know what taste to expect.

  16.2.  Serial Monitoring/Diagnostics

  A few Linux programs will monitor the modem control lines and indicate
  if they are positive (1) or negative (0).  See section ``Serial

  16.3.  (The following subsections are in both the Serial and Modem

  16.4.  Serial Port Can't be Found

  There are 3 possibilities:

  1. Your port is disabled since both the BIOS and Linux failed to
     enable it.  It has no IO address.

  2. Your port is enabled and has an IO address but it has no ttyS
     device number (like ttyS14) assigned to that address so the port
     can't be used.  As a last resort, you may need to use "setserial"
     to assign a ttyS number to it.

  3. Your port does have a ttyS number assigned to it (like ttyS14) but
     you don't know which physical connector it is (on the back of your

     See ``Which Connector on the Back of my PC is ttyS1, etc?''

  First check BIOS messages at boot-time (and possibly the BIOS menu for
  the serial port).  Then for the PCI bus type lspci -v.  If this shows
  something like "LPC Bridge" then your port is likely on the LPC bus
  which is not well supported by Linux yet (but the BIOS might find it)
  ??  If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
  and/or see Plug-and-Play-HOWTO.  If the port happens to be enabled
  then the following two paragraphs may help find the IO port:

  16.4.1.  Scanning/probing legacy ports

  This is mainly for legacy non-PCI ports and ISA ports that are not

  Using "scanport" (Debian only ??) will scan all enabled bus ports and
  may discover an unknown port that could be a serial port (but it
  doesn't probe the port).  It could hang your PC.  If you suspect that
  your port may be at a certain address, you may try manually probing
  with setserial, but it's a slow tedious task if you have several
  addresses to probe.  See ``Probing''.

  16.5.  Linux Creates an Interrupt Conflict (your PC has an ISA slot)

  If your PC has a BIOS that handles ISA (and likely PCI too) then if
  you find a IRQ conflict, it might be due to a shortage of free IRQs.
  The BIOS often maintains a list of reserved IRQs, reserved for legacy
  ISA cards.  If too many are reserved, the BIOS may not be able to find
  a free IRQ and will erroneously assign an IRQ to the serial port that
  creates a conflict.  So check to see if all the reserved IRQs are
  really needed and if not, unreserve an IRQ that the serial port can
  use.  For more details, see Plug-and-Play-HOWTO.

  16.6.  Extremely Slow: Text appears on the screen slowly after long

  It's likely mis-set/conflicting interrupts.  Here are some of the
  symptoms which will happen the first time you try to use a modem,
  terminal, or serial printer.  In some cases you type something but
  nothing appears on the screen until many seconds later.  Only the last
  character typed may show up.  It may be just an invisible <return>
  character so all you notice is that the cursor jumps down one line.
  In other cases where a lot of data should appear on the screen, only a
  batch of about 16 characters appear.  Then there is a long wait of
  many seconds for the next batch of characters.  You might also get
  "input overrun" error messages (or find them in logs).

  For more details on the symptoms and why this happens see ``Interrupt
  Problem Details'' and/or ``Interrupt Conflicts'' and/or ``Mis-set
  Interrupts''.  If it involves Plug-and-Play devices, see also the

  As a quick check to see if it really is an interrupt problem, set the
  IRQ to 0 with "setserial".  This will tell the driver to use polling
  instead of interrupts.  If this seems to fix the "slow" problem then
  you had an interrupt problem.  You should still try to solve the
  problem since polling uses excessive computer resources.

  Checking to find the interrupt conflict may not be easy since Linux
  supposedly doesn't permit any interrupt conflicts and will send you a
  ``/dev/ttyS?: Device or resource busy'' error message if it thinks you
  are attempting to create a conflict.  But a real conflict can be
  created if "setserial" has told the kernel incorrect info.  The kernel
  has been lied to and thus doesn't think there is any conflict.  Thus
  using "setserial" will not reveal the conflict (nor will looking at
  /proc/interrupts which bases its info on "setserial").  You still need
  to know what "setserial" thinks so that you can pinpoint where it's
  wrong and change it when you determine what's really set in the

  What you need to do is to check how the hardware is set by checking
  jumpers or using PnP software to check how the hardware is actually
  set.  For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
  "lspci" (if PCI bus).  Compare this to how Linux (e.g. "setserial")
  thinks the hardware is set.

  16.7.  Somewhat Slow: I expected it to be a few times faster

  An obvious reason is that the baud rate is set too slow.  It's claimed
  that this once happened by trying to set the baud rate to a speed
  higher than the hardware can support (such as 230400).

  Another reason may be that whatever is on the serial port (such as a
  modem, terminal, printer) doesn't work as fast as you thought it did.

  Another possible reason is that you have an obsolete serial port: UART
  8250, 16450 or early 16550 (or the serial driver thinks you do).  See

  ``What Are UARTS?''  Use "setserial -g /dev/ttyS*".  If it shows
  anything less than a 16550A, this may be your problem.  If you think
  that "setserial" has it wrong check it out.  See ``What is Setserial''
  for more info.  If you really do have an obsolete serial port, lying
  about it to setserial will only make things worse.

  16.8.  The Startup Screen Shows Wrong IRQs for the Serial Ports

  For non-PnP ports, Linux does not do any IRQ detection on startup.
  When the serial module loads it only does serial device detection.
  Thus, disregard what it says about the IRQ, because it's just assuming
  the standard IRQs.  This is done, because IRQ detection is unreliable,
  and can be fooled.  But if and when setserial runs from a start-up
  script, it changes the IRQ's and displays the new (and hopefully
  correct) state on on the startup screen.  If the wrong IRQ is not
  corrected by a later display on the screen, then you've got a problem.

  So, even though I have my ttyS2 set at IRQ 5, I still see

       ttyS02 at 0x03e8 (irq = 4) is a 16550A

  at first when Linux boots.  (Older kernels may show "ttyS02" as
  "tty02" which is the same as ttyS2).  You may need to use setserial to
  tell Linux the IRQ you are using.

  16.9.  "Cannot open /dev/ttyS?: Device or resource busy

  See ``/dev/tty? Device or resource busy''

  16.10.  "Cannot open /dev/ttyS?: Permission denied"

  Check the file permissions on this port with "ls -l /dev/ttyS?"_ If
  you own the ttyS? then you need read and write permissions: crw with
  the c (Character device) in col. 1.  It you don't own it then it will
  work for you if it shows rw- in cols. 8 & 9 which means that everyone
  has read and write permission on it.  Use "chmod" to change
  permissions.  There are more complicated (and secure) ways to get
  access like belonging to a "group" that has group permission.  Some
  programs change the permissions when they run but restore them when
  the program exists normally.  But if someone pulls the plug on your PC
  it's an abnormal exit and correct permissions may not be restored.

  16.11.  "Cannot open /dev/ttyS?"

  Unless stty is set for clocal, the CD pin may need to be asserted in
  order to open a serial port.  If the physical port is not connected to
  anything, or if it's connected to something that is not powered on
  (such an external modem) then there will be no voltage on CD from that
  device.  Thus the "cannot open" message.  Either set clocal or connect
  the serial port connector to something and power it on.

  Even if a device is powered on and connected to a port, it may
  sometimes prevent opening the port.  An example of this is where the
  device has negated CD and the CD pin on your PC is negated (negative

  16.12.  "Operation not supported by device" for ttyS?

  This means that an operation requested by setserial, stty, etc.
  couldn't be done because the kernel doesn't support doing it.
  Formerly this was often due to the "serial" module not being loaded.
  But with the advent of PnP, it may likely mean that there is no modem
  (or other serial device) at the address where the driver (and
  setserial) thinks it is.  If there is no modem there, commands (for
  operations) sent to that address obviously don't get done.  See ``What
  is set in my serial port hardware?''

  If the "serial" module wasn't loaded but "lsmod" shows you it's now
  loaded it might be the case that it's loaded now but wasn't loaded
  when you got the error message.  In many cases the module will
  automatically loaded when needed (if it can be found).  To force
  loading of the "serial" module it may be listed in the file:
  /etc/modules.conf or /etc/modules.  The actual module should reside
  in: /lib/modules/.../misc/serial.o.

  16.13.  "Cannot create lockfile. Sorry"

  Sometimes when it can't create a lockfile you get the erroneous
  message: "... Device or resource busy" instead of the one above.  When
  a port is "opened" by a program a lockfile is created in /var/lock/.
  Wrong permissions for the lock directory will not allow a lockfile to
  be created there.  Use "ls -ld /var/lock" to see if the permissions
  are OK.  Giving rwx permissions for the root owner and the group
  should work, provided that the users that need to dialout belong to
  that group.  Others should have r-x permission.  Even with this
  scheme, there may be a security risk.  Use "chmod" to change
  permissions and "chgrp" to change groups.  Of course, if there is no
  "lock" directory no lockfile can be created there.  For more info on
  lockfiles see ``What Are Lock Files''

  16.14.  "Device /dev/ttyS? is locked."

  This means that someone else (or some other process) is supposedly
  using the serial port.  There are various ways to try to find out what
  process is "using" it.  One way is to look at the contents of the
  lockfile (/var/lock/LCK...).  It should be the process id.  If the
  process id is say 100 type "ps 100" to find out what it is.  Then if
  the process is no longer needed, it may be gracefully killed by "kill
  100".  If it refuses to be killed use "kill -9 100" to force it to be
  killed, but then the lockfile will not be removed and you'll need to
  delete it manually.  Of course if there is no such process as 100 then
  you may just remove the lockfile but in most cases the lockfile should
  have been automatically removed if it contained a stale process id
  (such as 100).

  16.15.  "/dev/tty? Device or resource busy"

  This means that the device you are trying to access (or use) is
  supposedly busy (in use) or that a resource it needs (such as an IRQ)
  is supposedly being used by another device and can't be shared.  This
  message is easy to understand if it only means that the device is busy
  (in use).  But it sometimes means that a needed resource is already in
  use (busy).  What makes it even more confusing is that in some cases
  neither the device nor the resources that it needs are actually

  In olden days, if a PC was shutdown by just turning off the power, a
  bogus lockfile might remain and then later on one would get this bogus
  message and not be able to use the serial port.  Software today is
  supposed to automatically remove such bogus lockfiles, but as of 2006
  there is still a problem with the "wvdial" dialer program related to
  lockfiles.  If wvdial can't create a lockfile because it doesn't have
  write permission in the /var/lock/ directory, you will see this
  erroneous message.  See ``Cannot create lockfile. Sorry''
  The following example is where interrupts can't be shared (at least
  one of the interrupts is on the ISA bus).  The ``resource busy'' part
  often means (example for ttyS2) ``You can't use ttyS2 since another
  device is using ttyS2's interrupt.'' The potential interrupt conflict
  is inferred from what "setserial" thinks.  A more accurate error
  message would be ``Can't use ttyS2 since the setserial data (and
  kernel data) indicates that another device is using ttyS2's
  interrupt''.  If two devices use the same IRQ and you start up only
  one of the devices, everything is OK because there is no conflict yet.
  But when you next try to start the second device (without quitting the
  first device) you get a "... busy" error message.  This is because the
  kernel only keeps track of what IRQs are actually in use and actual
  conflicts don't happen unless the devices are in use (open).   The
  situation for I/O address (such as 0x3f8) conflict is similar.

  This error is sometimes due to having two serial drivers: one a module
  and the other compiled into the kernel.  Both drivers try to grab the
  same resources and one driver finds them "busy".

  There are two possible cases when you see this message:

  1. There may be a real resource conflict that is being avoided.

  2. Setserial has it wrong and the only reason ttyS2 can't be used is
     that setserial erroneously predicts a conflict.

  What you need to do is to find the interrupt setserial thinks ttyS2 is
  using.  Look at /proc/tty/driver/serial.  You should also be able to
  find it with the "setserial" command for ttyS2.

  Bug in old versions:  Prior to 2001 there was a bug which wouldn't let
  you see it with "setserial".  Trying to see it would give the same
  "... busy" error message.

  To try to resolve this problem reboot or: exit or gracefully kill all
  likely conflicting processes.   If you reboot: 1. Watch the boot-time
  messages for the serial ports.  2. Hope that the file that runs
  "setserial" at boot-time doesn't (by itself) create the same conflict

  If you think you know what IRQ say ttyS2 is using then you may look at
  /proc/interrupts to find what else (besides another serial port) is
  currently using this IRQ.  You might also want to double check that
  any suspicious IRQs shown here (and by "setserial") are correct (the
  same as set in the hardware).  A way to test whether or not it's a
  potential interrupt conflict is to set the IRQ to 0 (polling) using
  "setserial".  Then if the busy message goes away, it was likely a
  potential interrupt conflict.  It's not a good idea to leave it
  permanently set at 0 since it will put more load on the CPU.

  16.16.  "Input/output error" from setserial, stty, pppd, etc.

  This means that communication with the serial port isn't working
  right.  It could mean that there isn't any serial port at the IO
  address that setserial thinks your port is at.  It could also be an
  interrupt conflict (or an IO address conflict).  It also may mean that
  the serial port is in use (busy or opened) and thus the attempt to
  get/set parameters by setserial or stty failed.  It will also happen
  if you make a typo in the serial port name such as typing "ttys"
  instead of "ttyS".

  16.17.  "LSR safety check engaged"

  LSR is the name of a hardware register.  It usually means that there
  is no serial port at the address where the driver thinks your serial
  port is located.  You need to find your serial port and possibly
  configure it.  See ``Locating the Serial Port: IO address IRQs''
  and/or ``What is Setserial''

  16.18.  Overrun errors on serial port

  This is an overrun of the hardware FIFO buffer and you can't increase
  its size.  Bug note (reported in 2002): Due to a bug in some kernel
  2.4 versions, the port number may be missing and you will only see
  "ttyS" (no port number).  But if devfs notation such as "tts/2" was
  being used, there was no bug.  See ``Higher Serial Thruput''.

  16.19.  Port gets characters only sporadically

  There could be some other program running on the port.  Use "top"
  (provided you've set it to display the port number) or type "ps
  -alxw".  Look at the results to see if the port is being used by
  another program.  Be on the lookout for the gpm mouse program which
  often runs on a serial port.

  16.20.  Troubleshooting Tools

  These are some of the programs you might want to use in

  В·  "lsof /dev/ttyS*" will list serial ports which are open.

  В·  "setserial" shows and sets the low-level hardware configuration of
     a port (what the driver thinks it is).  See ``What is Setserial''

  В·  "stty" shows and sets the configuration of a port (except for that
     handled by "setserial").  See the section ``Stty''

  В·  "modemstat" or "statserial" or "watch head /proc/tty/driver/serial"
     will show the current state of various modem signal lines (such as
     DTR, CTS, etc.).  The one in /proc also shows byte flow and errors.

  В·  "irqtune" will give serial port interrupts higher priority to
     improve performance.

  В·  "hdparm" for hard-disk tuning may help some more.

  В·  "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.

  В·  "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
     PnP devices on the ISA bus.

  В·  Some "files" in the /proc tree (such as ioports, interrupts, and

  16.21.  Almost all characters are wrong; Many missing or many extras

  Perhaps a baud mismatch.  If one port sends at twice the speed that
  the other port is set to receive, then every two characters sent will
  be received as one character.  Each bit of this received character
  will be a sample of two bits of what is sent and will be wrong.  Also,
  only half the characters sent seem to get received.  For flow in the
  reverse direction, it's just the opposite.  Twice as many characters
  get received than were sent.  A worse mismatch will produce even worse

  A speed mismatch is not likely to happen with a modem since the modem
  autodetects the speed.  One cause of a mismatch may be due to serial
  port hardware that has been set to run at very fast speeds.  It may
  actually operate at a speed say 8 times that of which you (or an
  application) set it via software.  See ``Very High Speeds''

  17.  Interrupt Problem Details

  While the section ``Troubleshooting'' lists problems by symptom, this
  section explains what will happen if interrupts are set incorrectly.
  This section helps you understand what caused the symptom, what other
  symptoms might be due to the same problem, and what to do about it.

  17.1.  Types of interrupt problems

  The "setserial" program will show you how serial driver thinks the
  interrupts are set.  If the serial driver (and setserial) has it right
  then everything regarding interrupts should be OK.  Of course a
  /dev/ttyS must exist for the device and Plug-and-Play (or jumpers)
  must have set an address and IRQ in the hardware.  Linux will not
  knowingly permit an interrupt conflict and you will get a "Device or
  resource busy" error message if you attempt to do something that would
  create a conflict.

  Since the kernel tries to avoid interrupt conflicts and gives you the
  "resource busy" message if you try to create a conflict, how can
  interrupt conflicts happen?  Easy.  "setserial" may have it wrong and
  erroneously predicts no conflict when there will actually be a real
  conflict based on what is set in the hardware.  When this happens
  there will be no "... busy" message but a conflict will physically
  happen.  Performance is likely to be extremely slow.  Both devices
  will send identical interrupt signals on the same wire and the CPU
  will erroneously think that the interrupts only come from one device.
  This will be explained in detail in the following sections.

  Linux doesn't complain when you assign two devices the same IRQ
  provided that neither device is in use.  As each device starts up
  (initializes), it asks Linux for permission to use its hardware
  interrupt.  Linux keeps track of which interrupt is assigned to whom,
  and if your interrupt is already in use, you'll see this "... busy"
  error message.  Thus if two devices use the same IRQ and you start up
  only one of the devices, everything is OK.  But when you next try to
  start the second device (without quitting the first device) you get
  "... busy" error message.

  17.2.  Symptoms of Mis-set or Conflicting Interrupts

  The symptoms depend on whether or not you have a modern serial port
  with FIFO buffers or an obsolete serial port without FIFO buffers.
  It's important to understand the symptoms for the obsolete ones also
  since sometimes modern ports seem to behave that way.

  For the obsolete serial ports, only one character gets thru every
  several seconds.  This is so slow that it seems almost like nothing is
  working (especially if the character that gets thru is invisible (such
  a space or newline).  For the modern ports with FIFO buffers you will
  likely see bursts of up to 16 characters every several seconds.

  If you have a modem on the port and dial a number, it seemingly may
  not connect since the CONNECT message may not make it thru.  But after
  a long wait it may finally connect and you may see part of a login
  message (or the like).  The response from your side of the connection
  may be so delayed that the other side gives up and disconnects you,
  resulting in a NO CARRIER message.

  If you use minicom, a common test to see if things are working is to
  type the simplest "AT" command and see if the modem responds.  Typing
  just at<enter> should normally (if interrupts are OK) result in an
  immediate "OK" response from the modem.  With bad interrupts you type
  at<enter> and may see nothing.  But then after 10 seconds or so you
  see the cursor drop down one line.  What is going on is that the FIFO
  is behaving like it can only hold one byte.  The "at" you typed caused
  it to overrun and both letters were lost.  But the final <enter>
  eventually got thru and you "see" this invisible character by noticing
  that the cursor jumped down one line.  If you were to type a single
  letter and then wait about 10 seconds, you should see it echo back to
  the screen.  This is fine if your typing speed is less that one word
  per minute :-)

  17.3.  Mis-set Interrupts

  If you don't understand what an interrupt does see ``Interrupts''.  If
  a serial port has one IRQ set in the hardware but a different one set
  in the device driver, the device driver will not catch any interrupts
  sent by the serial port.  Since the serial port uses interrupts to
  call its driver to service the port (fetching bytes from its 16-byte
  receive buffer or putting another 16-bytes in its transmit buffer) one
  might expect that the serial port would not work at all.

  But it still may work anyway --sort of.  Why?  Well, besides the
  interrupt method of servicing the port there's an undocumented slow
  polling method that doesn't need interrupts.  The way it works is that
  every so often the device driver checks the serial port to see if it
  needs anything such as if it has some bytes that need fetching from
  its receive buffer.  If interrupts don't work, the serial driver falls
  back to this polling method.  But this polling method was not intended
  to be used a substitute for interrupts.  It's so slow that it's not
  practical to use and may cause buffer overruns.  Its purpose may have
  been to get things going again if just one interrupt is lost or fails
  to do the right thing.  It's also useful in showing you that
  interrupts have failed.  Don't confuse this slow polling method with
  the fast polling method that operates on ports that have their IRQs
  set to 0.

  For the 16-byte transmit buffer, 16 bytes will be transmitted and then
  it will wait until the next polling takes place (several seconds
  later) before the next 16 bytes are sent out.  Thus transmission is
  very slow and in small chunks.  Receiving is slow too since bytes that
  are received by the receive buffer are likely to remain there for
  several seconds until it is polled.

  This explains why it takes so long before you see what you typed.
  When you type say AT to a modem, the AT goes out the serial port to
  the modem.  The modem then echos the AT back thru the serial port to
  the screen.  Thus the AT characters have to pass twice thru the serial
  port.  Normally this happens so fast that AT seems to appear on the
  screen at the same time you hit the keys on the keyboard.  With slow
  polling delays at the serial port, you don't see what you typed until
  perhaps 15 seconds later.  Even then, you don't often see all you
  typed but only the first several characters.

  What about overruns of the 16-byte receive buffer?  This will happen
  with an external modem since the modem just sends to the serial port
  at high speed which is likely to overrun the 16-byte buffer.  But for
  an internal modem, the serial port is on the same card and it's likely
  to check that this 16-byte receive buffer has room for more bytes
  before putting received bytes into it.  In this case there will be no
  overrun of this receive buffer, but text will just appear on your
  screen in 16-byte chunks spaced at intervals of several seconds.

  Even with an external modem you might not get overruns.  If just a few
  characters (under 16) are sent you don't get overruns since the buffer
  likely has room for them.  But attempts to send a larger number of
  bytes from your modem to your screen may result in overruns.  However,
  more than 16 (with no gaps) can get thru without overruns if the
  timing is right.  For example, suppose a burst of 32 bytes is sent
  into the port from the external cable.  The polling might just happen
  after the first 16 bytes came in so it would pick up these 16 bytes
  OK.  Then there would be space for the next 16 bytes so that entire 32
  bytes gets thru OK.  While this scenario is not very likely, similar
  cases where 17 to 31 bytes make thru are more likely.  But it's even
  more likely that only an occasional 16-byte chunk will get thru with
  possible loss of data.

  If you have an obsolete serial port with only a 1-byte buffer (or it's
  been incorrectly set to work like a 1-byte buffer) then the situation
  will be much worse than described above and only one character will
  occasionally make it thru the port.  Every character received causes
  an overrun (and is lost) except for the last character received.  This
  character is likely to be just a line-feed since this is often the
  last character to be transmitted in a burst of characters sent to your
  screen.  Thus you may type AT<return> to the modem but never see AT on
  the screen.  All you see several seconds later is that the cursor
  drops down one line (a line feed).  This has happened to me with a
  16-byte FIFO buffer that was behaving like a 1-byte buffer.

  When a communication program starts up, it expects interrupts to be
  working.  It's not geared to using this slow polling-like mode of
  operation.  Thus all sorts of mistakes may be made such as setting up
  the serial port and/or modem incorrectly.  It may fail to realize when
  a connection has been made.  If a script is being used for login, it
  may fail (caused by timeout) due to the polling delays.

  17.4.  Interrupt Conflicts

  When two devices have the same IRQ number it's called sharing
  interrupts.  Under some conditions this sharing works out OK.
  Starting with kernel version 2.2, ISA serial ports may, if the
  hardware is designed for this, share interrupts with other serial
  ports.  Devices on the PCI bus may share the same IRQ interrupt with
  other devices on the PCI bus (provided the software supports this).
  In other cases where there is potential for conflict, there should be
  no problem if no two devices with the same IRQ are ever "in use" at
  the same time.   More precisely, "in use" really means "open" (in
  programmer jargon).  In cases other than the exceptions mentioned
  above (unless special software and hardware permit sharing), sharing
  is not allowed and conflicts arise if sharing is attempted.

  Even if two processes with conflicting IRQs run at the same time, one
  of the devices will likely have its interrupts caught by its device
  driver and may work OK.  The other device will not have its interrupts
  caught by the correct driver and will likely behave just like a
  process with mis-set interrupts.  See ``Mis-set Interrupts'' for more

  17.5.  Resolving Interrupt Problems

  If you are getting a very slow response as described above, then one
  test is to change the IRQ to 0 (uses fast polling instead of
  interrupts) and see if the problem goes away.  Note that the polling
  due to IRQ=0 is orders of magnitude faster than the slow "polling" due
  to bad interrupts.  If IRQ=0 seems to fix the problem, then there was
  likely something wrong with the interrupts.  Using IRQ=0 is very
  resource intensive and is only a temporary fix.  You should try to
  find the cause of the interrupt problem and not permanently use IRQ=0.

  Check /proc/interrupts to see if the IRQ is currently in use by
  another process. If it's in use by another serial port you could try
  "top" (type f and then enable the TTY display) or "ps -e" to find out
  which serial ports are in use.  If you suspect that setserial has a
  wrong IRQ then see ``What is the current IO address and IRQ of my
  Serial Port ?''

  18.  What Are UARTs?  How Do They Affect Performance?

  18.1.  Introduction to UARTS

  UARTs (Universal Asynchronous Receiver Transmitter) are serial chips
  on your PC motherboard (or on an internal modem card).   The UART
  function may also be done on a chip that does other things as well.
  On older computers like many 486's, the chips were on the disk IO
  controller card.  Still older computer have dedicated serial boards.

  When PCs all had parallel bus architecture, the UART's purpose was to
  convert bytes from the PC's parallel bus to a serial bit-stream.  The
  cable going out of the serial port is serial and has only one wire for
  each direction of flow.  The serial port sends out a stream of bits,
  one bit at a time.  Conversely, the bit stream that enters the serial
  port via the external cable was converted to parallel bytes that the
  computer can understand.  UARTs deal with data in byte sized pieces,
  which is conveniently also the size of ASCII characters.

  Say you have a terminal hooked up to a serial port on your PC.  When
  you type a character, the terminal gives that character to its
  transmitter (also a UART).  The transmitter sends that byte out onto
  the serial line, one bit at a time, at a specific rate.  On the PC
  end, the receiving UART takes all the bits and reconstruct the byte
  (parallel on older PCs) and puts it in a buffer.  For newer PCs that
  might have a PCI-e serial port, the UART doesn't need to convert
  parallel-to-serial since the PCI-e "bus" is already a serial line.
  But the PCI-e line carries an encoded signal which must be decoded and
  then greatly slowed down to the speed of the RS-232 serial line.

  Along with converting between serial and parallel, the UART does some
  other things as a byproduct (side effect) of its primary task.  The
  voltage used to represent bits is also converted (changed).  Extra
  bits (called start and stop bits) are added to each byte before it is
  transmitted.  See the Serial-HOWTO section ``Voltage Waveshapes'' for
  details.  Also, while the flow rate (in bytes/sec) on the parallel bus
  inside the computer is very high, the flow rate out the UART on the
  serial port side of it is much lower.  The UART has a fixed set of
  rates (speeds) which it can use at its serial port interface.

  18.2.  Two Types of UARTs

  There are two basic types of UARTs: dumb UARTS and FIFO UARTS.  Dumb
  UARTs are the 8250, 16450, early 16550, and early 16650.  They are
  obsolete but if you understand how they work it's easy to understand
  how the modern ones work with FIFO UARTS ( late 16550, 16550A, and
  higher numbers).  Note that the driver for all of them is still
  labeled a "8250" driver in Linux where you may see it in compile
  options if you compile your own kernel, etc.

  There is some confusion regarding 16550.  Early models had a bug and
  worked properly only as 16450's (no FIFO).  Later models with the bug
  fixed were named 16550A but many manufacturers did not accept the name
  change and continued calling it a 16550.  Most all 16550's in use
  today are like 16550A's.  Linux will report it as being a 16550A even
  though your hardware manual (or a label note) says it's a 16550.  A
  similar situation exists for the 16650 (only it's worse since the
  manufacturer allegedly didn't admit anything was wrong).  Linux will
  report a late 16650 as being a 16650V2.  If it reports it as 16650 it
  is bad news and only is used as if it had a one-byte buffer.

  18.3.  FIFOs

  To understand the differences between dumb and FIFO (First In, First
  Out queue discipline) first let's examine what happens when a UART has
  sent or received a byte.  The UART itself can't do anything with the
  data passing thru it, it just receives and sends it.  For the obsolete
  dumb UARTS, the CPU gets an interrupt from the serial device every
  time a byte has been sent or received.  The CPU then moves the
  received byte out of the UART's buffer and into memory somewhere, or
  gives the UART another byte to send.  The obsolete 8250 and 16450
  UARTs only have a 1 byte buffer.  That means, that every time 1 byte
  is sent or received, the CPU is interrupted.  At low transfer rates,
  this is OK.  But, at high transfer rates, the CPU gets so busy dealing
  with the UART, that is doesn't have time to adequately tend to other
  tasks.  In some cases, the CPU does not get around to servicing the
  interrupt in time, and the byte is overwritten, because they are
  coming in so fast.  This is called an "overrun" or "overflow".

  FIFO UARTs help solve this problem.  The 16550A (or 16550) FIFO chip
  comes with 16 byte FIFO buffers.  This means that it can receive up to
  14 bytes (or send 16 bytes) before it has to interrupt the CPU.  Not
  only can it wait for more bytes, but the CPU then can transfer all (14
  to 16) bytes at a time.  This is a significant advantage over the
  obsolete UARTs, which only had 1 byte buffers.  The CPU receives less
  interrupts, and is free to do other things.  Data is rarely lost.
  Note that the interrupt threshold of FIFO buffers (trigger level) may
  be set at less than 14.  1, 4 and 8 are other possible choices.  As of
  late 2000 there was no way the Linux user could set these directly
  (setserial can't do it).  While many PC's only have a 16550 with
  16-byte buffers, better UARTS have even larger buffers.

  Note that the interrupt is issued slightly before the buffer gets full
  (at say a "trigger level" of 14 bytes for a 16-byte buffer).  This
  allows room for a couple more bytes to be received before the
  interrupt service routine is able to actually fetch all these bytes.
  The trigger level may be set to various permitted values by kernel
  software.  A trigger level of 1 will be almost like an obsolete UART
  (except that it still has room for 15 more bytes after it issues the

  Now consider the case where you're on the Internet.  It's just sent
  you a short webpage of text.  All of this came in thru the serial
  port.  If you had a 16-byte buffer on the serial port which held back
  characters until it had 14 of them, some of the last several
  characters on the screen might be missing as the FIFO buffer waited to
  get the 14th character.  But the 14th character doesn't arrive since
  you've been sent the entire page (over the phone line) and there are
  no more characters to send to you.  It could be that these last
  characters are part of the HTML formatting, etc. and are not
  characters to display on the screen but you don't want to lose format

  There is a "timeout" to prevent the above problem.  The "timeout"
  works like this for the receive UART buffer: If characters arrive one
  after another, then an interrupt is issued only when say the 14th
  character reaches the buffer.  But if a character arrives and the next
  character doesn't arrive soon thereafter, then an interrupt is issued
  anyway.  This results in fetching all of the characters in the FIFO
  buffer, even if only a few (or only one) are present.  There is also
  "timeout" for the transmit buffer as well.

  18.4.  Why FIFO Buffers are Small

  You may wonder why the FIFO buffers are not larger.  After all, memory
  is cheap and it wouldn't cost much more to use buffers in the kilo-
  byte range.  The reason is flow control.  Flow control stops the flow
  of data (bytes) on serial line when necessary.  If a stop signal is
  sent to serial port, then the stop request is handled by software
  (even if the flow control is "hardware").  The serial port hardware
  knows nothing about flow control.

  If the serial port buffer contains 64 bytes ready to send when it
  receives a flow control signal to stop sending, it will send out the
  64 bytes anyway in violation of the stop request.  There is no
  stopping it since it doesn't know about flow control.  If the buffer
  was large, then many more bytes would be sent in violation of flow
  control's request to stop.

  18.5.  UART Model Numbers

  Here's a list of some UARTs.  TL is Trigger Level

  В·  8250, 16450, early 16550: Obsolete with 1-byte buffers

  В·  16550, 16550A, 16C552: 16-byte buffers, TL=1,4,8,14; 115.2 kbps
     standard, many support 230.4 or 460.8 kbps

  В·  16650:  32-byte buffers. 460.8 kbps

  В·  16750:  64-byte buffer for send, 56-byte for receive. 921.6 kbps

  В·  16850, 16C850: 128-byte buffers.  460.8 kbps or 1.5 mbps

  В·  16950

  В·  Hayes ESP: 1k-byte buffers.

  For V.90 56k modems, it may be a several percent faster with a 16650
  (especially if you are downloading large uncompressed files).  The
  main advantage of the 16650 is its larger buffer size as the extra
  speed isn't needed unless the modem compression ratio is high.  Some
  56k internal modems may come with a 16650 ??

  Non-UART, and intelligent multiport boards use DSP chips to do
  additional buffering and control, thus relieving the CPU even more.
  For example, the Cyclades Cyclom, and Stallion EasyIO boards use a
  Cirrus Logic CD1400 RISC UART, and many boards use 80186 CPUs or even
  special RISC CPUs, to handle the serial IO.
  Many 486 PCs (old) and all Pentiums (or the like) should have at least
  16550As (usually called just 16550's) with FIFOs.  Some better
  motherboards today (2000) even have 16650s.  For replacing obsolete
  UARTs with newer ones in pre 1990 hardware see the Appendix: Obsolete

  19.  Pinout and Signals

  19.1.  Pinout of 9-pin and 25-pin serial connectors

  The pin numbers are often engraved in the plastic of the connector but
  you may need a magnifying glass to read them.  Note DCD is sometimes
  labeled CD.  The numbering of the pins on a female connector is read
  from right to left, starting with 1 in the upper right corner (instead
  of 1 in the upper left corner for the male connector as shown below).
  --> direction is out of PC.

         ___________                    ________________________________________
         \1 2 3 4 5/  Looking at pins   \1  2  3  4  5  6  7  8  9  10 11 12 13/
          \6 7 8 9/  on male connector   \14 15 16 17 18 19 20 21 22 23 24 25/
           ------                         -----------------------------------
       Pin #   Pin #   Acronym  Full-Name   Direction  What-it-May-Do/Mean
       9-pin   25-pin
        3       2      TxD     Transmit Data     -->   Transmits bytes out of PC
        2       3      RxD     Receive Data      <--   Receives bytes into PC
        7       4      RTS     Request To Send   -->   RTS/CTS flow control
        8       5      CTS     Clear To Send     <--   RTS/CTS flow control
        6       6      DSR     Data Set Ready    <--   I'm ready to communicate
        4      20      DTR     Data Terminal Ready-->  I'm ready to communicate
        1       8      DCD     Data Carrier Detect<--  Modem connected to another
        9      22      RI      Ring Indicator    <--   Telephone line ringing
        5       7      SG      Signal Ground

         9-Pin DB9 Connector                    25-Pin DB-25 Connector
       1     DCD   Carrier Detect             1           Chassis Ground
       2     RxD   Receive Data               2     TxD   Transmit Data
       3     TxD   Transmit Data              3     RxD   Receive Data
       4     DTR   Data Terminal Ready        4     RTS   Request To Send
       5     SG    Signal Ground              5     CTS   Clear To Send
       6     DSR   Data Set Ready             6     DSR   Data Set Ready
       7     RTS   Request To Send            7     SG    Signal Ground
       8     CTS   Clear To Send              8     DCD   Carrier Detect
       9     RI    Ring Indicator            20     DTR   Data Terminal Ready
                                             22     RI    Ring Indicator

  19.2.  Signals May Have No Fixed Meaning

  Only 3 of the 9 pins have a fixed assignment: transmit, receive and
  signal ground.  This is fixed by the hardware and you can't change it.
  But the other signal lines are controlled by software and may do (and
  mean) almost anything at all.  However they can only be in one of two
  states: asserted (+12 volts) or negated (-12 volts).  Asserted is "on"
  and negated is "off".  For example, Linux software may command that
  DTR be negated and the hardware only carries out this command and puts
  -12 volts on the DTR pin.  A modem (or other device) that receives
  this DTR signal may do various things.  If a modem has been configured
  a certain way it will hang up the telephone line when DTR is negated.
  In other cases it may ignore this signal or do something else when DTR
  is negated (turned off).
  It's like this for all the 6 signal lines.  The hardware only sends
  and receives the signals, but what action (if any) they perform is up
  to the Linux software and the configuration/design of devices that you
  connect to the serial port.  However, most pins have certain functions
  which they normally perform but this may vary with the operating
  system and the device driver configuration.  Under Linux, one may
  modify the source code to make these signal lines behave differently
  (some people have).

  19.3.  Cabling Between Serial Ports

  A cable from a serial port always connects to another serial port.  An
  external modem or other device that connects to the serial port has a
  serial port built into it.  For modems, the cable is always straight
  thru: pin 2 goes to pin 2, etc.  The modem is said to be DCE (Data
  Communications Equipment) and the computer is said to be DTE (Data
  Terminal Equipment).  Thus for connecting DTE-to-DCE you use straight-
  thru cable.  For connecting DTE-to-DTE you must use a null-modem cable
  (also called a crossover cable).  There are many ways to wire such
  cable (see examples in Text-Terminal-HOWTO subsection: "Direct Cable

  There are good reasons why it works this way.  One reason is that the
  signals are unidirectional.  If pin 2 sends a signal out of it (but is
  unable to receive any signal) then obviously you can't connect it to
  pin 2 of the same type of device.  If you did, they would both send
  out signals on the same wire to each other but neither would be able
  to receive any signal.  There are two ways to deal with this
  situation.  One way is to have a two different types of equipment
  where pin 2 of the first type sends the signal to pin 2 of the second
  type (which receives the signal).  That's the way it's done when you
  connect a PC (DTE) to a modem (DCE).  There's a second way to do this
  without having two different types of equipment: Connect pin sending
  pin 2 to a receiving pin 3 on same type of equipment.  That's the way
  it's done when you connect 2 PCs together or a PC to a terminal (DTE-
  to-DTE).  The cable used for this is called a null-modem cable since
  it connects two PCs without use of a modem.  A null-modem cable may
  also be called a cross-over cable since the wires between pins 2 and 3
  cross over each other (if you draw them on a sheet of paper).  The
  above example is for a 25 pin connector but for a 9-pin connector the
  pin numbers 2 and 3 are just the opposite.

  The serial pin designations were originally intended for connecting a
  dumb terminal to a modem.  The terminal was DTE (Data Terminal
  Equipment) and the modem was DCE (Data Communication Equipment).
  Today the PC is usually used as DTE instead of a terminal (but real
  terminals may still be used this way).  The names of the pins are the
  same on both DTE and DCE.  The words: "receive" and "transmit" are
  from the "point of view" of the PC (DTE).  The transmit pin from the
  PC transmits to the "transmit" pin of the modem (but actually the
  modem is receiving the data from this pin so from the point of view of
  the modem it would be a receive pin).

  The serial port was originally intended to be used for connecting DTE
  to DCE which makes cabling simple: just use a straight-thru cable.
  Thus when one connects a modem one seldom needs to worry about which
  pin is which.  But people wanted to connect DTE to DTE (for example a
  computer to a terminal) and various ways were found to do this by
  fabricating various types of special null-modem cables.  In this case
  what pin connects to what pin becomes significant.

  19.4.  RTS/CTS and DTR/DSR Flow Control

  This is "hardware" flow control.  Flow control was previously
  explained in the ``Flow Control'' subsection but the pins and voltage
  signals were not.  Linux only supports RTS/CTS flow control at present
  (but a special driver may exist for a specific application which
  supports DTR/DSR flow control).  Only RTS/CTS flow control will be
  discussed since DTR/DSR flow control works the same way.  To get
  RTS/CTS flow control one needs to either select hardware flow control
  in an application program or use the command:
  stty -F /dev/ttyS2 crtscts (or the like).  This enables RTS/CTS
  hardware flow control in the Linux device driver.

  Then when a DTE (such as a PC) wants to stop the flow into it, it
  negates RTS.  Negated "Request To Send" (-12 volts) means "request NOT
  to send to me" (stop sending).  When the PC is ready for more bytes it
  asserts RTS (+12 volts) and the flow of bytes to it resumes.  Flow
  control signals are always sent in a direction opposite to the flow of
  bytes that is being controlled.  DCE equipment (modems) works the same
  way but sends the stop signal out the CTS pin.  Thus it's RTS/CTS flow
  control using 2 lines.

  On what pins is this stop signal received?  That depends on whether we
  have a DCE-DTE connection or a DTE-DTE connection.  For DCE-DTE it's a
  straight-thru connection so obviously the signal is received on a pin
  with the same name as the pin it's sent out from.  It's RTS-->RTS (PC
  to modem) and CTS<--CTS (modem to PC).  For DTE-to-DTE the connection
  is also easy to figure out.  The RTS pin always sends and the CTS pin
  always receives.  Assume that we connect two PCs (PC1 and PC2)
  together via their serial ports.  Then it's RTS(PC1)-->CTS(PC2) and
  CTS(PC1)<--RTS(PC2).  In other words RTS and CTS cross over.  Such a
  cable (with other signals crossed over as well) is called a "null
  modem" cable.  See ``Cabling Between Serial Ports''

  What is sometimes confusing is that there is the original use of RTS
  where it means about the opposite of the previous explanation above.
  This original meaning is: I Request To Send to you.  This request was
  intended to be sent from a terminal (or computer) to a modem which, if
  it decided to grant the request, would send back an asserted CTS from
  its CTS pin to the CTS pin of the computer: You are Cleared To Send to
  me.  Note that in contrast to the modern RTS/CTS bi-directional flow
  control, this only protects the flow in one direction: from the
  computer (or terminal) to the modem.  This original use appears to be
  little used today on modern equipment (including modems).

  19.4.1.  The DTR and DSR Pins

  Just like RTS and CTS, these pins are paired.  For DTE-to-DTE
  connections they are likely to cross over.  There are two ways to use
  these pins.  One way is to use them as a substitute for RTS/CTS flow
  control.  The DTR pin is just like the RTS pin while the DSR pin
  behaves like the CTS pin.  Although Linux doesn't support DTR/DSR flow
  control, it can be obtained by connecting the RTS/CTS pins at the PC
  to the DSR/DTR pins at the device that uses DTR/DSR flow control.  DTR
  flow control is the same as DTR/DSR flow control but it's only one-way
  and only uses the DTR pin at the device.  Many text terminals and some
  printers use DTR/DSR (or just DTR) flow control.  In the future, Linux
  may support DTR/DSR flow control.  The software has already been
  written but it's not clear when (or if) it will incorporated into the
  serial driver.

  The normal use of DTR and DSR (not for flow control) is as follows: A
  device asserting DTR says that its powered on and ready to operate.
  For a modem, the meaning of a DTR signal from the PC depends on how
  the modem is configured.  Negating DTR is sometimes called "hanging
  up" but it doesn't always do this.  One way to "hang up" (negate DTR)
  is to set the baud rate to 0 using the command "stty 0".  Trying to do
  this from a "foreign" terminal may not work due to the two-interface
  problem.  See ``Two interfaces at a terminal''.  For internal modem-
  serial_ports it worked OK with a port using minicom but didn't work if
  the port was using wvdial.  Why?

  19.5.  Preventing a Port From Opening

  If "stty -clocal" (or getty is used with the "local" flag negated)
  then a serial port can't open until DCD gets an assert (+12 volts)

  20.  Voltage Waveshapes

  20.1.  Voltage for a Bit

  At the RS-232 serial port, voltages are bipolar (positive or negative
  with respect to ground) and should be about 12 volts in magnitude
  (some are 5 or 10 volts).   For the transmit and receive pins +12
  volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit
  (sometimes called "mark").  This is known as inverted logic since
  normally a 0-bit is both false and negative while a one is normally
  both true and positive.  Although the receive and transmit pins are
  inverted logic, other pins (modem control lines) are normal logic with
  a positive voltage being true (or "on" or "asserted") and a negative
  voltage being false (or "off" or "negated").  Zero voltage has no
  meaning (except it usually means that the unit is powered off).

  A range of voltages is allowed.  The specs say the magnitude of a
  transmitted signal should be between 5 and 15 volts but must never
  exceed 25 V.  Any voltage received under 3 V is undefined (but some
  devices will accept a lower voltage as valid).  One sometimes sees
  erroneous claims that the voltage is commonly 5 volts (or even 3
  volts) but it's usually 11-12 volts.  If you are using a EIA-422
  (RS-422) port on a Mac computer as an RS-232 (requires a special
  cable) or EIA-423 (RS-423) then the voltage will actually be only 5 V.
  The discussion here assumes 12 V.

  Note that normal computer logic normally is just a few volts (5 volts
  was once the standard) so that if you try to use test equipment
  designed for testing 3-5 volt computer logic (TTL) on the 12 volts of
  a serial port, it may damage the test equipment.

  20.2.  Voltage Sequence for a Byte

  The transmit pin (TxD) is held at -12 V (mark) at idle when nothing is
  being sent.  To start a byte it jumps to +12 V (space) for the start
  bit and remains at +12 V for the duration (period) of the start bit.
  Next comes the low-order bit of the data byte.  If it's a 0-bit
  nothing changes and the line remains at +12 V for another bit-period.
  If it's a 1-bit the voltage jumps from +12 to -12 V.  After that comes
  the next bit (-12 V if a 1 or +12 V if a 0), etc., etc.  After the
  last data bit, a parity bit may be sent and then a -12 V (mark) stop
  bit.  Then the line remains at -12 V (idle) until the next start bit.
  Note that there is no return to 0 volts and thus there is no simple
  way (except by a synchronizing signal) to tell where one bit ends and
  the next one begins for the case where 2 consecutive bits are the same
  polarity (both zero or both one).

  A 2nd stop bit would also be -12 V, just the same as the first stop
  bit.  Since there is no signal to mark the boundaries between these
  bits, the only effect of the 2nd stop bit is that the line must remain
  at -12 V idle twice as long.  The receiver has no way of detecting the
  difference between a 2nd stop bit and a longer idle time between
  bytes.  Thus communications works OK if one end uses one stop bit and
  the other end uses 2 stop bits, but using only one stop bit is
  obviously faster.  In rare cases 1 1/2 stop bits are used.  This means
  that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
  50% wider than normal).

  20.3.  Parity Explained

  Characters are normally transmitted with either 7 or 8 bits of data.
  An additional parity bit may (or may not) be appended to this
  resulting in a byte length of 7, 8 or 9 bits.  Some terminal emulators
  and older terminals do not allow 9 bits.  Some prohibit 9 bits if 2
  stop bits are used (since this would make the total number of bits too
  large: 12 bits total after adding the start bit).

  The parity may be set to odd, even or none (mark and space parity may
  be options on some terminals or other serial devices).  With odd
  parity, the parity bit is selected so that the number of 1-bits in a
  byte, including the parity bit, is odd.  If a such a byte gets
  corrupted by a bit being flipped, the result is an illegal byte of
  even parity.  This error will be detected and if it's an incoming byte
  to the terminal an error-character symbol will appear on the screen.
  Even parity works in a similar manner with all legal bytes (including
  the parity bit) having an even number of 1-bits.  During set-up, the
  number of bits per character usually means only the number of data
  bits per byte (7 for true ASCII and 8 for various ISO character sets).

  A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic
  0).  For mark parity, the parity bit is always a one-bit.  For space
  parity it's always a zero-bit.  Mark or space parity (also known as
  "sticky parity") only wastes bandwidth and should be avoided if
  feasible.  The stty command can't set sticky parity but it's supported
  by serial hardware and can be dealt with by programming in C.  "No
  parity" means that no parity bit is added.   For terminals that don't
  permit 9 bit bytes, "no parity" must be selected when using 8 bit
  character sets since there is no room for a parity bit.

  20.4.  Forming a Byte (Framing)

  In serial transmission of bytes via RS-232 ports, the low-order bit is
  always sent first (the bit-order).  Serial ports on PC's use
  asynchronous communication where there is a start bit and a stop bit
  to mark the beginning and end of a byte.  This is called framing and
  the framed byte is sometimes called a frame.  As a result a total of
  9, 10, or 11 bits are sent per byte with 10 being the most common.
  8-N-1 means 8 data bits, No parity, 1 stop bit.  This adds up to 10
  bits total when one counts the start bit.  One stop bit is almost
  universally used.  At 110 bits/sec (and sometimes at 300 bits/sec) 2
  stop bits were once used but today the 2nd stop bit is used only in
  very unusual situations (or by mistake since it still works OK that
  way but wastes bandwidth).

  Don't confuse this type of framing with the framing used for a packet
  of bytes on a network.  The serial port just frames every byte.  For a
  network, many bytes are framed into a packet (sometimes called a
  frame).  For a network frame, instead of a start bit, there is a
  sequence of bytes called a header.  On a network that uses serial
  ports (with modems), a report of a frame error usually refers to a
  multi-byte frame and not the serial port frame of a single byte.

  20.5.  How "Asynchronous" is Synchronized

  The RS-232 serial port as implemented on PC is asynchronous which in
  effect means that there is no "clock" signal sent with "ticks" to mark
  when each bit is sent..  There are only two states of the transmit (or
  receive) wire: mark (-12 V) or space (+12 V).  There is no state of 0
  V.  Thus a sequence of 1-bits is transmitted by just a steady -12 V
  with no markers of any kind between bits.  For the receiver to detect
  individual bits it must always have a clock signal which is in
  synchronization with the transmitter clock.  Such a clock would
  generate a "tick" in synchronization with each transmitted (or
  received) bit.

  For asynchronous transmission, synchronization is achieved by framing
  each byte with a start bit and a stop bit (done by hardware).  The
  receiver listens on the negative line for a positive start bit and
  when it detects one it starts its clock ticking.  It uses this clock
  tick to time the reading of the next 7, 8 or 9 bits.  (It actually is
  a little more complex than this since several samples of a bit are
  normally taken and this requires additional timing ticks.)  Then the
  stop bit is read, the clock stops and the receiver waits for the next
  start bit.  Thus async is actually synchronized during the reception
  of a single byte but there is no synchronization between one byte and
  the next byte.

  21.  Other Serial Devices (not async RS-232)

  21.1.  Successors to RS-232

  A number of EIA (or RS) standards have been established for higher
  speeds and longer distances using twisted-pair (balanced) technology.
  Balanced transmission make possible higher speeds, and can be a
  thousand times faster than unbalanced RS-232.  For a given speed, the
  distance (maximum cable length) may be many times longer with twisted
  pair.  But PC's, prior to about 2004??, kept being made with the
  quasi-obsolete RS-232 since it works OK with modems and mice since the
  cable length is short.

  High speed serial ports (over 460.8 kbps) will often support both
  RS-232 and EIA-485/EIA-422 (RS-485/RS-422) modes .  (Note that for
  non-RS-232 I've used the "EIA" designation instead of the more
  commonly used "RS" but they both mean the same thing.)  At such high
  speeds RS-232 is not of much use (except for a very short cable).

  21.2.  EIA-422-A (balanced) and EIA-423-A (unbalanced)

  EIA-423 is just like the unbalanced RS-232 except that the voltage is
  only 5 volts.  Since this falls within RS-232 specs it can be
  connected to a RS-232 port.  Its specs call for somewhat higher speeds
  than the RS-232 (but this may be of little help on a long run where
  it's the unbalance that causes interference).  Since EIA-423 is not
  much of an improvement over RS-232, it is seldom used except on old
  Mac computers.

  EIA-422 is twisted pair (known as "balanced" or "differential) and is
  (per specs) exactly 100 times as fast as EIA-423 (which in turn is
  somewhat faster than RS-232).  Apple's Mac computer used it prior to
  mid-1998 with its RS-232/EIA-422 port.  The Mac used a small round
  "mini-DIN-8" connector and named these serial ports as "modem port",
  "printer port", and/or "GeoPort".

  Mac also provided conventional RS-232 but at only at 5 volts (which is
  still legal RS-232).  To make it work like at RS-232 one must use a
  special cable which (signal) grounds RxD+ (one side of a balanced
  pair) and use RxD- as the receive pin.  While TxD- is used as the
  transmit pin, for some reason TxD+ should not be grounded.  See
  Macintosh Communications FAQ <
  faq.html>.  However, due to the fact that Macs (and upgrades for them)
  cost more than PC's, they are not widely as host computers for Linux.

  21.3.  EIA-485

  This is like EIA-422 (balanced = differential).  It is half-duplex.
  It's not just point-to-point but is like ethernet or the USB since all
  devices (nodes) on it share the same "bus".  It may be used for a
  multidrop LAN (up to 32 nodes or more).  Unfortunately, Linux
  currently doesn't support this and you can only use it under Linux
  only for point-to-point where it behaves like RS-232.  So read further
  only if you are curious about how its features would work if only
  Linux supported them.

  Since many nodes share the same twisted pair, there's a need to use
  the electrical tri-state mode.  Thus, besides the 0 and 1 binary
  states, there is also an open circuit state to permit other nodes to
  use the twisted pair line.  Instead of a transmitter keeping a 1-state
  voltage on the line during line idle, the line is open circuited and
  all nodes just listen (receive mode).

  The most common architecture is master/slave.  The master polls the
  slaves to see if they have anything to send.  A slave can only
  transmit just after it's been polled.  But EIA-485 is just an
  electrical specification and doesn't specify any protocol for the
  master/slave interaction.  In fact, it doesn't even specify that there
  must be a master and slaves.  So various protocols have been used.
  Based on a discussion of 485 on the linux-serial mailing list in March
  2003, it seems likely that none of these master/slave protocols are
  currently supported by Linux.

  There is an alternative implementation where two pair of wires are
  used for sending data.  One pair is only for the Master to send to the
  Slaves.  Since no one transmits on this line except the master, there
  is no need for it to be tri-state.  Thus the Master may just be RS-232
  but the slaves must still be EIA-485.  See
  <> for more details.

  21.4.  EIA-530

  EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
  (balanced) was intended to be a replacement for RS-232 but few have
  been installed.  It uses the same 25-pin connector as RS-232.

  21.5.  EIA-612/613

  The High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin
  connector and goes up to about 50 Mbits/s but the distance is limited
  to only several meters.  For Linux there are PCI cards supporting
  HSSI.  The companies that sell the cards often provide (or point you
  to) a Linux driver.  A howto or the like is needed for this topic.

  21.6.  The Universal Serial Bus (USB)

  The Universal Serial Bus (USB) is being built into PCI chips.  Newer
  PC's have them.  It was originally 12 Mbps but is now 480 Mbps over a
  twisted pair with a 4-pin connector (2 wires are power supply).  It
  also is limited to short distances of at most 5 meters (depends on
  configuration).  Linux supports the bus, although not all devices that
  can plug into the bus are supported.

  It is synchronous and transmits in special packets like a network.
  Just like a network, it can have several devices physically attached
  to it, including serial ports.  Each device on it gets a time-slice of
  exclusive use for a short time.  A device can also be guaranteed the
  use of the bus at fixed intervals.  One device can monopolize it if no
  other device wants to use it.  It's not simple to describe in detail.

  For serial ports on the USB bus, there are numerous configuration
  options to use when compiling the kernel.  They all start with:
  CONFIG_USB_SERIAL.  Each one is usually for a certain brand/model of
  serial port, although generic is also an option.  See the
  Configuration Help file in the kernel documentation.

  For documentation, see the USB directory in /usr/share/doc/kernel ...
  and look at the file: usb-serial.txt.  The modules that support usb
  serial devices are found in the modules tree:
  kernel/drivers/usb/serial.  It would be nice to have a HOWTO on the
  USB.  See also  <> and/or

  21.7.  Firewire

  Firewire (IEEE 1394) is something like the USB only faster (800 Mbps
  is planned).  The protocol on the bus is claimed to be more efficient
  than USB's.  It uses two twisted pair for data plus two power
  conductors (6 conductors in all).  A variants uses only 4 conductors.
  You may compile firewire support into the Linux kernel.  Like USB,
  it's also limited to short distances.

  21.8.  MIDI

  Sound cards often have a 15-pin game port connector used for MIDI.
  They are for connecting a musical keyboard to a PC so that you can
  create musical recordings.  You could also connect a MIDI sound
  system.  The MIDI standard uses 31250 baud (1M/32) which is not
  available on an ordinary serial port.  Some MIDI devices are designed
  so that they can be connected directly to an ordinary serial port.

  Besides the 15-pin connector, a 5-pin DIN connector is also a MIDI
  standard but the flow of sound is only one way thru it so for
  bidirectional sound you need 2 of them.  Breakout cables often have a
  15-pin connector on one end and 2 or more 5-pin connectors on the
  other end.  The /dev/midi00 is for MIDI.

  21.9.  Synchronization & Synchronous

  Beside the asynchronous RS-232 (and others) there are a number of
  synchronous serial port standards.  In fact RS-232 includes
  synchronous specifications but they aren't normally implemented for
  serial ports on PC's.  But first we'll explain what a synchronous

  21.9.1.  Defining Asynchronous vs Synchronous

  Asynchronous (async) means "not synchronous".  In practice, an async
  signal is what the async serial port sends and receives which is a
  stream of bytes with each byte framed by a start and stop bit.
  Synchronous (sync) is most everything else.  But this doesn't explain
  the basic concepts.

  In theory, synchronous means that bytes are sent out at a constant
  rate one after another in step with a clock signal tick.  There is
  often a separate wire or channel for sending the clock signal.  The
  clock signal might also be embedded in the transmitted bytes.
  Asynchronous bytes may be sent out erratically with various time
  intervals between bytes (like someone typing characters at a

  When a file is being sent thru the async serial port, the flow of
  bytes will likely be at the speed of the port (say 115.2k) which is a
  constant rate.  This flow may frequently start and stop due to flow
  control.  Is this sync or async?  Ignoring the flow control stops, it
  might seem like sync since it's a steady flow.  But it's not because
  there is no clock signal and the bytes could have been sent
  erratically since they are framed by start/stop bits.

  Another case is where data bytes (without any start-stop bits) are put
  into packets with possible erratic spacing between one packet and the
  next.  This is called sync since the bytes within each packet are
  transmitted synchronously.

  21.9.2.  Synchronous Communication

  Did you ever wonder what all the unused pins are for on a 25-pin
  connector for the serial port?  Most of them are for use in
  synchronous communication which is seldom implemented in chips for
  PC's.  There are pins for sync timing signals as well as for a sync
  reverse channel.  The RS-232 spec provides for both sync and async but
  PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip
  such as a 16450, 16550A, or 16650 and can't deal with sync.  For sync
  one needs a USRT chip or the equivalent where the "S" stands for
  Synchronous.  A USART chip supports both synchronous and asynchronous.
  Since sync is a niche market, a sync serial port is likely to be quite

  SCC stands for "Serial Communication Controller" or "Serial Controller
  Chip".  It's likely old terminology and since it doesn't say "sync" or
  "async" it might support both.

  Besides the sync part of the RS-232, there are various other EIA
  synchronous standards.  For RS-232, 3 pins of the connector are
  reserved for clock (or timing) signals.  Sometimes it's a modem's task
  to generate some timing signals making it impossible to use
  synchronous communications without a synchronous modem (or without a
  device called a "synchronous modem eliminator" which provides the
  timing signals).

  Although few serial ports are sync, synchronous communication does
  often take place over telephone lines using modems which use V.42
  error correction.  This strips off the start/stop bits and puts the
  data bytes in packets resulting in synchronous operation over the
  phone line.

  22.  Other Sources of Information

  22.1.  Books

  1. Axleson, Jan: Serial Port Complete, Lakeview Research, Madison, WI,

  2. Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
     Computer Society Press, Los Alamitos, CA, 1996.

  3. Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.

  4. Campbell, Joe: C Programmer's Guide to Serial Communications, 2nd
     ed., Unknown Publisher, 1993.

  5. Levine, Donald:  POSIX Programmer's Guide
     <>, O'Reilly, 1991.

  6. Nelson, Mark: Serial Communications Developer's Guide, 2nd ed.,
     Hungry Minds, 2000.

  7. Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.

  8. Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall, 1991.

  9. Stevens, Richard W.: Advanced Programming in the UNIX Environment
     <http://heg->, (ISBN
     0-201-56317-7; Addison-Wesley)

     Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996.
     Chapter 7: Serial Ports

  Notes re books:

  1. "... Complete" has hardware details (including register) but the
     programming aspect is Window oriented.

  2. "Physical Layer ..." covers much more than just RS-232.

  22.2.  Serial Software

  If it's not available in your Linux distribution try:
  Serial Software <> for
  Linux software for the serial ports including getty and port monitors.
  Serial Communications
  <> for communication

  В·  irqtune will give serial port interrupts higher priority to improve
     performance.  Using hdparm for hard-disk tuning may help some more.

  В·  modemstat and statserial show the current state of various modem
     control lines.  See ``Serial Monitoring/Diagnostics''

  22.3.  Related Linux Documents

  В·  man pages for: setserial and stty

  В·  Low-Level Terminal Interface
     <> part of
     "GNU C Library Reference manual" (in libc (or glibc) docs package).
     It covers the detailed meaning of "stty" commands, etc.

  В·  Modem-HOWTO: modems on the serial port

  В·  PPP-HOWTO: help with PPP (using a modem on the serial port)

  В·  Printing-HOWTO: for setting up a serial printer

  В·  Serial-Programming-HOWTO: for some aspects of serial-port

  В·  Text-Terminal-HOWTO: how they work and how to install and configure

  В·  UPS-HOWTO: setting up UPS sensors connected to your serial port

  В·  UUCP-HOWTO: for information on setting up UUCP

  22.4.  Serial Mailing List

  The Linux serial mailing list.  To join, send email to, with ``subscribe linux-serial'' in the
  message body.  If you send ``help'' in the message body, you get a
  help message.  The server also serves many other Linux lists.  Send
  the ``lists'' command for a list of mailing lists.

  22.5.  Internet

  В·  Serial port - Wikipedia <>

  В·  Linux Serial  Driver home page <>
     Includes info about PCI support.

  В·  Serial-Programming-HOWTO (not yet available from the Linux
     Documentation Project).  It's now on my website:
     <> See
     also: Serial-Programming-HOWTO by Peter Baumann

  В·  Terminal IO by Vern Hoxie
     <> Termios man page
     revision by Vern Hoxie

  В·  A white paper discussing serial communications and multiport serial
     boards was available from Cyclades at

  В·  Serial and UART Tutorial (FreeBSD)

  23.  Appendix A: Very Obsolete Hardware/Software

  23.1.  Replacing pre 1990 UARTS

  Many 486 PCs (old) and all Pentiums (or the like) should have modern
  16550As (usually called just 16550's) with FIFOs.  If you have
  something really old (pre 1990), the chip may unplug so that you may
  be able to upgrade by finding a plug-in 16550A chip and replacing your
  old existing 16450 UART.  If the functionality has been built into
  another type of chip, you are out of luck.  If the UART is socketed,
  then upgrading would be easy if you could find a replacement.  The new
  and old are pin-to-pin compatible.  It may be more feasible to just
  buy a new serial card on the Internet (few retail stores stock them as
  of 2000) or find a used one.

  23.2.  Two Ports with the Same IO address

  Modern kernels should not allow the opening of ports with the same IO
  address.  But one may probe for ports even though they are not open.
  If two ports have the same IO address then old fashioned probing by
  sending commands to the address  will erroneously indicate only one
  port.  But modem device detection at boot-time should discover both
  ports and report the conflict.  In olden days, all sorts of errors
  were reported/observed for devices illegally attempting to use the
  same IO address.  See ``Probing''.

  23.3.  Configuring by modifying source code

  In the past, to get a certain serial port supported, one might need to
  modify the C source code, perhaps by adding a #define to it.  Today,
  the use of parameters for modules or the kernel, or the use of
  configuration options should handle all cases (except possible for
  antique hardware ??).

  23.4.  Modems on Multiport Cards Obsolete for Sending at 56k

  For a modem to transmit at nearly 56k requires that it be a special
  digital modem and have a digital connection to a digital phone line
  (such as a T1 line).  Modems used with serial cards (the modems may
  either be on the serial card or on another card) normally have no such
  digital connection so they can't be used at the 56k speed, and thus
  are obsolete unless one doesn't need to send at 56k.  In other words
  they are obsolete for ISP servers but might be OK for small business
  or home use.

  A partial exception to the above are modem banks that connect to
  multiport serial cards where the modem bank can access multiplexed
  digital phone lines.   Thus one could use a multiport serial card with
  a few 56k digital modems for sending at 56k.  For both analog and
  digital modems there is one modem on each serial port so there needs
  to be an external cable (modem bank to multiport) for each modem.
  This can lead to a large number of cables.  So it's less clutter (and
  cheaper) to use internal modems without a multiport card.  This makes
  even this "exception" obsolete for high volume work.  It's somewhat
  analogous to the lower cost of an internal modem for a desktop PC as
  compared to the higher cost (and more cabling) for an external modem.
  See Modem-HOWTO: Modem Pools, Digital Modems.

  23.5.  Lock-Files if you used the depreciated devfs

  The abandoned device-filesystem (devfs) has the /dev directory with
  subdirectories.  As of late 2001, there were problems with lockfiles.
  For example, the lockfile mechanism considered dev/usb/tts/0 and
  /dev/tts/0 to be the same device with name "0".  Ditto for all other
  devices that had the same "leaf" name.  Also, if some applications use
  the old name for a device and other applications use the devfs name
  for the same  device, then the lockfiles will have different names.
  But the serial driver should know they are the same.

  23.6.  Devfs (The deprecated Device File System.  History)

  Kernel 2.4 introduced the now obsolete optional "device file system"
  (devfs) with a whole new set of names for everything.  Some people
  used it but the majority probably didn't.  But in 2003-4, it was
  claimed that devfs had unsolvable problems and starting with kernel
  2.6.12 it was replaced with "udev" (kernels prior to 2.6.12 also could
  use udev but with some problems).  Although udev doesn't provide all
  the functionality of devfs, it does handle hot plugging.  Also, the
  use of udev isn't required to run Linux so some people don't use it.
  But many distributions install it by default.

  Devfs was a good idea and was claimed to be more efficient than udev.
  But unfortunately, the author of devfs didn't maintain it for long and
  it allegedly became not too well maintained.  So for better or worse
  we now have udev instead although the debate of devfs vs. udev still
  continues.  For a detailed description of devfs see:
  <> Also see the
  kernel documentation tree: filesystems/devfs.

  The names of devices for the devfs can be used in udev, but usually
  are not and may not be simple to activate.  Here's the devfs names for
  serial devices: ttyS1 becomes tts/1, ttyUSB1 becomes /usb/tts/1, and
  ttyACM1 is /usb/acm/1.  Note that the number 1 above is just an
  example.  It could be replaced by 0, 2, 3, 4, etc.  Some more examples
  of udev names: ttyS2 becomes tts/2 (Serial port), tty3 becomes vc/3
  (Virtual Console), ptyp1 becomes pty/m1 (PTY master), ttyp2 becomes
  pty/s2 (PTY slave).  "tts" looks like a directory which contains
  devices "files": 0, 1, 2, etc.  All of these new names were put in the
  /dev directory although optionally one may put them elsewhere.

  For devfs, device names in the /dev directory are created
  automatically by the corresponding driver.  Thus, if serial support
  comes from a module and that module isn't loaded yet, there will not
  be any serial devices in the /dev directory.  This can be confusing:
  you physically have serial ports but don't see them in the /dev
  directory.  However, if a device name is requested (attempt to open
  it) by a communication program and the serial module isn't loaded, the
  kernel is supposed to try to find a driver for it and create a name
  for it in the /dev directory.

  This works OK if it finds a driver.  But suppose there is no driver
  found for it.  For example, if you try to use "setserial" to configure
  a port that the driver failed to detect, it claims there is no such
  port.  How does one create a devfs port in this case?

  For multiport devices for example, /dev/ttyF9 becomes /dev/ttf/9, or
  in a later version /dev/tts/F9.  Substitute for F (or f) whatever
  letter(s) your multiport board uses for this purpose.  A multiport
  driver is supposed to create a devfs name similar to the above and put
  it into the /dev directory


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