Serial-HOWTO (Linux)

Az alábbi fájl Slackware Linux-ban a "/usr/doc/faq/howto" directory "Serial-HOWTO.gz" fájljának "zcat"-tal történõ kicsomagolásával keletkezett. (A directory további hasznos fájlokat tartalmaz, nézegetni "zless"-el célszerû õket.)

  The Linux Serial HOWTO
  by Greg Hankins, greg.hankins@cc.gatech.edu
  v1.8a, 5 June 1995

  This document describes how to set up serial communications devices
  on a Linux box.

  1.  Introduction

  This is the Linux Serial HOWTO.  All about how to set up modems and
  terminals under Linux, some serial tips, and troubleshooting.


  1.1.  Copyright

  The Linux Serial HOWTO is copyright (C) 1993 - 1995 by Greg Hankins.
  Linux HOWTO documents may be reproduced and distributed in whole or in
  part, in any medium physical or electronic, as long as this copyright
  notice is retained on all copies. Commercial redistribution is allowed
  and encouraged; however, the author would like to be notified of any
  such distributions.

  All translations, derivative works, or aggregate works incorporating
  any Linux HOWTO documents must be covered under this copyright notice.
  That is, you may not produce a derivative work from a HOWTO and impose
  additional restrictions on its distribution. Exceptions to these rules
  may be granted under certain conditions; please contact the Linux
  HOWTO coordinator at the address given below.

  In short, we wish to promote dissemination of this information through
  as many channels as possible. However, we do wish to retain copyright
  on the HOWTO documents, and would like to be notified of any plans to
  redistribute the HOWTOs.

  If you have questions, please contact Greg Hankins, the Linux HOWTO
  coordinator, at
  gregh@sunsite.unc.edu via email, or at +1 404 853 9989.


  1.2.  Other sources of information

  o  man pages for: agetty(8), getty(1m), gettydefs(5), init(1),
     login(1), mgetty(8), setserial(8)

  o  Your modem manual

  o  UUCP HOWTO: for information on setting up UUCP

  o  Printing HOWTO: for setting up a serial printer

  o  NET-2 HOWTO: all about networking, including SLIP, CSLIP, PLIP and
     PPP

  o  BUPS HOWTO: setting up UPS boxen connected to your serial port

  o  Term HOWTO: everything you wanted to know about the term program

  o  USENET newsgroups:

  comp.os.linux.advocacy            Benefits of Linux compared to other operating systems.
  comp.os.linux.announce            Announcements important to the Linux community.
  comp.os.linux.answers             FAQs, How-To's, READMEs, etc. about Linux.
  comp.os.linux.development.apps    Writing Linux applications, porting to Linux.
  comp.os.linux.development.system  Linux kernels, device drivers, modules.
  comp.os.linux.hardware            Hardware compatibility with the Linux operating system.
  comp.os.linux.misc                Linux-specific topics not covered by other groups.
  comp.os.linux.networking          Networking and communications under Linux.
  comp.os.linux.setup               Linux installation and system administration.
  comp.os.linux.x                   Linux X Window System servers, clients, libs and fonts.


  o  the Linux serial mailing list.  To join, send email to
     majordomo@vger.rutgers.edu, with

     ``subscribe linux-serial'' in the message body.  If you send
     ``help'' in the message body, you get a help message.



  1.3.  New versions of this document

  New versions of the Serial-HOWTO will be placed on

  sunsite.unc.edu (ftp://sunsite.unc.edu:/pub/Linux/docs/HOWTO/Serial-
  HOWTO) and mirror sites.  There are other formats, such as a
  PostScript and dvi version in the other-formats directory.  The Serial
  HOWTO (http://sunsite.unc.edu/mdw/HOWTO/Serial-HOWTO.html) is also
  available for WWW clients such as mosaic.  It will also be posted
  regularly to comp.os.linux.answers.

  If you don't have FTP access, you can get Linux help files via email.
  Bill Riemers runs a mail handler on his account.  Send mail to
  bcr@physics.purdue.edu with a subject of ``help'' for more infomation,
  and to get an index file.

  1.4.  Feedback

  Please send me any questions, comments, suggestions, or additional
  material.  I'm always eager to hear about what you think about the
  HOWTO.  I'm also always on the lookout for improvements!  Tell me
  exactly what you don't understand, or what could be clearer.  You can
  reach me at greg.hankins@cc.gatech.edu via email.  I can also be
  reached at:

  Greg Hankins

  College of Computing

  801 Atlantic Drive

  Atlanta, GA 30332-0280

  via snail mail, and at my home page
  (http://www.cc.gatech.edu/staff/h/Greg.Hankins/) via the WWW.

  Please include the version number of the Serial HOWTO when writing,
  this is version 1.8a.


  1.5.  Disclaimer

  Your milage may vary.  The answers given may not work for all systems
  and all setup combinations.


  2.  Supported serial hardware

  Linux is known to work with the following serial hardware.


  o  standard PC serial boards (COM1 - COM4), to which external serial
     devices (modems, serial mice, etc...) can be connected

  o  standard PC internal modems (COM1 - COM4)

  o  Quickpath Systems Port-Folio 550e (allows IRQs of 3, 4, 5, 9, 10,
     11, 12, and 15)


  2.1.  Multiport serial boards (with 16450/16550A UARTs)


  o  AST FourPort and clones (4 port)

  o  Accent Async-4 (4 port)

  o  Arnet Multiport-8 (8 port)

  o  Bell Technologies HUB6 (6 port)

  o  Boca BB-1004 (4 port), BB-1008 (8 port), BB-2016 (16 port)

  o  Boca IOAT66 (6 port)

  o  Boca 2by4 (4S/2P)

  o  Computone ValuePort V4-ISA (AST FourPort compatible)

  o  PC-COMM (4 port)

  o  STB-4COM (4 port)

  o  Twincom ACI/550

  o  Usenet Serial Board II (4 port)


  In general, Linux will support any serial board which uses a 8250,
  16450, 16550, 16550A (or compatible) UART, or an internal modem which
  emulates one of the above UARTs.


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


  2.2.  Intelligent multiport serial boards


  o  Comtrol RocketPort (36Mhz ASIC - 4, 8, 16 or 32 port) (contact
     info@comtrol.com or Comtrol's Home Page (http://www.comtrol.com).
     Driver location: tsx-11.mit.edu/pub/linux/packages/comtrol)
  o  Computone IntelliPort II (16Mhz 80186 - 4, 8, or 16 port) Computone
     IntelliPort II EXpandable (20Mhz 80186 - modular 16 - 64 port)
     (pre-ALPHA driver, contact Michael H. Warfield,
     mhw@wittsend.atl.ga.us)

  o  Cyclades Cyclom 8Y (8 port), and 16Y (16 port) (Cirrus Logic
     CD-1400 RISC UARTs) (contact cyclades@netcom.com)

  o  DigiBoard PC/Xe (12.5MHz 80186 processor - 2, 4, 8, or 16 port),
     and PC/Xi (12.5MHz 80186 processor - 8, or 16 port) (contact Troy
     De Jongh, troyd@digibd.com.  Driver location:
     ftp.digibd.com:/digiline/drivers/linux)

  o  Digiboard COM/Xi (10MHz 80188 processor - 4 or 8 port)

     (pre-ALPHA driver contact Simon Park, si@wimpol.demon.co.uk)

  o  Hayes ESP8 (8 port)

     (pre-ALPHA driver, contact Dennis Boylan, dennis@lan.com)

  o  Omega COMM-8 (8 port)

     (contact Vance Petree, vpetree@infi.net)

  o  Specialix SIO - (modular, 4 - 32 port)

     (ALPHA driver, contact Simon Allen, simonallen@cix.compulink.co.uk)

  o  Stallion EasyIO-4 (4 port), EasyIO-8 (8 port), and Stallion
     EasyConnection (modular, 8 - 32 port) (Cirrus Logic CD-1400 RISC
     UARTs) (contact Greg Ungerer, gerg@stallion.oz.au)


  Drivers for the Cyclades, DigiBoard, Stallion and Specialix boards can
  be

  found on sunsite.unc.edu:/pub/Linux/kernel/patches/serial and mirror
  sites.


  3.  What are the names of the serial ports?

  There are the 4 serial devices corresponding to COM1 - COM4:


       /dev/cua0, /dev/ttyS0 (COM1) address 0x3f8 IRQ 4
       /dev/cua1, /dev/ttyS1 (COM2) address 0x2f8 IRQ 3
       /dev/cua2, /dev/ttyS2 (COM3) address 0x3e8 IRQ 4
       /dev/cua3, /dev/ttyS3 (COM4) address 0x2e8 IRQ 3


  The /dev/ttySN devices are for incoming connections and /dev/cuaN
  devices for outgoing connections.  N is the serial port number.  In
  this document, I refer to COM1 as ttyS0, COM2 as ttyS1, COM3 as ttyS2,
  and COM4 as ttyS3.  If I am refering to a specific device in /dev, I
  will always prepend /dev to avoid confusing you.


  On some installations, two extra devices will be created, /dev/modem
  for your modem and /dev/mouse for your mouse.  Both of these are
  symbolic links to the appropriate /dev/cuaN device which you specified
  during the installation (unless you have a bus mouse, then /dev/mouse
  will point to the bus mouse device).


  There has been some discussion on the merits of /dev/mouse and
  /dev/modem.  I strongly discourage the use of these links.  In
  particular, if you are planning on using your modem for dialin you
  will run into problems because the lock files will not work correctly
  if you use /dev/modem. Use them if you like, but be sure they point to
  the right device.


  3.1.  Major and minor device numbers of serial devices in /dev

       /dev/ttyS0 major 4, minor 64    /dev/cua0 major 5, minor 64
       /dev/ttyS1 major 4, minor 65    /dev/cua1 major 5, minor 65
       /dev/ttyS2 major 4, minor 66    /dev/cua2 major 5, minor 66
       /dev/ttyS3 major 4, minor 67    /dev/cua3 major 5, minor 67


  Note that all distributions should come with these devices already
  made correctly.


  3.1.1.  Creating devices in /dev

  If you don't have a device, you will have to create it with the mknod
  command.

  Example, suppose you needed to create devices for ttyS0:


       linux# mknod -m 666 /dev/cua0 c 5 64
       linux# mknod -m 666 /dev/ttyS0 c 4 64


  You can also get the MAKEDEV script, available on the usual FTP sites.
  This simplifies the making of devices.  For example, if you needed to
  make the devices for ttyS0 you would type:

       linux# cd /dev
       linux# MAKEDEV ttyS0


  This handles the devices creation for the incoming and outgoing
  devices.


  3.1.2.  Notes for multiport boards

  The devices your multiport board uses depends on what kind of board
  you have.  These are listed in detail in the rc.serial which comes
  with the setserial program.  You will probably need to create these
  devices.  Either use the mknod command, or get the MAKEDEV script.
  Devices for mulitport boards are made by adding ``64 + the port
  number''.  So, if you wanted to create devices for ttyS17, you would
  type:

       linux# mknod -m 666 /dev/cua17 c 5 81
       linux# mknod -m 666 /dev/ttyS17 c 4 81


  Note that ``64 + 17 = 81''.  Using the MAKEDEV script, you would type:

       linux# cd /dev
       linux# MAKEDEV ttyS17


  4.  What is getty?

  getty is a program that handles the login process when you log onto a
  Unix box.  There are 3 versions that are commonly used with Linux:
  agetty, getty_ps and mgetty.


  4.1.  About getty_ps


  This version of getty was written by Paul Sutcliffe Jr.,
  paul@devon.lns.pa.us.  Kris Gleason, gleasokr@boulder.colorado.edu
  currently maintains it.  2.0.7e is the latest version, and supercedes
  any older versions.  Most distributions come with the getty_ps package
  installed as /sbin/getty.


  The getty_ps package contains two getties.  getty is used for console,
  and terminal devices - and uugetty which is used for modems.  I use
  this version of getty, so I will focus on the getty_ps package in this
  HOWTO.


  4.2.  About mgetty

  mgetty is a version of getty written by Gert Doering
  gert@greenie.muc.de.  In addition to allowing logins, mgetty also
  provides class 2 FAX support through sendfax, which accompanies
  mgetty.  mgetty+sendfax 0.22 is the latest version of this package.
  The mgetty documentation is quite good, and does not need
  supplementing.  Please refer to it for installation instructions.


  4.3.  About agetty

  agetty is the third variation of getty.  It was original written by
  W.Z. Venema, wietse@wzv.win.tue.nl.  It's a simple implementation of
  getty.


  5.  What is setserial?

  setserial is a program which allows you to look at and change various
  attributes of a serial device, including its port address, its
  interrupt, and other serial port options.  It was initially written
  Rick Sladkey, and was heavily modified by Ted T'so tytso@mit.edu, who
  also maintains it.  The newest version is 2.10, and can be found on
  the Linux FTP sites.  You can find out what version you have by
  running setserial with no arguments.


  When your Linux system boots, only ttyS{0-3} are configured, using the
  default IRQs of 4 and 3.  So, if you have any other serial ports
  provided by other boards or if ttyS{0-3} have a non-standard IRQ, you
  must use this program in order to configure those serial ports.  For
  the full listing of options, consult the man page.



  6.  How do I dial out with my modem?

  6.1.  Hardware requirements

  First, make sure you have the right cable.  Your modem requires a
  straight through cable, with no pins crossed.  Any computer store
  should have these.  Make sure you get the correct gender.  If you are
  using the DB25 serial port, it will always be the male DB25.  Do not
  confuse it with the parallel port, which is the female DB25.  Hook up
  your modem to one of your serial ports.  Consult your modem manual on
  how to do this if you need help.


  6.1.1.  Notes on internal modems

  For an internal modem, you will not need a cable.  An internal modem
  does not need a serial port, it has one built in.  All you need to do
  is configure it to use an interrupt that is not being used, and
  configure the port I/O address.  Consult your modem manual if you get
  stuck.  Also, see section ``Can I use more than   2 serial devices?''
  if you need help on choosing interrupts or addresses.

  Due to a bit of stupidity on IBM's part, you may encounter problems if
  you want your internal modem to be on ttyS3.  If Linux does not detect
  your internal modem on ttyS3, you can use setserial and the modem will
  work fine.  Internal modems on ttyS{0-2} should not have any problems
  being detected.


  6.2.  Talking to your modem

  Use kermit or some other simple comm program to test the setup, before
  you go jumping into complex comm programs.  (For legel reasons, kermit
  is not distributed with commercial distributions.  You can find the
  lastest version of kermit on sunsite.unc.edu:/pub/Linux/apps/comm and
  mirror sites.)  For example, say your modem was on ttyS3, and it could
  handle 38400 bps.  You would do the following:


  linux# kermit
  C-Kermit 5A(188), 23 Nov 92, POSIX
  Type ? or HELP for help
  C-Kermit>set line /dev/cua3
  C-Kermit>set speed 38400
  /dev/cua3, 38400 bps
  C-Kermit>c
  Connecting to /dev/cua3, speed 38400.
  The escape character is Ctrl-\ (ASCII 28, FS)
  Type the escape character followed by C to get back,
  or followed by ? to see other options.
  AT
  OK
  -\-C
  (Back at linux)
  C-Kermit>quit
  linux#


  If your modem responds to AT commands, you can assume your modem is
  working correctly on the Linux side.  Try calling another modem.  If
  you don't like kermit, try one of the more advanced comm programs.
  Check out section ``Communications programs'' about comm programs if
  you need some pointers.

  When you dial out with your modem, set the speed to the highest bps
  rate that your modem supports.  Since there is no speed named 57600 or
  115200 bps, you must use the setserial program to set your serial port
  to a higher speed.  See section ``How do I set up my serial ports for
  higher speeds?'' for how to do this.  Then, set the speed to 38400 bps
  in your comm program.


  6.3.  Dial out modem configuration

  For dial out use only, you can configure your modem however you want.
  If you intend to use your modem for dialin, you must configure your
  modem at the same speed that you intend to run getty at.  So, if you
  want to run getty at 38400 bps, set your speed to 38400 bps when you
  configure your modem.  This is done to prevent speed mismatches
  between your computer and modem.


  I like to see result codes, so I set Q0 - result codes are reported.
  To set this on my modem, I would have to preceed the register name
  with an AT command.  Using kermit or some comm program, connect to
  your modem and type the following: ATQ0.  If your modem says OK back
  to you, then the register is set.  Do this for each register you want
  to set.


  I also like to see what I'm typing, so I set E1 - command echo on.  If
  your modem has data compression capabilities, you probably want to
  enable them.  Consult your modem manual for more help, and a full
  listing of options.  If your modem supports a stored profile, be sure
  to write the configuration to the modem (often done with AT&W, but
  varies between modem manufacturers) if not you will have to set the
  registers everytime you turn on, or reset your modem.


  6.4.  Hardware flow control

  If your modem supports hardware flow control (RTS/CTS), I highly
  recommend you use it.  This is particularly important for modems that
  support data compression.  First, you have to enable RTS/CTS flow
  control on the serial port itself.  This is best done on startup, like
  in /etc/rc.d/rc.local or /etc/rc.d/rc.serial.  Make sure that these
  files are being run from the main rc.M file!  You need to do the
  following for each serial port you want to enable hardware flow
  control on:

       stty crtscts < /dev/cuaN


  You must also enable RTS/CTS flow control on your modem.  Consult your
  modem manual on how to do this, as it varies between modem
  manufacturers.  Be sure to save your modem configuration if your modem
  supports stored profiles.


  7.  How do I dial in and out with my modem using getty_ps?

  Get your modem to dial out correctly.  If you haven't read section
  ``How do I dial out with my modem'' go read it now!  It contains very
  important setup information.


  7.1.  Dial in and out modem configuration

  For dialin and dialout use, you have to set up your modem a certain
  way (again, using AT commands on your modem):

       E1       command echo ON
       Q0       result codes are reported
       V1       verbose ON
       S0=0     never answer (uugetty handles this with the WAITFOR option)


  If you don't set these correctly, your INIT string in your config file
  may fail, hosing the whole process.  But, more on config files
  below...

       &C1     DCD is on after connect only
       &S0     DSR is always on
               DTR on/off resets modem (depends on manufacturer - RTFM)


  These affect what your modem does when calls start and end.


  If your modem does not support a stored profile, you can set these
  through the INIT string in your config file.  See below.  Some modems
  come with DIP switches that affect register settings.  Be sure these
  are set correctly, too.


  I have started a collection of modem setups for different types of
  modems.  So far, I only have a few of them, if you would like to send
  me your working configuration, please do so!  If you would like me to
  send you one of the configurations, just mail me and ask.  I'm not
  listing them here due to space concerns.  I have setups for Supra,
  Telebit T1600, USR Courier and Sportster, and Zoom 14.4/28.8 modems.


  7.2.  Installing getty_ps

  By default, getty_ps will be configured to be Linux FSSTND (FileSystem
  STaNDard) compliant, which means that the binaries will be in /sbin,
  and the config files will be named /etc/conf.{uu}getty.ttySN.  This is
  not apparant from the documentation!  It will also expect lock files
  to go in /var/lock.  Make sure you have the /var/lock directory.


  If you don't want FSSTND compliance, binaries will go in /etc, config
  files will go in /etc/default/{uu}getty.ttySN, and lock file will go
  in /usr/spool/uucp.  I recommend doing things this way if you are
  using UUCP, because Taylor UUCP will have problems if you move the
  lock files to where it isn't looking for them.


  getty_ps also uses syslogd to log messages.  See the man pages for
  syslogd(1) and syslog.conf(5) for setting up syslogd, if you don't
  have it running already.  Messages are logged with priority LOG_AUTH,
  errors use LOG_ERR, and debugging uses LOG_DEBUG.  If you don't want
  to use syslogd you can edit tune.h in the getty_ps source files to use
  a log file for messages instead, namely /var/adm/getty.log by default.


  When you have decided if you want FSSTND, and syslog, edit tune.h and
  the Makefile in the getty_ps source directory to reflect you
  decisions.  Now, install according to the instructions.


  From this point on, all references to getty will refer to getty_ps.
  References to uugetty will refer to the uugetty that comes with the
  getty_ps package.


  7.3.  Setting up uugetty

  For dialing into, and out from your modem, we want to use uugetty.
  uugetty does important lock file checking.  Update /etc/gettydefs to
  include entries for modems (note that the entries point to each other,
  these are not for fixed speed):


       # Modem entries
       38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S @L @B login: #19200
       19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S @L @B login: #9600
       9600# B9600 CS8 # B9600 SANE -ISTRIP HUPCL #@S @L @B login: #2400
       2400# B2400 CS8 # B2400 SANE -ISTRIP HUPCL #@S @L @B login: #1200
       1200# B1200 CS8 # B1200 SANE -ISTRIP HUPCL #@S @L @B login: #300
       300# B300 CS8 # B300 SANE -ISTRIP HUPCL #@S @L @B login: #38400


  If you have a 9600 bps or faster modem, with data compression, you can
  lock your serial port speed and let the modem handle the transitions
  to other bps rates. Then, instead of the step down series of lines
  listed above, /etc/gettydefs only needs to contain one line for the
  modem:

       # 38400 fixed speed
       F38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S @L @B login: #F38400
       # 19200 fixed speed
       F19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S @L @B login: #F19200
       # 9600 fixed speed
       F9600# B9600 CS8 # B9600 SANE -ISTRIP HUPCL #@S @L @B login: #F9600


  If you have your modem set up to do RTS/CTS hardware flow control, you
  can add CRTSCTS to the entries:


       # 38400 fixed speed with hardware flow control
       F38400# B38400 CS8 CRTSCTS # B38400 SANE -ISTRIP HUPCL CRTSCTS #@S @L @B login: #F38400
       # 19200 fixed speed with hardware flow control
       F19200# B19200 CS8 CRTSCTS # B19200 SANE -ISTRIP HUPCL CRTSCTS #@S @L @B login: #F19200
       # 9600 fixed speed with hardware flow control
       F9600# B9600 CS8 CRTSCTS # B9600 SANE -ISTRIP HUPCL CRTSCTS #@S @L @B login: #F9600


  If you want, you can make uugetty print interesting things in the
  login banner.  In my examples, I have the system name, the serial
  line, and the current bps rate.  You can add other things:


              @B    The current (evaluated at the time the @B is seen) bps rate.
              @D    The current date, in MM/DD/YY.
              @L    The serial line to which getty is attached.
              @S    The system name.
              @T    The current time, in HH:MM:SS (24-hour).
              @U    The number of currently signed-on users.  This is  a
                    count of the number of entries in the /etc/utmp file
                    that have a non-null ut_name field.
              @V    The value of VERSION, as given in the defaults file.
              To display a single '@' character, use either '\@' or '@@'.


  Next, make sure that you have an outgoing and incoming device for the
  serial port your modem is on.  If you have your modem on ttyS3 you
  will need the /dev/cua3, and /dev/ttyS3 devices.  If you don't have
  the correct devices, see section ``Creating devices in /dev''
  on how to create devices, and create the devices.


  7.4.  Customizing uugetty

  There are lots of parameters you can tweak for each port you have.
  These are implemented in seperate config files for each port.  The
  file /etc/conf.uugetty will be used by all instances of uugetty, and
  /etc/conf.uugetty.ttySN will only be used by that one port.  Sample
  default config files can be found with the getty_ps source files,
  which come with most Linux distributions.  Due to space concerns, they
  are not listed here.  Note that if you are using older versions of
  getty (older than 2.0.7e), or aren't using FSSTND, then the default
  file will be /etc/default/uugetty.ttySN.  My /etc/conf.uugetty.ttyS3
  looks like this:


       # sample uugetty configuration file for a Hayes compatible modem to allow
       # incoming modem connections
       #
       # alternate lock file to check... if this lock file exists, then uugetty is
       # restarted so that the modem is re-initialized
       ALTLOCK=cua3
       ALTLINE=cua3
       # line to initialize
       INITLINE=cua3
       # timeout to disconnect if idle...
       TIMEOUT=60
       # modem initialization string...
       # format:   ... (chat sequence)
       INIT="" AT\r OK\r\n
       WAITFOR=RING
       CONNECT="" ATA\r CONNECT\s\A
       # this line sets the time to delay before sending the login banner
       DELAY=1
       #DEBUG=010


  Add the following line to your /etc/inittab, so that uugetty is run on
  your serial port (substituting in the correct information for your
  environment - port, speed, and default terminal type):

       S3:456:respawn:/sbin/uugetty ttyS3 F38400 vt100


  Restart init:

       linux# init q


  For the speed parameter in your inittab, you want to use the highest
  bps rate that your modem supports.  Since there is no speed named
  57600 or 115200, you must use the setserial program to set your serial
  port to a higher speed.  See section ``How do I set up my serial ports
  for higher speeds?'' for doing this.  Then, use 38400 bps in your
  inittab.


  Now Linux will be watching your serial port for connections.  Dial in
  from another site and login to you Linux system.  Rejoice.


  uugetty has a lot more options, see the man page for getty(1m) for a
  full description.  Among other things there is a scheduling feature,
  and a ringback feature.  RTFM :-).


  7.5.  US Robotics Notes

  To get my USR Courier modem to reset correctly when DTR drops, I had
  to set &D2 and S13=1.


  7.6.  Supra Notes

  Supra modems treat DCD differently than other modems.  If you are
  using a Supra, you must set &C0 and not &C1.  You must also set &D2 to
  handle DTR correctly.



  8.  How do I set up a terminal connected to my PC?

  8.1.  Hardware requirements

  Make sure you have the right kind of cable.  A null modem cable bought
  at a computer store will do it.  But it must be a null modem cable!
  Make sure you are using your serial port, the male DB25 or the DB9,
  and not your parallel port.


  At a minimum, you should have (for a DB25 connector):

          RxD   Receive Data          2 - 3       TxD   Transmit Data
          TxD   Transmit Data         3 - 2       RxD   Receive Data
          GND   Signal Ground         7 - 7       GND   Signal Ground


  If you want to have hardware handshaking signals, you must have a full
  null modem cable:

                Frame Ground          1 - 1             Frame Ground
          RxD   Receive Data          2 - 3       TxD   Transmit Data
          TxD   Transmit Data         3 - 2       RxD   Receive Data
          RTS   Request To Send       4 - 5       CTS   Clear To Send
          CTS   Clear To Send         5 - 4       RTS   Request To Send
          DSR   Data Set Ready        6 - 20      DTR   Data Terminal Ready
          GND   Signal Ground         7 - 7       GND   Signal Ground
          DCD   Carrier Detect        8 - 20      DTR   Data Terminal Ready
          DTR   Data Terminal Ready  20 - 6       DSR   Data Set Ready
          DTR   Data Terminal Ready  20 - 8       DCD   Carrier Detect



  If you have a DB9 connector on your serial port, try the following:

                                    DB9   DB25
          RxD   Receive Data          2 - 2       TxD   Transmit Data
          TxD   Transmit Data         3 - 3       RxD   Receive Data
          GND   Signal Ground         5 - 7       GND   Signal Ground


  Alternatively, a full DB9-DB25 null modem cable:

                                    DB9   DB25
          DCD   Carrier Detect        1 - 20      DTR   Data Terminal Ready
          RxD   Receive Data          2 - 2       TxD   Transmit Data
          TxD   Transmit Data         3 - 3       RxD   Receive Data
          DTR   Data Terminal Ready   4 - 6       DSR   Data Set Ready
          DTR   Data Terminal Ready   4 - 8       DCD   Carrier Detect
          GND   Signal Ground         5 - 7       GND   Signal Ground
          DSR   Data Set Ready        6 - 20      DTR   Data Terminal Ready
          RTS   Request To Send       7 - 5       CTS   Clear To Send
          CTS   Clear To Send         8 - 4       RTS   Request To Send
          (RI Ring Indicator          9 not needed)



  If you are not using a full null modem cable, you might have to do the
  following trick: on your computer side of the connector, connect RTS
  and CTS together, and also connect DSR, DCD and DTR together.  This
  way, when the computer wants a certain handshaking signal, it will get
  it (from itself).

  Now that you have the right kind of cable, connect your terminal to
  your computer.  If you can, tell you terminal to ignore modem control
  signals.  Try using 9600 bps, 8 data bits, 1 stop bit, no parity bits
  for the terminal's setup.


  8.2.  Setting up getty

  Install getty_ps as described in REFERENCE.  Add an entry for getty to
  use for your terminal in /etc/gettydefs:



       # 38400 bps Dumb Terminal entry
       DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400
       # 19200 bps Dumb Terminal entry
       DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200
       # 9600 bps Dumb Terminal entry
       DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600


  If you want, you can make getty print interesting things in the login
  banner.  In my examples, I have the system name and the serial line
  printed.  You can add other things:



       @B    The current (evaluated at the time the @B is seen) bps rate.
       @D    The current date, in MM/DD/YY.
       @L    The serial line to which getty is attached.
       @S    The system name.
       @T    The current time, in HH:MM:SS (24-hour).
       @U    The number of currently signed-on users.  This is  a
             count of the number of entries in the /etc/utmp file
             that have a non-null ut_name field.
       @V    The value of VERSION, as given in the defaults file.
       To display a single '@' character, use either '\@' or '@@'.


  Edit your /etc/inittab file to run getty on the serial port
  (substituting in the correct information for your environment - port,
  speed, and default terminal type):

       S1:456:respawn:/sbin/getty ttyS1 DT9600 vt100


  Restart init:

       linux# init q

  At this point, you should see a login prompt on your terminal.  You
  may have to hit return to get the terminal's attention.  Rejoice.


  8.3.  Notes on setting up a PC as a terminal


  Many people set up other PCs as terminals connected to Linux boxen.
  For example, old 8088 or 286 PCs are perfect for this purpose.  All
  you need is a DOS boot disk containing a version of DOS suitable for
  your terminal-PC, and a communications program for your terminal-PC to
  run.  kermit works very well for this purpose.  You can find
  precompiled versions of kermit for nearly every OS in existence at
  watsun.cc.columbia.edu:/pub/ftp/kermit.  Other popular DOS comm
  programs such as telix and procomm will work equally well.  Be sure to
  input correct serial port information into your terminal-PC's
  communications setup.


  9.  Can I use more than 2 serial devices?

  You don't need to read this section, unless you want to use 3 or more
  serial devices... (assuming you don't have a multiport board).


  Providing you have another spare serial port, yes, you can.


  The number of serial ports you can use is limited by the number of
  interrupts (IRQ) and port I/O addresses we have to use.  This is not a
  Linux limitation, but a limitation of the PC bus.  Each serial devices
  must be assigned it's own interrupt and address.  A serial device can
  be a serial port, an internal modem, or a multiport serial board.


  Multiport serial boards are specially designed to have multiple serial
  ports that share the same IRQ for all serial ports on the board.
  Linux gets data from them by using a different I/O address for each
  port on the card.


  9.1.  Choosing serial device interrupts

  Your PC will normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1
  and ttyS3 at IRQ 3.  To use more than 2 serial devices, you will have
  to give up an interrupt to use.  A good choice is to reassign an
  interrupt from your parallel port.  Your PC normally comes with IRQ 5
  and IRQ 7 set up as interrupts for your parallel ports, but few people
  use 2 parallel ports.  You can reassign one of the interrupts to a
  serial device, and still happily use a parallel port.  You will need
  the setserial program to do this.  In addition, you have to play with
  the jumpers on your boards, check the docs for your board.  Set the
  jumpers to the IRQ you want for each port.


  You will need to set things up so that there is one, and only one
  interrupt for each serial device.  Here is how I set mine up in
  /etc/rc.d/rc.local - you should do it upon startup somewhere:

               /etc/setserial /dev/cua0 irq 3          # my serial mouse
               /etc/setserial /dev/cua1 irq 4          # my Wyse dumb terminal
               /etc/setserial /dev/cua2 irq 5          # my Zoom modem
               /etc/setserial /dev/cua3 irq 9          # my USR modem


  Standard IRQ assignments:

                     IRQ  0    Timer channel 0
                     IRQ  1    Keyboard
                     IRQ  2    Cascade for controller 2
                     IRQ  3    Serial port 2
                     IRQ  4    Serial port 1
                     IRQ  5    Parallel port 2
                     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 coprocessor
                     IRQ 14    Hard disk controller
                     IRQ 15    not assigned


  There is really no Right Thing to do when choosing interrupts.  Just
  make sure it isn't being used by the motherboard, or your other cards.
  2, 3, 4, 5, or 7  is a good choice.  ``not assigned'' means that
  currently nothing standard uses these IRQs.  Also 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 serial card with a 16-bit bus connector, you can also
  use IRQ 10, 11, 12 or 15.


  Just make sure you don't use IRQ 0, 1, 6, 8, 13 or 14!  These are used
  by your mother board.  You will make her very unhappy by taking her
  IRQs.


  9.2.  Setting serial device addresses

  Next, you must set the port address.  Check the manual on your board
  for the jumper settings.  Like interrupts, there can only be one
  serial device at each address.  Your ports will usually come
  configured as follows:

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


  Choose which address you want each serial device to have and set the
  jumpers accordingly.  I have my modem on ttyS3, my mouse on ttyS0, and
  my terminal on ttyS2.

  When you reboot, Linux should see your serial ports at the address you
  set them.  The IRQ Linux sees may not correspond to the IRQ you set
  with the jumpers.  Don't worry about this.  Linux does not do any IRQ
  detection when it boots, because IRQ detection is dicy and can be
  fooled.  Use setserial to tell Linux what IRQ the port is using.


  10.  How do I set up my serial ports for higher        speeds?  What
  speed should I use with my modem?

  This section should help you figure out what speed to use when using
  your modem with a communications program, or with a getty program.

  o  If you have something slower than a 9600 bps (V.32) modem, set your
     speed to the highest speed your modem supports.  For example 300,
     1200, or 2400 bps.

  o  If you have a 9600 bps (V.32) modem, with V.42bis data compression.
     use 38400 as your speed.  V.42bis compression has a theoretical
     rate of 4:1, thus ``4 * 9600 = 38400''.

  o  If you have a 14400 bps (V.32bis) modem, with V.42bis data
     compression, use setserial, with the spd_hi flag to configure your
     serial port to use 57600 bps (4 * 14400 = 57600).

     Use the spd_vhi flag if you have a 28800 (V.FC or V.34) modem (4 *
     28800 = 115200).

     Then, use 38400 as the speed in your comm program, or inittab.
     This is now the high speed you have set.  There is no speed named
     57600 or 115200 (although support was added in 1.1.65 and will be
     used soon).  Make sure you have 16550A UARTs :-).

  Put your modifications into /etc/rc.d/rc.serial or /etc/rc.d/rc.local
  so that they are done at startup.  In my /etc/rc.d/rc.local, I set
  ttyS3 to 115200 bps by doing:

       /sbin/setserial /dev/cua3 spd_vhi


  11.  Communications programs and utilities

  Once you get everything working, you may want to check out these more
  advanced programs, all are available on the usual FTP sites, if they
  didn't come with your distribution.


  o  ecu - a communications program

  o  minicom - telix-like comm program

  o  procomm - procommish comm program with zmodem

  o  seyon - X based comm program

  o  xc - xcomm communications package


  These programs offer more features than just kermit alone, including
  telephone directories, auto-dialing and so on.


  o  Another useful program is term.  term multiplexes many connections
     over one serial line.  It is somewhat similar to SLIP, and offers
     some SLIP functionality.  These include rlogin, telnet, ftp,
     finger, rdate, xmosaic and tredir.  tredir is a special program
     which lets you redirect remote TCP/IP ports to your local machine.
     This allows for remote NNTP, and SMTP access.  The good thing about
     term is that is runs entirely in user space, meaning it requires no
     kernel support, or sysadmin support (like SLIP does).

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

  o  callback is a program that will have your modem call you back
     immediately from where you just called.

  o  mgetty+fax handles FAX stuff, and provides an alternate getty

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


  o  Other things can be found on
     sunsite.unc.edu:/pub/Linux/system/Serial and

     sunsite.unc.edu:/pub/Linux/apps/comm or one of the many mirrors.
     These are the directories where all the serial type things are
     kept.


  12.  Serial Tips


  Here are some serial tips you might find helpful...


  12.1.  kermit and zmodem

  To use zmodem with kermit, add the following to your .kermrc:


  define rz !rz < /dev/cua3 > /dev/cua3
  define sz !sz \%0 > /dev/cua3 < /dev/cua3


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


  12.2.  Setting terminal types automagically

  To set your terminal type automagically when you log in, add the
  terminal type to the entry in /etc/inittab.  If I have a vt100
  terminal on ttyS1, I would add ``vt100'' to the getty command:


       S1:456:respawn:/sbin/getty ttyS1 DT9600 vt100


  You can also get tset from sunsite.unc.edu:/pub/Linux/system/Terminal-
  management or a mirror site.  See the docs that come with tset to
  learn how to use it.  tset can establish terminal characteristics when
  you log in, and doesn't depend on any defaults.


  12.3.  Color ls on serial connections

  If ls is screwing up your terminal emulation with the color feature,
  turn it off.  ls --color, and ls --colour all use the color feature.
  Some installations have ls set to use color by default.  Check
  /etc/profile and /etc/csh.cshrc for ls aliases.  You can also alias ls
  to ls --no-color, if you don't want to change the system defaults.


  12.4.  Printing to a printer connected to a terminal

  There is a program called vtprint that will do this, written by
  Garrett D'Amore garrett@sdsu.edu.

  It is available from ftp.sdsu.edu:/pub/vtprint, and also from
  http://www.sdsu.edu/~garrett/.  The following is from the README file
  that comes with the program:


       vtprint is a program that allows users to print from a
       remote UNIX host to a printer attached to their local termi-
       nal or emulator, which makes it great for printing files at
       home, etc. (It only does text files, though.)


  12.5.  Can Linux configure the serial devices automagically?

  Yes.  To get Linux to detect and set up the serial devices
  automatically on startup, add the line:

       /sbin/setserial /dev/cuaN auto_irq skip_test autoconfig

  to your /etc/rc.d/rc.local or /etc/rc.d/rc.serial file.  Do this for
  every serial port you want to auto configure.  Be sure to give a
  device name that really does exist on your machine.


  12.5.1.  Notes for multiport boards

  For board addresses, and IRQs, look at the rc.serial that comes with
  the setserial program.  It has a lot of detail on multiport boards,
  including I/O addresses and device names.


  13.  Linux FTP sites

       sunsite.unc.edu [152.2.22.81]:/pub/Linux        (NC, USA)
       tsx-11.mit.edu [18.172.1.2]:/pub/linux          (MA, USA)
       nic.funet.fi [128.214.6.100]:/pub/OS/Linux      (Finland, Europe)


  sunsite.unc.edu is the official Linux FTP site, and has many mirrors.
  Please use a mirror site if at all possible to save sunsite some
  traffic.


  sunsite mirrors (sunsite.unc.edu:/pub/Linux/MIRRORS):


  * CONTINENT
  - COUNTRY
     CITY...  FTP Site   Directory
  ------------------------------------------------------------------------
  * Africa
  - None so far
  * Asia
  - Thailand
     Bangkok...  ftp.nectec.or.th  /pub/mirrors/linux/
  - Hong Kong
      ...  ftp.cs.cuhk.hk  /pub/Linux/
  - Republic of Singapore
     Singapore...  ftp.nus.sg  /pub/unix/Linux/
  - Japan
     Unknown...  ftp.spin.ad.jp  /pub/linux/sunsite.unc.edu/
  * Australia
     Adelaide...  smug.student.adelaide.edu.au  /pub/sunsite.linux/
     Brisbane...  ftp.dstc.edu.au  /pub/linux/
  * Europe
  - Austria
     Graz...  ftp.tu-graz.ac.at  /pub/Linux/
  - Czech Republic
     Brno...  ftp.fi.muni.cz  /pub/UNIX/linux/
     Prague...  pub.vse.cz  /pub/386-unix/linux/
  - France
     Angers...  ftp.univ-angers.fr  /pub/linux/
     Nancy...  ftp.loria.fr  /pub/linux/sunsite/
  - Germany (Deutschland)
     Aachen...  ftp.dfv.rwth-aachen.de  /pub/linux/sunsite/
     Dortmund...  ftp.germany.eu.net  /pub/os/Linux/Mirror.SunSITE/
     Dresden...  ftp.tu-dresden.de  /pub/Linux/sunsite/
     Erlangen...  ftp.uni-erlangen.de  /pub/Linux/MIRROR.sunsite/
     Mannheim...  ftp.ba-mannheim.de  /pub/linux/mirror.sunsite/
     Paderborn...  ftp.uni-paderborn.de  /pub/Mirrors/sunsite.unc.edu/
     Rostock...  ftp.uni-rostock.de  /Linux/sunsite/
     Stuttgart...  ftp.rus.uni-stuttgart.de  /pub/unix/systems/linux/MIRROR.sunsite/
     Tuebingen...  ftp.uni-tuebingen.de  /pub/linux/Mirror.sunsite/
     Ulm...  ftp.rz.uni-ulm.de  /pub/mirrors/linux/sunsite/
     Unknown...  ftp.gwdg.de  /pub/linux/mirrors/sunsite/
  - Hungary
     Budapest...  ftp.kfki.hu  /pub/linux/
  - Italy
     Pisa...  cnuce-arch.cnr.it  /pub/Linux/
  - Switzerland
     Zurich...  ftp.switch.ch  /mirror/linux/
  - Turkey (Turkiye)
     Ankara...  ftp.metu.edu.tr  /pub/linux/sunsite/
  - United Kingdom
     Coventry...  ftp.maths.warwick.ac.uk  /mirrors/linux/sunsite.unc-mirror/
     London...  src.doc.ic.ac.uk  /packages/linux/sunsite.unc-mirror/
     Mildenhall...  ftp.dungeon.com  /pub/linux/sunsite-mirror/
  * North America
  - United States
     Atlanta, GA...  ftp.cc.gatech.edu  /pub/linux/
     Chapel Hill, NC...  sunsite.unc.edu  /pub/Linux/
     Fayetteville, AR...  ftp.engr.uark.edu  /pub/linux/sunsite/
     Flagstaff, AZ...  ftp.infomagic.com  /pub/mirrors/linux/sunsite/
     Midwest...  ftp.wit.com  /systems/unix/linux/
     Mt. Pleasant, MI...  ftp.cps.cmich.edu  /pub/linux/sunsite/
     Rochester, NY...  ftp.rge.com  /pub/linux/sunsite/
     Salt Lake City, UT...  ftp.pht.com  /mirrors/linux/sunsite/
     Urbana, IL...  mrcnext.cso.uiuc.edu  /pub/linux/
     Unknown...  ftp.linux.org  /pub/mirrors/sunsite/
     Unknown...  ftp.orst.edu  /pub/mirrors/sunsite.unc.edu/linux/
     Unknown...  ftp.iquest.com  /pub/linux/sunsite/
     Unknown...  ftp.yggdrasil.com  mirrors/sunsite/
  * South America
  - Chile
    ftp.inf.utfsm.cl              /pub/Linux
  * Unknown
  - If you know where these sites are, please mail ewt@sunsite.unc.edu.
      ...  ftp.linux.org  /pub/mirrors/sunsite/
      ...  ftp.gwdg.de  /pub/linux/mirrors/sunsite/
      ...  ftp.orst.edu  /pub/mirrors/sunsite.unc.edu/linux/
      ...  ftp.iquest.com  /pub/linux/sunsite/
      ...  ftp.spin.ad.jp  /pub/linux/sunsite.unc.edu/
      ...  ftp.yggdrasil.com  mirrors/sunsite/


  These FTP sites support annonymous FTP, which means login as ftp, and
  password as your email address (ie logname@yourhost.yourdomain).


  14.  One step further...

  This section is not required reading, but may give you some further
  insight into Unix, and the world of telecommunications.


  14.1.  What are lock files?

  Lock file are simply a file saying that a particular device is in use.
  They are kept in /usr/spool/uucp, or /var/lock.  Linux lock files are
  named LCK..name, where name is either a device name, or a UUCP site
  name.  Certain processes create these locks so that they can have
  exclusive access to devices, for instance if you dial out on your
  modem, a lock will appear telling other processes that someone is
  using the modem already.  Locks mainly contain the PID of the process
  that has locked the device.  Most programs look at the lock, and try
  to determine if that lock is still valid by checking the process table
  for the process that has locked the device.  If the lock is found to
  be valid, the program (should) exit.  If not, some programs remove the
  stale lock, and use the device, creating their own lock in the
  process.  Other programs just exit and tell you that the device is in
  use.


  14.2.  ``baud'' vs. ``bps''

  ``baud'' and ``bps'' are prehaps one of the most misused terms in the
  computing/telecom field.  Many people use these terms interchangeably,
  when in fact they are not!


     baud
        The baud rate is a measure of how many times per second the
        signal sent by a modem (modulator-demodulator) changes.  For
        example, a baud rate of 1200 implies one signal change every 833
        microseconds.  Common baud rates are 50, 75, 110, 300, 600,
        1200, and 2400.  Most high speed modems run at 2400 baud.
        Because of the bandwidth limitations on voice-grade phone lines,
        baud rates greater than 2400 are harder to achieve, and only
        work under very pristine phone line quality.  ``baud'' is named
        after Emile Baudot, the inventor of the asynchronous telegraph
        printer.


     bps
        The bps rate is a measure of how many bits per second are
        transmitted.  Common bps rates are 50, 75, 110, 300, 1200, 2400,
        9600, ... 115200.  With modems using V.42bis compression (4:1
        compression), theoretical bps rates are possible up to 115200.
        This is what most people mean when they misuse the word
        ``baud''.


  So, if high speed modems are running at 2400 baud, how can they send
  14400 bps?  The modems achive a bps > baud rate by encoding a number
  of bits per baud.  Thus, when 2 or more bits are encoded per baud, the
  bps rate exceeds the baud rate.  If your modem connects at 14400 bps,
  it's going to be sending 6 bits per baud.

  How did this confusion start?  Well, back when today's low speed
  modems were yesterday's high speed modems, the bps rate actually did
  equal the baud rate.  One bit would be encoded per baud.  People would
  use bps and baud interchangeably, because they were the same number.
  For example, a 300 bps modem also had a baud rate of 300.  This all
  changed when faster modems came around, and the bit rate exceeded the
  baud rate.



  14.3.  What are UARTs?  How do they affect performance?

  UARTs (Universal Asyncronous Receiver Transmitter) are chips on your
  PC serial card.  Their purpose is to convert data to bits, send the
  bits down the serial line, and then rebuild the data again on the
  other end.  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 your PC.  When you type a
  character, the terminal gives that character to it's transmitter (also
  a UART of some sort).  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 rebuilds the byte and puts
  it in a buffer.


  There are two different types of UARTs.  You have probably heard of
  dumb UARTs - the 8250 and 16450, and FIFO UARTs - the 16550A.  To
  understand their differences, 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, it just sends and
  receives it.  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 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 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 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.


  That's where the 16550A UARTs are useful.  These chips come with 16
  byte FIFOs.  This means that it can receive or transmit up to 16 bytes
  before it has to interrupt the CPU.  Not only can it wait, but the CPU
  then can transfer all 16 bytes at a time.  Although the interrupt
  threshold is seldom set at 16, this is still a significant advantage
  over the other UARTs, which only have the 1 byte buffer.  The CPU
  receives less interrupts, and is free to do other things.  Data is not
  lost, and everyone is happy.  (There is also a 16550 UART, but it is
  treated as a 16450)


  In general, the 8250 and 16450 UARTs should be fine for speeds up to
  38400 bps.  At speeds greater than 38400 bps, you might start seeing
  data loss, and a reduction in interactive response time.  Other PC
  operating systems (definition used loosely here), like DOS aren't
  multitasking, so they might be able to cope better with 8250 or
  16450s.  That's why some people don't see data loss, until they switch
  to Linux.

  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 CD-1400 RISC chip.

  Keep in mind that these dumb UART types are not bad, they just aren't
  good for high speeds.  You should have no problem connecting a
  terminal, or a mouse to these UARTs.  But, for a high speed modem, the
  16550A is definately a must.


  You can buy serial cards with the 16550A UARTs for a little more
  money, just ask your dealer what type of UART is on the card.  Or if
  you want to upgrade your existing card, you can simply purchase 16550A
  chips and replace your existing 16450 UARTs.  They are pin-to-pin
  compatible.  Some cards come with socketed UARTs for this purpose, if
  not you can solder.  Note, that you'll probably save yourself a lot of
  trouble by just getting a new card, if you've got the money, they are
  under US$ 50.


  14.4.  /dev/ttySN devices?  What's the real difference between the
  /dev/cuaN and

  The only difference is the way that the devices are opened.  The
  dialin devices /dev/ttySN are opened in blocking mode, until CD is
  asserted (ie someone connects).  So, when someone wants to use a
  /dev/cuaN device, there is no conflict with a program watching the
  /dev/ttySN device.

  The distinction is made to allow dialin and dialout use of the same
  serial port.


  15.  Troubleshooting

  15.1.  I keep getting ``line NNN of inittab invalid''.

  Make sure you are using the correct syntax for your version of init.
  The different init's that are out there use different syntax in the
  /etc/inittab file.  Make sure you are using the correct syntax for
  your version of getty.


  15.2.  When I try to dial out, it says ``/dev/cuaN: Device or resource
  busy''.

  This problem can arise when DCD or DTR are not set correctly.  DCD
  should only be set when there is an actual connection (ie someone is
  dialed in), not when getty is watching the port.  Check to make sure
  that your modem is configured to only set DCD when there is a
  connection.  DTR should be set whenever something is using, or
  watching the line, like getty, kermit, or some other comm program.

  Another common cause of ``device busy'' errors, is that you set up
  your serial port with an interrupt already taken by something else.
  As each device 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 taken, your device won't be
  able to initialize properly.  The device really doesn't have much of
  any way to tell you that this happened, except that when you try to
  use it, it will return a ``device-busy'' error.  Check the interrupts
  on all of your cards (serial, ethernet, SCSI, etc.).  Look for IRQ
  conflicts.


  15.3.  I keep getting ``Id SN respawning too fast: disabled for 5 min-
  utes''.

  Make sure your modem is configured correctly.  Look at registers E and
  Q.  This can occur when your modem is chatting with getty.

  Make sure you are calling getty correctly from your /etc/inittab.
  Using the wrong syntax or device names will cause serious problems.

  This can also happen when the uugetty initialization is failing.  Go
  to the ``getty or uugetty still doesn't work'' question.


  15.4.  Serial devices are slow or serial devices can only send in one
  direction.

  You probably have an IRQ conflict.  Make sure there are no IRQs being
  shared.  Check all your cards (serial, ethernet, SCSI, etc...).  Make
  sure the jumper settings, and the setserial parameters are correct for
  all your serial devices.


  15.5.  respawn.  My modem is hosed after someone hangs up, or uugetty
  doesn't

  This can happen when your modem doesn't reset when DTR is dropped.  I
  saw my RD and SD LEDs go nuts when this happened to me.  You need to
  have your modem reset.  Most Hayes compatible modems do this with &D3,
  but on my USR Courier, I had to set &D2 and S13=1.  Check your modem
  manual.


  15.6.  it just locks up.  I have my terminal connected to my PC, but
  after I type in a login name,

  You probably don't have CLOCAL in your /etc/gettydefs entry for the
  terminal, and you're probably not using a full null modem cable.  You
  need CLOCAL,  which tells Linux to ignore modem control signals.  Here
  is what it should look like:


       # 38400 bps Dumb Terminal entry
       DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400
       # 19200 bps Dumb Terminal entry
       DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200
       # 9600 bps Dumb Terminal entry
       DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600


  Next, kill the getty process so a new one will be spawned with the new
  entry.


  15.7.  At high speeds, my modem loses data.

  If you are trying to run your modem at > 19200 bps, and you don't have
  16550A UARTs, you should upgrade them.  See section ``What are
  UARTs?'' about UARTs.


  15.8.  them configured.  On startup, Linux doesn't report the serial
  devices the way I have

  This is true.  Linux does not do any IRQ detection on startup, 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.


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


       Jan 23 22:25:28 misfits vmunix: tty02 at 0x03e8 (irq = 4) is a 16550A


  You have to use setserial to tell Linux the IRQ you are using.


  15.9.  rz and/or sz don't work when I call my Linux box on my modem.

  If Linux looks for /dev/modem when you try to transfer files, look at
  /etc/profile, and /etc/csh.cshrc.  There are a bunch of aliases
  defined there on some distributions, most notably Slackware.  These
  aliases mess up the zmodem programs.  Take them out, or correct them.


  15.10.  My screen is printing funny looking characters.

  This happens on virtual consoles when you send binary data to your
  screen, or sometimes on serial connections.  The way to fix this is to
  type echo ^v^[c.  For the control-character-impaired, thats echo
  vc.


  15.11.  getty or uugetty still doesn't work.

  There is a DEBUG option that comes with getty_ps.  Edit your config
  file /etc/conf.{uu}getty.ttySN and add DEBUG=NNN.  Where NNN is one of
  the following combination of numbers according to what you are trying
  to debug:


  D_OPT   001            option settings
  D_DEF   002            defaults file processing
  D_UTMP  004            utmp/wtmp processing
  D_INIT  010            line initialization (INIT)
  D_GTAB  020            gettytab file processing
  D_RUN   040            other runtime diagnostics
  D_RB    100            ringback debugging
  D_LOCK  200            uugetty lockfile processing
  D_SCH   400            schedule processing
  D_ALL   777            everything


  Setting DEBUG=010 is a good place to start.


  If you are running syslogd, debugging info will appear in your log
  files.  If you aren't running syslogd info will appear in
  /tmp/getty:ttySN for debugging getty and /tmp/uugetty:ttySN for
  uugetty, and in /var/adm/getty.log.  Look at the debugging info and
  see what is going on.  Most likely, you will need to tune some of the
  parameters in your config file, and reconfigure your modem.


  You could also try mgetty.  Some people have better luck with it.


  16.  Contributions


  There was no possible way to write this HOWTO alone.  Although a lot
  of the HOWTO is my writing, I have rewritten many contributions to
  maintain continuity in the writing style and flow.  Thanks to everyone
  who has contributed or commented, the list of people has gotten too
  long to list (somewhere over fifty).  Special thanks to Ted T'so for
  answering questions about the serial drivers, Kris Gleason who
  maintains getty_ps, and Gert Doering who maintains mgetty.

  END OF Serial-HOWTO