Serial CommunicationsSynopsisserial communications&unix; has always had support for serial communications as
the very first &unix; machines relied on serial lines for user
input and output. Things have changed a lot from the days
when the average terminal consisted of a 10-character-per-second
serial printer and a keyboard. This chapter covers some of the
ways serial communications can be used on &os;.After reading this chapter, you will know:How to connect terminals to a &os; system.How to use a modem to dial out to remote hosts.How to allow remote users to login to a &os; system
with a modem.How to boot a &os; system from a serial console.Before reading this chapter, you should:Know how to configure and
install a custom kernel.Understand &os; permissions
and processes.Have access to the technical manual for the serial
hardware to be used with &os;.Serial Terminology and HardwareThe following terms are often used in serial
communications:bpsBits per
Secondbits-per-second
(bps) is the rate at which data is
transmitted.DTEData Terminal
EquipmentDTE
(DTE) is one of two endpoints in a
serial communication. An example would be a
computer.DCEData Communications
EquipmentDCE
(DTE) is the other endpoint in a
serial communication. Typically, it is a modem or serial
terminal.RS-232The original standard which defined hardware serial
communications. It has since been renamed to
TIA-232.When referring to communication data rates, this section
does not use the term baud. Baud refers
to the number of electrical state transitions made in a period
of time, while bps is the correct term to
use.To connect a serial terminal to a &os; system, a serial port
on the computer and the proper cable to connect to the serial
device are needed. Users who are already familiar with serial
hardware and cabling can safely skip this section.Serial Cables and PortsThere are several different kinds of serial cables. The
two most common types are null-modem cables and standard
RS-232 cables. The documentation for the
hardware should describe the type of cable required.These two types of cables differ in how the wires are
connected to the connector. Each wire represents a signal,
with the defined signals summarized in . A standard serial
cable passes all of the RS-232C signals
straight through. For example, the Transmitted
Data pin on one end of the cable goes to the
Transmitted Data pin on the other end. This is
the type of cable used to connect a modem to the &os; system,
and is also appropriate for some terminals.A null-modem cable switches the Transmitted
Data pin of the connector on one end with the
Received Data pin on the other end. The
connector can be either a DB-25 or a
DB-9.A null-modem cable can be constructed using the pin
connections summarized in ,
, and . While the standard calls for
a straight-through pin 1 to pin 1 Protective
Ground line, it is often omitted. Some terminals
work using only pins 2, 3, and 7, while others require
different configurations. When in doubt, refer to the
documentation for the hardware.null-modem cable
RS-232C Signal NamesAcronymsNamesRDReceived DataTDTransmitted DataDTRData Terminal ReadyDSRData Set ReadyDCDData Carrier DetectSGSignal GroundRTSRequest to SendCTSClear to Send
When one pin at one end connects to a pair of pins at
the other end, it is usually implemented with one short wire
between the pair of pins in their connector and a long wire
to the other single pin.Serial ports are the devices through which data is
transferred between the &os; host computer and the terminal.
Several kinds of serial ports exist. Before purchasing or
constructing a cable, make sure it will fit the ports on the
terminal and on the &os; system.Most terminals have DB-25 ports.
Personal computers may have DB-25 or
DB-9 ports. A multiport serial card may
have RJ-12 or RJ-45/
ports. See the documentation that accompanied the hardware
for specifications on the kind of port or visually verify the
type of port.In &os;, each serial port is accessed through an entry in
/dev. There are two different kinds of
entries:Call-in ports are named
/dev/ttyuN
where N is the port number,
starting from zero. If a terminal is connected to the
first serial port (COM1), use
/dev/ttyu0 to refer to the terminal.
If the terminal is on the second serial port
(COM2), use
/dev/ttyu1, and so forth. Generally,
the call-in port is used for terminals. Call-in ports
require that the serial line assert the Data
Carrier Detect signal to work correctly.Call-out ports are named
/dev/cuauN
on &os; versions 8.X and higher and
/dev/cuadN
on &os; versions 7.X and lower. Call-out ports are
usually not used for terminals, but are used for modems.
The call-out port can be used if the serial cable or the
terminal does not support the Data Carrier
Detect signal.&os; also provides initialization devices
(/dev/ttyuN.init
and
/dev/cuauN.init
or
/dev/cuadN.init)
and locking devices
(/dev/ttyuN.lock
and
/dev/cuauN.lock
or
/dev/cuadN.lock).
The initialization devices are used to initialize
communications port parameters each time a port is opened,
such as crtscts for modems which use
RTS/CTS signaling for flow control. The
locking devices are used to lock flags on ports to prevent
users or programs changing certain parameters. Refer to
&man.termios.4;, &man.sio.4;, and &man.stty.1; for information
on terminal settings, locking and initializing devices, and
setting terminal options, respectively.Serial Port ConfigurationBy default, &os; supports four serial ports which are
commonly known as COM1,
COM2, COM3, and
COM4. &os; also supports dumb multi-port
serial interface cards, such as the BocaBoard 1008 and 2016,
as well as more intelligent multi-port cards such as those
made by Digiboard. However, the default kernel only looks for
the standard COM ports.To see if the system recognizes the serial ports, look for
system boot messages that start with
uart:&prompt.root; grep uart /var/run/dmesg.bootIf the system does not recognize all of the needed serial
ports, additional entries can be added to
/boot/device.hints. This file already
contains hint.uart.0.* entries for
COM1 and hint.uart.1.*
entries for COM2. When adding a port
entry for COM3 use
0x3E8, and for COM4
use 0x2E8. Common IRQ
addresses are 5 for
COM3 and 9 for
COM4.ttyucuauTo determine the default set of terminal
I/O settings used by the port, specify its
device name. This example determines the settings for the
call-in port on COM2:&prompt.root; stty -a -f /dev/ttyu1System-wide initialization of serial devices is controlled
by /etc/rc.d/serial. This file affects
the default settings of serial devices. To change the
settings for a device, use stty. By
default, the changed settings are in effect until the device
is closed and when the device is reopened, it goes back to the
default set. To permanently change the default set, open and
adjust the settings of the initialization device. For
example, to turn on mode, 8 bit
communication, and flow control for
ttyu5, type:&prompt.root; stty -f /dev/ttyu5.init clocal cs8 ixon ixoffrc filesrc.serialTo prevent certain settings from being changed by an
application, make adjustments to the locking device. For
example, to lock the speed of ttyu5 to
57600 bps, type:&prompt.root; stty -f /dev/ttyu5.lock 57600Now, any application that opens ttyu5
and tries to change the speed of the port will be stuck with
57600 bps.TerminalsSeanKellyContributed by terminalsTerminals provide a convenient and low-cost way to access
a &os; system when not at the computer's console or on a
connected network. This section describes how to use terminals
with &os;.The original &unix; systems did not have consoles. Instead,
users logged in and ran programs through terminals that were
connected to the computer's serial ports.The ability to establish a login session on a serial port
still exists in nearly every &unix;-like operating system
today, including &os;. By using a terminal attached to an
unused serial port, a user can log in and run any text program
that can normally be run on the console or in an
xterm window.Many terminals can be attached to a &os; system. An older
spare computer can be used as a terminal wired into a more
powerful computer running &os;. This can turn what might
otherwise be a single-user computer into a powerful
multiple-user system.&os; supports three types of terminals:Dumb terminalsDumb terminals are specialized hardware that connect
to computers over serial lines. They are called
dumb because they have only enough
computational power to display, send, and receive text.
No programs can be run on these devices. Instead, dumb
terminals connect to a computer that runs the needed
programs.There are hundreds of kinds of dumb terminals made by
many manufacturers, and just about any kind will work with
&os;. Some high-end terminals can even display graphics,
but only certain software packages can take advantage of
these advanced features.Dumb terminals are popular in work environments where
workers do not need access to graphical
applications.Computers Acting as TerminalsSince a dumb terminal has just enough ability to
display, send, and receive text, any spare computer can
be a dumb terminal. All that is needed is the proper
cable and some terminal emulation
software to run on the computer.This configuration can be useful. For example, if one
user is busy working at the &os; system's console, another
user can do some text-only work at the same time from a
less powerful personal computer hooked up as a terminal to
the &os; system.There are at least two utilities in the base-system of
&os; that can be used to work through a serial connection:
&man.cu.1; and &man.tip.1;.For example, to connect from a client system that runs
&os; to the serial connection of another system:&prompt.root; cu -l /dev/cuauNPorts are numbered starting from zero. This means that
COM1 is
/dev/cuau0.Additional programs are available through the Ports
Collection, such as
comms/minicom.X TerminalsX terminals are the most sophisticated kind of
terminal available. Instead of connecting to a serial
port, they usually connect to a network like Ethernet.
Instead of being relegated to text-only applications, they
can display any &xorg;
application.This chapter does not cover the setup, configuration,
or use of X terminals.Terminal ConfigurationThis section describes how to configure a &os; system to
enable a login session on a serial terminal. It assumes that
the system recognizes the serial port to which the terminal is
connected and that the terminal is connected with the correct
cable.In &os;, init reads
/etc/ttys and starts a
getty process on the available terminals.
The getty process is responsible for
reading a login name and starting the login
program. The ports on the &os; system which allow logins are
listed in /etc/ttys. For example, the
first virtual console, ttyv0, has an
entry in this file, allowing logins on the console. This file
also contains entries for the other virtual consoles, serial
ports, and pseudo-ttys. For a hardwired terminal, the serial
port's /dev entry is listed without the
/dev part. For example,
/dev/ttyv0 is listed as
ttyv0.The default /etc/ttys configures
support for the first four serial ports,
ttyu0 through
ttyu3:ttyu0 "/usr/libexec/getty std.9600" dialup off secure
ttyu1 "/usr/libexec/getty std.9600" dialup off secure
ttyu2 "/usr/libexec/getty std.9600" dialup off secure
ttyu3 "/usr/libexec/getty std.9600" dialup off secureWhen attaching a terminal to one of those ports, modify
the default entry to set the required speed and terminal type,
to turn the device on and, if needed, to
change the port's secure setting. If the
terminal is connected to another port, add an entry for the
port. configures two terminals in
/etc/ttys. The first entry configures a
Wyse-50 connected to COM2. The second
entry configures an old computer running
Procomm terminal software emulating
a VT-100 terminal. The computer is connected to the sixth
serial port on a multi-port serial card.Configuring Terminal Entriesttyu1 "/usr/libexec/getty std.38400" wy50 on insecure
ttyu5 "/usr/libexec/getty std.19200" vt100 on insecureThe first field specifies the device name of the
serial terminal.The second field tells getty to
initialize and open the line, set the line speed, prompt
for a user name, and then execute the
login program. The optional
getty type configures
characteristics on the terminal line, like
bps rate and parity. The available
getty types are listed in
/etc/gettytab. In almost all
cases, the getty types that start with
std will work for hardwired terminals
as these entries ignore parity. There is a
std entry for each
bps rate from 110 to 115200. Refer
to &man.gettytab.5; for more information.When setting the getty type, make sure to match the
communications settings used by the terminal. For this
example, the Wyse-50 uses no parity and connects at
38400 bps. The computer uses no parity and
connects at 19200 bps.The third field is the type of terminal. For
dial-up ports, unknown or
dialup is typically used since users
may dial up with practically any type of terminal or
software. Since the terminal type does not change for
hardwired terminals, a real terminal type from
/etc/termcap can be specified. For
this example, the Wyse-50 uses the real terminal type
while the computer running
Procomm is set to emulate a
VT-100.The fourth field specifies if the port should be
enabled. To enable logins on this port, this field must
be set to on.The final field is used to specify whether the port
is secure. Marking a port as secure
means that it is trusted enough to allow root to login from that
port. Insecure ports do not allow root logins. On an
insecure port, users must login from unprivileged
accounts and then use su or a similar
mechanism to gain superuser privileges, as described in
. For security
reasons, it is recommended to change this setting to
insecure.After making any changes to
/etc/ttys, send a SIGHUP (hangup) signal
to the init process to force it to re-read
its configuration file:&prompt.root; kill -HUP 1Since init is always the first process
run on a system, it always has a process ID
of 1.If everything is set up correctly, all cables are in
place, and the terminals are powered up, a
getty process should now be running on each
terminal and login prompts should be available on each
terminal.Troubleshooting the ConnectionEven with the most meticulous attention to detail,
something could still go wrong while setting up a terminal.
Here is a list of common symptoms and some suggested
fixes.If no login prompt appears, make sure the terminal is
plugged in and powered up. If it is a personal computer
acting as a terminal, make sure it is running terminal
emulation software on the correct serial port.Make sure the cable is connected firmly to both the
terminal and the &os; computer. Make sure it is the right
kind of cable.Make sure the terminal and &os; agree on the
bps rate and parity settings. For a video
display terminal, make sure the contrast and brightness
controls are turned up. If it is a printing terminal, make
sure paper and ink are in good supply.Use ps to make sure that a
getty process is running and serving the
terminal. For example, the following listing shows that a
getty is running on the second serial port,
ttyu1, and is using the
std.38400 entry in
/etc/gettytab:&prompt.root; ps -axww|grep ttyu
22189 d1 Is+ 0:00.03 /usr/libexec/getty std.38400 ttyu1If no getty process is running, make
sure the port is enabled in /etc/ttys.
Remember to run kill -HUP 1 after modifying
/etc/ttys.If the getty process is running but the
terminal still does not display a login prompt, or if it
displays a prompt but will not accept typed input, the
terminal or cable may not support hardware handshaking. Try
changing the entry in /etc/ttys from
std.38400 to
3wire.38400, then run kill -HUP
1 after modifying /etc/ttys.
The 3wire entry is similar to
std, but ignores hardware handshaking. The
baud rate may need to be reduced or software flow control
enabled when using 3wire to prevent buffer
overflows.If garbage appears instead of a login prompt, make sure
the terminal and &os; agree on the bps rate
and parity settings. Check the getty
processes to make sure the correct
getty type is in use. If not, edit
/etc/ttys and run kill
-HUP 1.If characters appear doubled and the password appears when
typed, switch the terminal, or the terminal emulation
software, from half duplex or local
echo to full duplex.Dial-in ServiceGuyHelmerContributed by SeanKellyAdditions by dial-in serviceConfiguring a &os; system for dial-in service is similar to
configuring terminals, except that modems are used instead of
terminal devices. &os; supports both external and internal
modems.External modems are more convenient because they often can
be configured via parameters stored in non-volatile
RAM and they usually provide lighted
indicators that display the state of important
RS-232 signals, indicating whether the modem
is operating properly.Internal modems usually lack non-volatile
RAM, so their configuration may be limited to
setting DIP switches. If the internal modem
has any signal indicator lights, they are difficult to view when
the system's cover is in place.modemWhen using an external modem, a proper cable is needed. A
standard RS-232C serial cable should
suffice.&os; needs the RTS and
CTS signals for flow control at speeds above
2400 bps, the CD signal to detect when a
call has been answered or the line has been hung up, and the
DTR signal to reset the modem after a session
is complete. Some cables are wired without all of the needed
signals, so if a login session does not go away when the line
hangs up, there may be a problem with the cable. Refer to for more information about these
signals.Like other &unix;-like operating systems, &os; uses the
hardware signals to find out when a call has been answered or a
line has been hung up and to hangup and reset the modem after a
call. &os; avoids sending commands to the modem or watching for
status reports from the modem.&os; supports the NS8250,
NS16450, NS16550, and
NS16550A-based RS-232C
(CCITT V.24) communications interfaces. The
8250 and 16450 devices have single-character buffers. The 16550
device provides a 16-character buffer, which allows for better
system performance. Bugs in plain 16550 devices prevent the use
of the 16-character buffer, so use 16550A devices if possible.
As single-character-buffer devices require more work by the
operating system than the 16-character-buffer devices,
16550A-based serial interface cards are preferred. If the
system has many active serial ports or will have a heavy load,
16550A-based cards are better for low-error-rate
communications.The rest of this section demonstrates how to configure a
modem to receive incoming connections, how to communicate with
the modem, and offers some troubleshooting tips.Modem ConfigurationgettyAs with terminals, init spawns a
getty process for each configured serial
port used for dial-in connections. When a user dials the
modem's line and the modems connect, the Carrier
Detect signal is reported by the modem. The kernel
notices that the carrier has been detected and instructs
getty to open the port and display a
login: prompt at the specified initial line
speed. In a typical configuration, if garbage characters are
received, usually due to the modem's connection speed being
different than the configured speed, getty
tries adjusting the line speeds until it receives reasonable
characters. After the user enters their login name,
getty executes login,
which completes the login process by asking for the user's
password and then starting the user's shell./usr/bin/loginThere are two schools of thought regarding dial-up modems.
One configuration method is to set the modems and systems so
that no matter at what speed a remote user dials in, the
dial-in RS-232 interface runs at a locked
speed. The benefit of this configuration is that the remote
user always sees a system login prompt immediately. The
downside is that the system does not know what a user's true
data rate is, so full-screen programs like
Emacs will not adjust their
screen-painting methods to make their response better for
slower connections.The second method is to configure the
RS-232 interface to vary its speed based on
the remote user's connection speed. As
getty does not understand any particular
modem's connection speed reporting, it gives a
login: message at an initial speed and
watches the characters that come back in response. If the
user sees junk, they should press Enter until
they see a recognizable prompt. If the data rates do not
match, getty sees anything the user types
as junk, tries the next speed, and gives the
login: prompt again. This procedure normally
only takes a keystroke or two before the user sees a good
prompt. This login sequence does not look as clean as the
locked-speed method, but a user on a low-speed connection
should receive better interactive response from full-screen
programs.When locking a modem's data communications rate at a
particular speed, no changes to
/etc/gettytab should be needed. However,
for a matching-speed configuration, additional entries may be
required in order to define the speeds to use for the modem.
This example configures a 14.4 Kbps modem with a top
interface speed of 19.2 Kbps using 8-bit, no parity
connections. It configures getty to start
the communications rate for a V.32bis connection at
19.2 Kbps, then cycles through 9600 bps,
2400 bps, 1200 bps, 300 bps, and back to
19.2 Kbps. Communications rate cycling is implemented
with the nx= (next table) capability. Each
line uses a tc= (table continuation) entry
to pick up the rest of the settings for a particular data
rate.#
# Additions for a V.32bis Modem
#
um|V300|High Speed Modem at 300,8-bit:\
:nx=V19200:tc=std.300:
un|V1200|High Speed Modem at 1200,8-bit:\
:nx=V300:tc=std.1200:
uo|V2400|High Speed Modem at 2400,8-bit:\
:nx=V1200:tc=std.2400:
up|V9600|High Speed Modem at 9600,8-bit:\
:nx=V2400:tc=std.9600:
uq|V19200|High Speed Modem at 19200,8-bit:\
:nx=V9600:tc=std.19200:For a 28.8 Kbps modem, or to take advantage of
compression on a 14.4 Kbps modem, use a higher
communications rate, as seen in this example:#
# Additions for a V.32bis or V.34 Modem
# Starting at 57.6 Kbps
#
vm|VH300|Very High Speed Modem at 300,8-bit:\
:nx=VH57600:tc=std.300:
vn|VH1200|Very High Speed Modem at 1200,8-bit:\
:nx=VH300:tc=std.1200:
vo|VH2400|Very High Speed Modem at 2400,8-bit:\
:nx=VH1200:tc=std.2400:
vp|VH9600|Very High Speed Modem at 9600,8-bit:\
:nx=VH2400:tc=std.9600:
vq|VH57600|Very High Speed Modem at 57600,8-bit:\
:nx=VH9600:tc=std.57600:For a slow CPU or a heavily loaded
system without 16550A-based serial ports, this configuration
may produce siosilo errors at 57.6 Kbps./etc/ttysThe configuration of /etc/ttys is
similar to , but a different
argument is passed to getty and
dialup is used for the terminal type.
Replace xxx with the process
init will run on the device:ttyu0 "/usr/libexec/getty xxx" dialup onThe dialup terminal type can be
changed. For example, setting vt102 as the
default terminal type allows users to use
VT102 emulation on their remote
systems.For a locked-speed configuration, specify the speed with
a valid type listed in /etc/gettytab.
This example is for a modem whose port speed is locked at
19.2 Kbps:ttyu0 "/usr/libexec/getty std.19200" dialup onIn a matching-speed configuration, the entry needs to
reference the appropriate beginning auto-baud
entry in /etc/gettytab. To continue the
example for a matching-speed modem that starts at
19.2 Kbps, use this entry:ttyu0 "/usr/libexec/getty V19200" dialup onAfter editing /etc/ttys, wait until
the modem is properly configured and connected before
signaling init:&prompt.root; kill -HUP 1rc filesrc.serialHigh-speed modems, like V.32,
V.32bis, and V.34
modems, use hardware (RTS/CTS) flow
control. Use stty to set the hardware flow
control flag for the modem port. This example sets the
crtscts flag on COM2's
dial-in and dial-out initialization devices:&prompt.root; stty -f /dev/ttyu1.init crtscts
&prompt.root; stty -f /dev/cuau1.init crtsctsTroubleshootingThis section provides a few tips for troubleshooting a
dial-up modem that will not connect to a &os; system.Hook up the modem to the &os; system and boot the system.
If the modem has status indication lights, watch to see
whether the modem's DTR indicator lights
when the login: prompt appears on the
system's console. If it lights up, that should mean that &os;
has started a getty process on the
appropriate communications port and is waiting for the modem
to accept a call.If the DTR indicator does not light,
login to the &os; system through the console and type
ps ax to see if &os; is running a
getty process on the correct port: 114 ?? I 0:00.10 /usr/libexec/getty V19200 ttyu0If the second column contains a d0
instead of a ?? and the modem has not
accepted a call yet, this means that getty
has completed its open on the communications port. This could
indicate a problem with the cabling or a misconfigured modem
because getty should not be able to open
the communications port until the carrier detect signal has
been asserted by the modem.If no getty processes are waiting to
open the port, double-check that the entry for the port is
correct in /etc/ttys. Also, check
/var/log/messages to see if there are
any log messages from init or
getty.Next, try dialing into the system. Be sure to use 8 bits,
no parity, and 1 stop bit on the remote system. If a prompt
does not appear right away, or the prompt shows garbage, try
pressing Enter about once per second. If
there is still no login: prompt,
try sending a BREAK. When using a
high-speed modem, try dialing again after locking the
dialing modem's interface speed.If there is still no login: prompt, check
/etc/gettytab again and double-check
that:The initial capability name specified in the entry in
/etc/ttys matches the name of a
capability in /etc/gettytab.Each nx= entry matches another
gettytab capability name.Each tc= entry matches another
gettytab capability name.If the modem on the &os; system will not answer, make
sure that the modem is configured to answer the phone when
DTR is asserted. If the modem seems to be
configured correctly, verify that the
DTR line is asserted by checking the
modem's indicator lights.If it still does not work, try sending an email
to the &a.questions; describing the modem and the
problem.Dial-out Servicedial-out serviceThe following are tips for getting the host to connect over
the modem to another computer. This is appropriate for
establishing a terminal session with a remote host.This kind of connection can be helpful to get a file on the
Internet if there are problems using PPP. If PPP is not
working, use the terminal session to FTP the needed file. Then
use zmodem to transfer it to the machine.Using a Stock Hayes ModemA generic Hayes dialer is built into
tip. Use at=hayes in
/etc/remote.The Hayes driver is not smart enough to recognize some of
the advanced features of newer modems messages like
BUSY, NO DIALTONE, or
CONNECT 115200. Turn those messages off
when using tip with
ATX0&W.The dial timeout for tip is 60
seconds. The modem should use something less, or else
tip will think there is a communication
problem. Try ATS7=45&W.Using AT Commands/etc/remoteCreate a direct entry in
/etc/remote. For example, if the modem
is hooked up to the first serial port,
/dev/cuau0, use the following
line:cuau0:dv=/dev/cuau0:br#19200:pa=noneUse the highest bps rate the modem
supports in the br capability. Then, type
tip cuau0 to connect to the modem.Or, use cu as root with the following
command:&prompt.root; cu -lline -sspeedline is the serial port, such
as /dev/cuau0, and
speed is the speed, such as
57600. When finished entering the AT
commands, type ~. to exit.The @ Sign Does Not WorkThe @ sign in the phone number
capability tells tip to look in
/etc/phones for a phone number. But, the
@ sign is also a special character in
capability files like /etc/remote, so it
needs to be escaped with a backslash:pn=\@Dialing from the Command LinePut a generic entry in
/etc/remote. For example:tip115200|Dial any phone number at 115200 bps:\
:dv=/dev/cuau0:br#115200:at=hayes:pa=none:du:
tip57600|Dial any phone number at 57600 bps:\
:dv=/dev/cuau0:br#57600:at=hayes:pa=none:du:This should now work:&prompt.root; tip -115200 5551234Users who prefer cu over
tip, can use a generic
cu entry:cu115200|Use cu to dial any number at 115200bps:\
:dv=/dev/cuau1:br#57600:at=hayes:pa=none:du:and type:&prompt.root; cu 5551234 -s 115200Setting the bps RatePut in an entry for tip1200 or
cu1200, but go ahead and use whatever
bps rate is appropriate with the
br capability.
tip thinks a good default is 1200 bps
which is why it looks for a tip1200 entry.
1200 bps does not have to be used, though.Accessing a Number of Hosts Through a Terminal
ServerRather than waiting until connected and typing
CONNECT host
each time, use tip's cm
capability. For example, these entries in
/etc/remote will let you type
tip pain or tip muffin
to connect to the hosts pain or
muffin, and tip
deep13 to connect to the terminal server.pain|pain.deep13.com|Forrester's machine:\
:cm=CONNECT pain\n:tc=deep13:
muffin|muffin.deep13.com|Frank's machine:\
:cm=CONNECT muffin\n:tc=deep13:
deep13:Gizmonics Institute terminal server:\
:dv=/dev/cuau2:br#38400:at=hayes:du:pa=none:pn=5551234:Using More Than One Line with
tipThis is often a problem where a university has several
modem lines and several thousand students trying to use
them.Make an entry in /etc/remote and use
@ for the pn
capability:big-university:\
:pn=\@:tc=dialout
dialout:\
:dv=/dev/cuau3:br#9600:at=courier:du:pa=none:Then, list the phone numbers in
/etc/phones:big-university 5551111
big-university 5551112
big-university 5551113
big-university 5551114tip will try each number in the listed
order, then give up. To keep retrying, run
tip in a while
loop.Using the Force CharacterCtrlP is the default force character,
used to tell tip that the next character is
literal data. The force character can be set to any other
character with the ~s escape, which means
set a variable.Type
~sforce=single-char
followed by a newline. single-char
is any single character. If
single-char is left out, then the
force character is the null character, which is accessed by
typing
Ctrl2
or CtrlSpace. A pretty good value for
single-char is
ShiftCtrl6, which is only used on some terminal
servers.To change the force character, specify the following in
~/.tiprc:force=single-charUpper Case CharactersThis happens when
CtrlA is pressed, which is tip's
raise character, specially designed for people
with broken caps-lock keys. Use ~s to set
raisechar to something reasonable. It can
be set to be the same as the force character, if neither
feature is used.Here is a sample ~/.tiprc for
Emacs users who need to type
Ctrl2 and CtrlA:force=^^
raisechar=^^The ^^ is
ShiftCtrl6.File Transfers with tipWhen talking to another &unix;-like operating system,
files can be sent and received using ~p
(put) and ~t (take). These commands run
cat and echo on the
remote system to accept and send files. The syntax is:~plocal-fileremote-file~tremote-filelocal-fileThere is no error checking, so another protocol, like
zmodem, should probably be used.Using zmodem with
tip?To receive files, start the sending program on the remote
end. Then, type ~C rz to begin receiving
them locally.To send files, start the receiving program on the remote
end. Then, type ~C sz
files to send them to the
remote system.Setting Up the Serial ConsoleKazutakaYOKOTAContributed by BillPaulBased on a document by serial console&os; has the ability to boot a system with a dumb
terminal on a serial port as a console. This configuration is
useful for system administrators who wish to install &os; on
machines that have no keyboard or monitor attached, and
developers who want to debug the kernel or device
drivers.As described in , &os; employs a three
stage bootstrap. The first two stages are in the boot block
code which is stored at the beginning of the &os; slice on the
boot disk. The boot block then loads and runs the boot loader
as the third stage code.In order to set up booting from a serial console, the boot
block code, the boot loader code, and the kernel need to be
configured.Quick Serial Console ConfigurationThis section provides a fast overview of setting up the
serial console. This procedure can be used when the dumb
terminal is connected to COM1.Configuring a Serial Console on
COM1Connect the serial cable to
COM1 and the controlling
terminal.To configure boot messages to display on the serial
console, issue the following command as the
superuser:&prompt.root; echo 'console="comconsole"' >> /boot/loader.confEdit /etc/ttys and change
off to on and
dialup to vt100 for
the ttyu0 entry. Otherwise, a
password will not be required to connect via the serial
console, resulting in a potential security hole.Reboot the system to see if the changes took
effect.If a different configuration is required, see the next
section for a more in-depth configuration explanation.In-Depth Serial Console ConfigurationThis section provides a more detailed explanation of the
steps needed to setup a serial console in &os;.Configuring a Serial ConsolePrepare a serial cable.null-modem cableUse either a null-modem cable or a standard serial
cable and a null-modem adapter. See for a discussion on serial
cables.Unplug the keyboard.Many systems probe for the keyboard during the
Power-On Self-Test (POST) and will
generate an error if the keyboard is not detected. Some
machines will refuse to boot until the keyboard is plugged
in.If the computer complains about the error, but boots
anyway, no further configuration is needed.If the computer refuses to boot without a keyboard
attached, configure the BIOS so that it
ignores this error. Consult the motherboard's manual for
details on how to do this.Try setting the keyboard to Not
installed in the BIOS.
This setting tells the BIOS not to
probe for a keyboard at power-on so it should not
complain if the keyboard is absent. If that option is
not present in the BIOS, look for an
Halt on Error option instead. Setting
this to All but Keyboard or to No
Errors will have the same effect.If the system has a &ps2; mouse, unplug it as well.
&ps2; mice share some hardware with the keyboard and
leaving the mouse plugged in can fool the keyboard probe
into thinking the keyboard is still there.While most systems will boot without a keyboard,
quite a few will not boot without a graphics adapter.
Some systems can be configured to boot with no graphics
adapter by changing the graphics adapter
setting in the BIOS configuration to
Not installed. Other systems do not
support this option and will refuse to boot if there is
no display hardware in the system. With these machines,
leave some kind of graphics card plugged in, even if it
is just a junky mono board. A monitor does not need to
be attached.Plug a dumb terminal, an old computer with a modem
program, or the serial port on another &unix; box into the
serial port.Add the appropriate hint.sio.*
entries to /boot/device.hints for the
serial port. Some multi-port cards also require kernel
configuration options. Refer to &man.sio.4; for the
required options and device hints for each supported
serial port.Create boot.config in the root
directory of the a partition on the
boot drive.This file instructs the boot block code how to boot
the system. In order to activate the serial console, one
or more of the following options are needed. When using
multiple options, include them all on the same
line:Toggles between the internal and serial
consoles. Use this to switch console devices. For
instance, to boot from the internal (video) console,
use to direct the boot loader
and the kernel to use the serial port as its console
device. Alternatively, to boot from the serial
port, use to tell the boot
loader and the kernel to use the video display as
the console instead.Toggles between the single and dual console
configurations. In the single configuration, the
console will be either the internal console (video
display) or the serial port, depending on the state
of . In the dual console
configuration, both the video display and the
serial port will become the console at the same
time, regardless of the state of
. However, the dual console
configuration takes effect only while the boot
block is running. Once the boot loader gets
control, the console specified by
becomes the only
console.Makes the boot block probe the keyboard. If no
keyboard is found, the and
options are automatically
set.Due to space constraints in the current
version of the boot blocks, is
capable of detecting extended keyboards only.
Keyboards with less than 101 keys and without F11
and F12 keys may not be detected. Keyboards on
some laptops may not be properly found because of
this limitation. If this is the case, do not use
.Use either to select the console
automatically or to activate the
serial console. Refer to &man.boot.8; and
&man.boot.config.5; for more details.The options, except for , are
passed to the boot loader. The boot loader will
determine whether the internal video or the serial port
should become the console by examining the state of
. This means that if
is specified but
is not specified in /boot.config, the
serial port can be used as the console only during the
boot block as the boot loader will use the internal video
display as the console.Boot the machine.When &os; starts, the boot blocks echo the contents of
/boot.config to the console. For
example:/boot.config: -P
Keyboard: noThe second line appears only if is
in /boot.config and indicates the
presence or absence of the keyboard. These messages go
to either the serial or internal console, or both,
depending on the option in
/boot.config:OptionsMessage goes tononeinternal consoleserial consoleserial and internal consolesserial and internal consoles, keyboard presentinternal console, keyboard absentserial consoleAfter the message, there will be a small pause before
the boot blocks continue loading the boot loader and
before any further messages are printed to the console.
Under normal circumstances, there is no need to interrupt
the boot blocks, but one can do so in order to make sure
things are set up correctly.Press any key, other than Enter, at
the console to interrupt the boot process. The boot
blocks will then prompt for further action:>> FreeBSD/i386 BOOT
Default: 0:ad(0,a)/boot/loader
boot:Verify that the above message appears on either the
serial or internal console, or both, according to the
options in /boot.config. If the
message appears in the correct console, press
Enter to continue the boot
process.If there is no prompt on the serial terminal,
something is wrong with the settings. Enter
then Enter or
Return to tell the boot block (and then
the boot loader and the kernel) to choose the serial port
for the console. Once the system is up, go back and check
what went wrong.During the third stage of the boot process, one can still
switch between the internal console and the serial console by
setting appropriate environment variables in the boot loader.
See &man.loader.8; for more
information.This line in /boot/loader.conf or
/boot/loader.conf.local configures the
boot loader and the kernel to send their boot messages to
the serial console, regardless of the options in
/boot.config:console="comconsole"That line should be the first line of
/boot/loader.conf so that boot messages
are displayed on the serial console as early as
possible.If that line does not exist, or if it is set to
console="vidconsole", the boot loader and
the kernel will use whichever console is indicated by
in the boot block. See
&man.loader.conf.5; for more information.At the moment, the boot loader has no option
equivalent to in the boot block, and
there is no provision to automatically select the internal
console and the serial console based on the presence of the
keyboard.While it is not required, it is possible to provide a
login prompt over the serial line. To
configure this, edit the entry for the serial port in
/etc/ttys using the instructions in
. If the speed of the serial
port has been changed, change std.9600 to
match the new setting.Setting a Faster Serial Port SpeedBy default, the serial port settings are 9600 baud, 8
bits, no parity, and 1 stop bit. To change the default
console speed, use one of the following options:Edit /etc/make.conf and set
BOOT_COMCONSOLE_SPEED to the new
console speed. Then, recompile and install the boot
blocks and the boot loader:&prompt.root; cd /sys/boot
&prompt.root; make clean
&prompt.root; make
&prompt.root; make installIf the serial console is configured in some other way
than by booting with , or if the serial
console used by the kernel is different from the one used
by the boot blocks, add the following option, with the
desired speed, to a custom kernel configuration file and
compile a new kernel:options CONSPEED=19200Add the
boot
option to /boot.config, replacing
19200 with the speed to
use.Add the following options to
/boot/loader.conf. Replace
115200 with the speed to
use.boot_multicons="YES"
boot_serial="YES"
comconsole_speed="115200"
console="comconsole,vidconsole"Entering the DDB Debugger from the Serial LineTo configure the ability to drop into the kernel debugger
from the serial console, add the following options to a custom
kernel configuration file and compile the kernel using the
instructions in . Note that
while this is useful for remote diagnostics, it is also
dangerous if a spurious BREAK is generated on the serial port.
Refer to &man.ddb.4; and &man.ddb.8; for more information
about the kernel debugger.options BREAK_TO_DEBUGGER
options DDB