Chapter 29. Serial Communications

There 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 RS-232C Signal Names. 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 FreeBSD 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 DB-25 to DB-25 Null-Modem Cable, DB-9 to DB-9 Null-Modem Cable, and DB-9 to DB-25 Null-Modem Cable. 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.

Serial ports are the devices through which data is transferred between the FreeBSD 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 FreeBSD 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 FreeBSD, each serial port is accessed through an entry in /dev. There are two different kinds of entries:

FreeBSD 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 termios(4), uart(4), and stty(1) for information on terminal settings, locking and initializing devices, and setting terminal options, respectively.

By default, FreeBSD supports four serial ports which are commonly known as COM1, COM2, COM3, and COM4. FreeBSD 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:

If 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.

To 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:

System-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 CLOCAL mode, 8 bit communication, and XON/XOFF flow control for ttyu5, type:

To 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:

Now, any application that opens ttyu5 and tries to change the speed of the port will be stuck with 57600 bps.

This section describes how to configure a FreeBSD 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 FreeBSD, 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 FreeBSD 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:

When 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.

Configuring Terminal Entries 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.

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:

Since 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.

Even 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 FreeBSD computer. Make sure it is the right kind of cable.

Make sure the terminal and FreeBSD 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:

If 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 bps may also 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 FreeBSD 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."

As 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.

There 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.

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:

For a slow CPU or a heavily loaded system without 16550A-based serial ports, this configuration may produce uart "silo" errors at 57.6 Kbps.

The configuration of /etc/ttys is similar to Configuring Terminal Entries, 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:

The 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:

In 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:

After editing /etc/ttys, wait until the modem is properly configured and connected before signaling init:

High-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:

This section provides a few tips for troubleshooting a dial-up modem that will not connect to a FreeBSD system.

Hook up the modem to the FreeBSD 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 FreeBSD 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 FreeBSD system through the console and type ps ax to see if FreeBSD is running a getty process on the correct port:

If 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:

If the modem on the FreeBSD 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 FreeBSD general questions mailing list describing the modem and the problem.

A 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.

Create 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:

Use 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:

line 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 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:

Put a "generic" entry in /etc/remote. For example:

This should now work:

Users who prefer cu over tip, can use a generic cu entry:

and type:

Put 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.

Rather 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.

This 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:

Then, list the phone numbers in /etc/phones:

tip will try each number in the listed order, then give up. To keep retrying, run tip in a while loop.

Ctrl+P 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 Ctrl+2 or Ctrl+Space. A pretty good value for single-char is Shift+Ctrl+6, which is only used on some terminal servers.

To change the force character, specify the following in ~/.tiprc:

This happens when Ctrl+A 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 Ctrl+2 and Ctrl+A:

The ^^ is Shift+Ctrl+6.

When 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: ~p local-file [ remote-file ] ~t remote-file [ local-file ]

There is no error checking, so another protocol, like zmodem, should probably be used.

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.

This section provides a fast overview of setting up the serial console. This procedure can be used when the dumb terminal is connected to COM1.

If a different configuration is required, see the next section for a more in-depth configuration explanation.

This section provides a more detailed explanation of the steps needed to setup a serial console in FreeBSD.

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 loader(8) for more information.

By default, the serial port settings are 115200 baud, 8 bits, no parity, and 1 stop bit. To change the default console speed, use one of the following options:

To 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 Configuring the FreeBSD Kernel. 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 ddb(4) and ddb(8) for more information about the kernel debugger.