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. Because 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 sio “silo” errors at 57.6 Kbps.

The configuration of /etc/ttys is similar to Example 26.1, “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:

ttyu0   "/usr/libexec/getty xxx"   dialup on

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:

ttyu0   "/usr/libexec/getty std.19200"   dialup on

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:

ttyu0   "/usr/libexec/getty V19200"   dialup on

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

# kill -HUP 1

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:

# stty -f /dev/ttyu1.init crtscts
# stty -f /dev/cuau1.init crtscts

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:

  114 ??  I      0:00.10 /usr/libexec/getty V19200 ttyu0

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.