Chapter 30. Serial Communications | FreeBSD Documentation Portal
Description: This chapter covers some of the ways serial communications can be used on FreeBSD
Keywords: serial,communications,terminal,modem,console

# Chapter 30. Serial Communications

## 30.1. Synopsis

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 FreeBSD.

Read this chapter to learn:

- How to connect terminals to a FreeBSD system.

- How to use a modem to dial out to remote hosts.

- How to allow remote users to login to a FreeBSD system with a modem.

- How to boot a FreeBSD system from a serial console.

Before reading this chapter:

- Know how to configure and install a custom kernel.

- Understand FreeBSD permissions and processes.

- Have access to the technical manual for the serial hardware to be used with FreeBSD.

## 30.2. Serial Terminology and Hardware

The following terms are often used in serial communications:

bps

  Bits per Second (bps) is the rate at which data is transmitted.

DTE

  Data Terminal Equipment (DTE) is one of two endpoints in a serial communication. An example would be a computer.

DCE

  Data Communications Equipment (DCE) is the other endpoint in a serial communication. Typically, it is a modem or serial terminal.

RS-232

  The 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 FreeBSD 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.

### 30.2.1. Serial Cables and Ports

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, 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.

Table 1. RS-232C Signal Names
Acronyms | Names
--- | ---
RD | Received Data
TD | Transmitted Data
DTR | Data Terminal Ready
DSR | Data Set Ready
DCD | Data Carrier Detect
SG | Signal Ground
RTS | Request to Send
CTS | Clear to Send

Table 2. DB-25 to DB-25 Null-Modem Cable
Signal | Pin # |  | Pin # | Signal
--- | --- | --- | --- | ---
SG | 7 | connects to | 7 | SG
TD | 2 | connects to | 3 | RD
RD | 3 | connects to | 2 | TD
RTS | 4 | connects to | 5 | CTS
CTS | 5 | connects to | 4 | RTS
DTR | 20 | connects to | 6 | DSR
DTR | 20 | connects to | 8 | DCD
DSR | 6 | connects to | 20 | DTR
DCD | 8 | connects to | 20 | DTR

Table 3. DB-9 to DB-9 Null-Modem Cable
Signal | Pin # |  | Pin # | Signal
--- | --- | --- | --- | ---
RD | 2 | connects to | 3 | TD
TD | 3 | connects to | 2 | RD
DTR | 4 | connects to | 6 | DSR
DTR | 4 | connects to | 1 | DCD
SG | 5 | connects to | 5 | SG
DSR | 6 | connects to | 4 | DTR
DCD | 1 | connects to | 4 | DTR
RTS | 7 | connects to | 8 | CTS
CTS | 8 | connects to | 7 | RTS

Table 4. DB-9 to DB-25 Null-Modem Cable
Signal | Pin # |  | Pin # | Signal
--- | --- | --- | --- | ---
RD | 2 | connects to | 2 | TD
TD | 3 | connects to | 3 | RD
DTR | 4 | connects to | 6 | DSR
DTR | 4 | connects to | 8 | DCD
SG | 5 | connects to | 7 | SG
DSR | 6 | connects to | 20 | DTR
DCD | 1 | connects to | 20 | DTR
RTS | 7 | connects to | 5 | CTS
CTS | 8 | connects to | 4 | RTS

| 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 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:

- 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 FreeBSD versions 8.X and higher and /dev/cuadN on FreeBSD 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.

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.

### 30.2.2. Serial Port Configuration

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`:

    # grep uart /var/run/dmesg.boot

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:

    # stty -a -f /dev/ttyu1

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:

    # stty -f /dev/ttyu5.init clocal cs8 ixon ixoff

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:

    # stty -f /dev/ttyu5.lock 57600

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

## 30.3. Terminals

Terminals provide a convenient and low-cost way to access a FreeBSD system when not at the computer’s console or on a connected network. This section describes how to use terminals with FreeBSD.

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 FreeBSD. 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 FreeBSD system. An older spare computer can be used as a terminal wired into a more powerful computer running FreeBSD. This can turn what might otherwise be a single-user computer into a powerful multiple-user system.

FreeBSD supports three types of terminals:

Dumb terminals

  Dumb 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 FreeBSD. 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 Terminals

  Since 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 FreeBSD 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 FreeBSD system.

  There are at least two utilities in the base-system of FreeBSD that can be used to work through a serial connection: cu(1) and tip(1).

  For example, to connect from a client system that runs FreeBSD to the serial connection of another system:

      # cu -l /dev/cuauN

  Ports 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 Terminals

  X 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.

### 30.3.1. Terminal Configuration

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:

    ttyu0   "/usr/libexec/getty std.115200"   dialup  off secure
    ttyu1   "/usr/libexec/getty std.115200"   dialup  off secure
    ttyu2   "/usr/libexec/getty std.115200"   dialup  off secure
    ttyu3   "/usr/libexec/getty std.115200"   dialup  off secure

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.

Example 1. Configuring Terminal Entries

    ttyu1  "/usr/libexec/getty std.38400"  wy50   on insecure
    ttyu5   "/usr/libexec/getty std.19200"  vt100  on insecure

The 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 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 “The Superuser Account”. 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:

    # kill -HUP 1

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.

### 30.3.2. Troubleshooting the Connection

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:

    # ps -axww|grep ttyu
    22189  d1  Is+    0:00.03 /usr/libexec/getty std.38400 ttyu1

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."

## 30.4. Dial-in Service

Configuring a FreeBSD system for dial-in service is similar to configuring terminals, except that modems are used instead of terminal devices. FreeBSD 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.

When using an external modem, a proper cable is needed. A standard RS-232C serial cable should suffice.

FreeBSD 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 Serial Cables and Ports for more information about these signals.

Like other UNIX®-like operating systems, FreeBSD 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. FreeBSD avoids sending commands to the modem or watching for status reports from the modem.

FreeBSD 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.

### 30.4.1. Modem Configuration

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.

    #
    # 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 `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:

    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

### 30.4.2. Troubleshooting

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:

- 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 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.

## 30.5. Dial-out Service

The 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.

### 30.5.1. Using a Stock Hayes Modem

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`.

### 30.5.2. Using `AT` Commands

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:

    cuau0:dv=/dev/cuau0:br#19200:pa=none

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:

    # cu -lline -sspeed

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.

### 30.5.3. The `@` Sign Does Not Work

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:

    pn=\@

### 30.5.4. Dialing from the Command Line

Put 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:

    # tip -115200 5551234

Users 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:

    # cu 5551234 -s 115200

### 30.5.5. Setting the bps Rate

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.

### 30.5.6. Accessing a Number of Hosts Through a Terminal Server

Rather than waiting until connected and typing `CONNECT host` each time, use `tip`'s `cm` capability. For example, these entries in /etc/remote allow typing `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:

### 30.5.7. Using More Than One Line with `tip`

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:

    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 5551114

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

### 30.5.8. Using the Force Character

`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:

    force=single-char

### 30.5.9. Upper Case Characters

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`:

    force=^^
    raisechar=^^

The `^^` is `Shift`+`Ctrl`+`6`.

### 30.5.10. File Transfers with `tip`

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.

### 30.5.11. 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.

## 30.6. Setting Up the Serial Console

FreeBSD 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 FreeBSD on machines that have no keyboard or monitor attached, and developers who want to debug the kernel or device drivers.

As described in The FreeBSD Booting Process, FreeBSD employs a three stage bootstrap. The first two stages are in the boot block code which is stored at the beginning of the FreeBSD 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.

### 30.6.1. Quick Serial Console Configuration

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.

Procedure: Configuring a Serial Console on COM1

1. Connect the serial cable to COM1 and the controlling terminal.

2. To configure boot messages to display on the serial console, issue the following command as the superuser:

       # echo 'console="comconsole"' >> /boot/loader.conf

3. Edit /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.

4. 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.

### 30.6.2. In-Depth Serial Console Configuration

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

Procedure: Configuring a Serial Console

1. Prepare a serial cable.

   Use either a null-modem cable or a standard serial cable and a null-modem adapter. See Serial Cables and Ports for a discussion on serial cables.

2. 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 PS/2® mouse, unplug it as well. PS/2® 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.

3. Plug a dumb terminal, an old computer with a modem program, or the serial port on another UNIX® box into the serial port.

4. Add the appropriate `hint.uart.*` entries to /boot/device.hints for the serial port. Some multi-port cards also require kernel configuration options. Refer to uart(4) for the required options and device hints for each supported serial port.

5. 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:

   `-h`

     Toggles between the internal and serial consoles. Use this to switch console devices. For instance, to boot from the internal (video) console, use `-h` 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 `-h` to tell the boot loader and the kernel to use the video display as the console instead.

   `-D`

     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 `-h`. 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 `-h`. However, the dual console configuration takes effect only while the boot block is running. Once the boot loader gets control, the console specified by `-h` becomes the only console.

   `-P`

     Makes the boot block probe the keyboard. If no keyboard is found, the `-D` and `-h` options are automatically set.

     | Due to space constraints in the current version of the boot blocks, `-P` 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 `-P`.

     Use either `-P` to select the console automatically or `-h` to activate the serial console. Refer to boot(8) and boot.config(5) for more details.

     The options, except for `-P`, 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 `-h`. This means that if `-D` is specified but `-h` 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.

6. Boot the machine.

   When FreeBSD starts, the boot blocks echo the contents of /boot.config to the console. For example:

       /boot.config: -P
       Keyboard: no

   The second line appears only if `-P` 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:

   Options | Message goes to
   --- | ---
   none | internal console
   `-h` | serial console
   `-D` | serial and internal consoles
   `-Dh` | serial and internal consoles
   `-P`, keyboard present | internal console
   `-P`, keyboard absent | serial console

   After 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 `-h` 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 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 `-h` in the boot block. See loader.conf(5) for more information. At the moment, the boot loader has no option equivalent to `-P` 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 Terminal Configuration. If the speed of the serial port has been changed, change `std.115200` to match the new setting.

### 30.6.3. Setting a Faster Serial Port Speed

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:

- 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:

      # cd /sys/boot
      # make clean
      # make
      # make install

  If the serial console is configured in some other way than by booting with `-h`, 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=19200

- Add the `-S19200` 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"

### 30.6.4. Entering the DDB Debugger from the Serial Line

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.

    options BREAK_TO_DEBUGGER
    options DDB

Last modified on: July 7, 2026 by Vladlen Popolitov

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