Chapter 14. Serial Communications

This section answers common questions about serial communications with FreeBSD. PPP is covered in the Networking section.

14.1. Which multi-port serial cards are supported by FreeBSD?
14.2. How do I get the boot: prompt to show on the serial console?
14.3. How do I tell if FreeBSD found my serial ports or modem cards?
14.4. How do I access the serial ports on FreeBSD?
14.5. How do I enable support for a multiport serial card?
14.6. Can I set the default serial parameters for a port?
14.7. How can I enable dialup logins on my modem?
14.8. How can I connect a dumb terminal to my FreeBSD box?
14.9. Why can I not run tip or cu?

14.1.

Which multi-port serial cards are supported by FreeBSD?

There is a list of these in the Serial Communications chapter of the handbook.

Most multi-port PCI cards that are based on 16550 or clones are supported with no extra effort.

Some unnamed clone cards have also been known to work, especially those that claim to be AST compatible.

Check uart(4) and sio(4) to get more information on configuring such cards.

14.2.

How do I get the boot: prompt to show on the serial console?

See this section of the handbook.

14.3.

How do I tell if FreeBSD found my serial ports or modem cards?

As the FreeBSD kernel boots, it will probe for the serial ports in your system for which the kernel was configured. You can either watch your system closely for the messages it prints or run this command after your system is up and running:

% dmesg | grep -E "^sio[0-9]"

Here is some example output from the above command:

sio0: <16550A-compatible COM port> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
sio0: type 16550A
sio1: <16550A-compatible COM port> port 0x2f8-0x2ff irq 3 on acpi0
sio1: type 16550A

This shows two serial ports. The first is on IRQ 4, is using port address 0x3f8, and has a 16550A-type UART chip. The second uses the same kind of chip but is on IRQ 3 and is at port address 0x2f8. Internal modem cards are treated just like serial ports — except that they always have a modem attached to the port.

The GENERIC kernel includes support for two serial ports using the same IRQ and port address settings in the above example. If these settings are not right for your system, or if you have added modem cards or have more serial ports than your kernel is configured for, just reconfigure your kernel. See section about building a kernel for more details.

14.4.

How do I access the serial ports on FreeBSD?

The third serial port, sio2 (see sio(4), known as COM3 in DOS), is on /dev/cuad2 for dial-out devices, and on /dev/ttyd2 for dial-in devices. What is the difference between these two classes of devices?

You use ttydX for dial-ins. When opening /dev/ttydX in blocking mode, a process will wait for the corresponding cuadX device to become inactive, and then wait for the carrier detect line to go active. When you open the cuadX device, it makes sure the serial port is not already in use by the ttydX device. If the port is available, it steals it from the ttydX device. Also, the cuadX device does not care about carrier detect. With this scheme and an auto-answer modem, you can have remote users log in and you can still dial out with the same modem and the system will take care of all the conflicts.

14.5.

How do I enable support for a multiport serial card?

Again, the section on kernel configuration provides information about configuring your kernel. For a multiport serial card, place an sio(4) line for each serial port on the card in the device.hints(5) file. But place the IRQ specifiers on only one of the entries. All of the ports on the card should share one IRQ. For consistency, use the last serial port to specify the IRQ. Also, specify the following option in the kernel configuration file:

options COM_MULTIPORT

The following /boot/device.hints example is for an AST 4-port serial card on IRQ 12:

hint.sio.4.at="isa"
hint.sio.4.port="0x2a0"
hint.sio.4.flags="0x701"
hint.sio.5.at="isa"
hint.sio.5.port="0x2a8"
hint.sio.5.flags="0x701"
hint.sio.6.at="isa"
hint.sio.6.port="0x2b0"
hint.sio.6.flags="0x701"
hint.sio.7.at="isa"
hint.sio.7.port="0x2b8"
hint.sio.7.flags="0x701"
hint.sio.7.irq="12"

The flags indicate that the master port has minor number 7 (0x700), and all the ports share an IRQ (0x001).

14.6.

Can I set the default serial parameters for a port?

See the Serial Communications section in the FreeBSD Handbook.

14.7.

How can I enable dialup logins on my modem?

Please read the section about Dial-in Services in the FreeBSD Handbook.

14.8.

How can I connect a dumb terminal to my FreeBSD box?

You can find this information in the Terminals section of the FreeBSD Handbook.

14.9.

Why can I not run tip or cu?

On your system, the programs tip(1) and cu(1) can only access the /var/spool/lock directory via user uucp and group dialer. You can use the group dialer to control who has access to your modem or remote systems. Just add yourself to group dialer.

Alternatively, you can let everyone on your system run tip(1) and cu(1) by typing:

# chmod 4511 /usr/bin/cu
# chmod 4511 /usr/bin/tip

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

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