Remote Serial Console HOWTO
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2.3. Select a serial speed and parameters

This HOWTO does not discuss the RS-232 standard, which is formally known as ANSI/TIA/EIA-232-F-1997 Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Data Interchange. For an explanation of ‘bits per second’, ‘start bits’, ‘data bits’, ‘parity’, ‘stop bits’ and ‘flow control’ refer to the Serial-HOWTO and the Modem-HOWTO.

The description of the command syntax for setting the serial parameters in the kernel, boot loaders and login applications uses the following variables which describe RS-232 parameters.

<speed>

The speed of the serial link in bits per second.

The Linux kernel on a modern PC supports 50, 75, 110, 134.5, 150, 200, 300, 600, 1200, 1800, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second for a serial port being used as a console. Higher bit rates may be possible depending upon the model of the serial port's semiconductor.

Very recent Linux kernels can also offer a serial console using a USB serial dongle at speeds of 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second.

Most boot loaders only support a subset of the range supported by the kernel. LILO 21.7.5 supports 110, 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 56000, 57600 and 115200 bits per second. SYSLINUX 1.67 supports 75 to 56000 bits per second. GRUB 0.90 supports 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second.

You must chose the same speed for both the boot loader and for the Linux kernel. An operating system may use more than one boot loader. For example, Red Hat Linux uses SYSLINUX to install or upgrade the operating system; LILO as the boot loader for Red Hat Linux 7.1 and earlier; and GRUB as the boot loader for Red Hat Linux 7.2 and later.

If you are using a serial terminal or if you are using a dumb modem then the bit rate of the terminal or dumb modem must also match the bit rate selected in the boot loader and kernel.

If the serial console is connected to a Hayes-style modem slower than 9600bps then configure the serial console with the same speed as the modem. Modems faster than 9600bps will generally automatically synchronize to the speed of the serial port.

The selected bit rate must also be supported by the serial port's semiconductor. Early model UARTs such as the 8250 series and the 16450 could only reliably recieve at up to 14400bps. The 16550 series and later models will work at all bit rates.

Unless you have good reason, use the popular bit rate of 9600 bits per second. This is the default bit rate of a great many devices.

The speeds that are supported by the kernel, the three common boot loaders, and all IBM PCs capable of running Linux are: 2400, 4800, 9600 and 19200 bits per second. This is a depressingly small selection: not slow enough to support a call over an international phone circuit and not fast enough to upload large files. You may need to choose a speed that will result in a less robust software configuration.

Figure 2-2. Syntax for serial bits per second rate, in extended Backus-Naur form

<speed> ::=  <digits>
<digits> ::= <digit> | <digit><digits>
<digit> ::= 0 | 1 | … | 9

<parity>

Number of parity bits and the interpretation of a parity bit if one is present.

Allowed values are n for no parity bit, e for one bit of even parity and o for one bit of odd parity.

Using no parity bit and eight data bits is recommended.

If parity is used then even parity is the common choice.

Parity is a simple form of error detection. Modern modems have much better error detection and correction. As a result the parity bit guards only the data on the cable between the modem and the serial port. If this cable has a low error rate, and it should, then the parity bit is not required.

Figure 2-3. Syntax for serial parity, in extended Backus-Naur form

<parity> ::= n | e | o

<data>

The number of data bits per character.

Allowed values are 7 bits or 8 bits, as Linux uses the ASCII character set which requires at least seven bits.

Eight data bits are recommended. This allows the link to easily be used for file transfers and allows non-English text to be presented.

Figure 2-4. Syntax for serial data bits, in extended Backus-Naur form

<data> ::= 7 | 8

<stop>

The number of stop bit-times.[1]

Allowed values are 1 or 2.

One stop bit-time is recommended.

If the RS-232 cable is very long then two stop bit-times may be needed.

You may occassionally see 1.5 stop bit-times. The intent is to gain 4% more data throughput when a link is too long for one stop bit-time but is too short to require two stop bit-times. 1.5 stop bit-times is now rare enough to be a hazard to use.

Figure 2-5. Syntax for serial stop bits, in extended Backus-Naur form

<stop> ::= 1 | 2

<flow_control>

The type of flow control to use.

The Linux kernel allows no flow control and CTS/RTS flow control.

No flow control is the default, this is indicated by omitting <flow_control>.

CTS/RTS flow control is recommended, especially if login access is also provided to the serial port. This is indicated by a <flow_control> of r.

CTS/RTS flow control regulates the flow of chatacters. The computer does not send characters until Clear To Send is asserted by the modem. If the computer is has enough buffering to recieve characters from the modem the computer asserts Ready to Send. Thus neither the computer nor the modem's buffers are filled to overflowing.

The kernel's CTS/RTS flow control is not perfect. Unlike a user application, the kernel cannot wait forever for Clear To Send to be asserted. The kernel tests Clear To Send a large number of times and then sends the message in any case. The kernel does not assert Ready To Send, as the kernel is never recieves key strokes. Of course, a user-space application such as getty can set Ready To Send to recieve keystrokes.

At present the RS-232 status lines are ignored by the kernel. A kernel message will be printed even if Data Carrier Detect and Data Set Ready are not asserted. This is a bug, which probably appeared because it is convenient for kernel developers to avoid the hassle of configuring their terminal emulators correctly.

Figure 2-6. Syntax for serial flow control, in extended Backus-Naur form

<flow_control> ::= <nil> | r

The Linux kernel uses the syntax in Figure 2-7 to describe the serial parameters. Many boot loaders use a variation of the syntax used by the Linux kernel.

Figure 2-7. Syntax for kernel serial parameters, in extended Backus-Naur form

<mode> ::= <speed><parity><data><flow_control>

Note that <mode> does not include <stop>. The kernel assumes the number of stop bits to be one. This shortcoming need to be considered when deploying long RS-232 cables.

Most boot loaders default to 9600n8. A common default found on older terminals is 9600e7.

Use 9600n8r if possible, as this is the default for most Linux software and modern devices.

This HOWTO always configures the serial speed and parameters, even where not strictly necessary. This is so that people configuring parameters other than the recommended and common default value 9600n8r will know what to alter.

Notes

[1]

A bit-time is the time taken to transmit one bit. The distinction between bit-times of signal and bits of data is apparent when you consider that 1.5 bit-times of signal is possible but that 1.5 bits of data is impossible.

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