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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 a serial console speeds of 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second.

The kernel supports a much wider range of serial bit rates when the serial interface is not being used as a serial console.[1]

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 different range of speeds than are 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 UART semiconductor chip. Early UARTs without on-chip receive buffers could only reliably receive at up to 14400bps, this includes models 8250A, 82510, 16450 and 16550 (with no A). Recent UARTs with receive buffers will work at all serial console bit rates, this includes models 16550A, 16552, 16650, 16654, 16750, 16850 and 16950.

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.[2]

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.

Caution

The kernel's CTS/RTS flow control is currently buggy. Machines can take a significant time to write console messages if flow control is enabled but CTS will never be asserted (as occurs when there is no call present on a modem or no session on a null modem cable or cable to a terminal server). As a result of the large number of kernel messages when the kernel is started a machine configured with the kernel's CTS/RTS flow control can take many minutes to reboot.

The kernel's CTS/RTS flow control cannot be recommended at this time. The HOWTO's author has a kernel patch available which he is seeking to have included in the mainstream kernel source code.

The CTS/RTS flow control in user-space applications does not share the kernel's bugs and CTS/RTS flow control is still recommended for getty.

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

<flow_control> ::= <nil> | r

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 leads to the kernel messages being sent to a modem which is idle and in command mode.

The console's slack interpretation of CTS, DSR and DCD makes it impossible to connect a serial console to an RS-232 multi-drop circuit. Multi-drop circuits have more than two computers on the circuit; they are traditionally four-wire, satelite or wireless services.

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 needs 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 9600n8 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 9600n8 will know what to alter.

Notes

[1]

There is no good reason for this difference. Feel free to submit a patch to the linux-kernel mailing list to correct this oddity.

[2]

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.