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.
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.
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.
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
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. |