Now let's spend some time looking at the two most useful serial device
configuration utilities: setserial and
The kernel will make its best effort to correctly determine how your
serial hardware is configured, but the variations on serial device
configuration makes this determination difficult to achieve 100
percent reliably in practice. A good example of where this is a
problem is the internal modems we talked about earlier. The UART they
use has a 16-byte FIFO buffer, but it looks like a 16450 UART to the
kernel device driver: unless we specifically tell the driver that this
port is a 16550 device, the kernel will not make use of the extended
buffer. Yet another example is that of the dumb 4-port cards that
allow sharing of a single IRQ among a number of serial devices. We may
have to specifically tell the kernel which IRQ port it's supposed to
use, and that IRQs may be shared.
setserial was created to configure the serial driver
at runtime. The setserial command is most commonly
executed at boot time from a script called 0setserial
on some distributions, and rc.serial on others. This
script is charged with the responsibility of initializing the serial driver
to accommodate any nonstandard or unusual serial hardware in the machine.
The general syntax for the setserial command is:
setserial device [parameters]
in which the device is one of the serial devices, such as
The setserial command has a large number of
parameters. The most common of these are described in Table 4-1. For information on the
remainder of the parameters, you should refer to the
setserial manual page.
Table 4-1. setserial Command-Line Parameters
Specify the I/O port address of the serial device. Port numbers
should be specified in hexadecimal notation, e.g., 0x2f8.
Specify the interrupt request line the serial device is
Specify the UART type of the serial device. Common values are
16450, 16550, etc. Setting this
value to none will disable this serial device.
Specifying this parameter instructs the kernel serial driver that
this port is one port of an AST Fourport card.
Program the UART to use a speed of 57.6 kbps when a process
requests 38.4 kbps.
Program the UART to use a speed of 115 kbps when a process
requests 38.4 kbps.
Program the UART to use the default speed of 38.4 kbps when
requested. This parameter is used to reverse the effect of a
spd_hi or spd_vhi performed on
the specified serial device.
This parameter will cause the kernel to attempt to automatically
determine the IRQ of the specified device. This attempt may not be
completely reliable, so it is probably better to think of this as a
request for the kernel to guess the IRQ. If you know the IRQ of the
device, you should specify that it use the irq
This parameter must be specified in conjunction with the
port parameter. When this parameter is supplied,
setserial instructs the kernel to attempt to
automatically determine the UART type located at the supplied port
address. If the auto_irq parameter is also
supplied, the kernel attempts to automatically determine the IRQ,
This parameter instructs the kernel not to bother performing
the UART type test during auto-configuration. This is necessary when
the UART is incorrectly detected by the kernel.
A typical and simple rc file to configure your serial
ports at boot time might look something like that shown in
Example 4-1. Most Linux distributions
will include something slightly more sophisticated than this one.
Example 4-1. Example rc.serial setserial Commands
# /etc/rc.serial - serial line configuration script.
# Configure serial devices
/sbin/setserial /dev/ttyS0 auto_irq skip_test autoconfig
/sbin/setserial /dev/ttyS1 auto_irq skip_test autoconfig
/sbin/setserial /dev/ttyS2 auto_irq skip_test autoconfig
/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig
# Display serial device configuration
/sbin/setserial -bg /dev/ttyS*
The -bg /dev/ttyS* argument in the last
command will print a neatly formatted summary of the hardware configuration
of all active serial devices. The output will look like that shown in
Example 4-2. Output of setserial -bg /dev/ttyS Command
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
The name stty probably means “set tty,”
but the stty command can also be used to display a
terminal's configuration. Perhaps even more so than
setserial, the stty command
provides a bewildering number of characteristics you can
configure. We'll cover the most important of these in a moment. You
can find the rest described in the stty manual
The stty command is most commonly used to configure
terminal parameters, such as whether characters will be echoed or what
key should generate a break signal. We explained earlier that serial
devices are tty devices and the stty command is therefore
equally applicable to them.
One of the more important uses of the stty for
serial devices is to enable hardware handshaking on the device. We
talked briefly about hardware handshaking earlier. The default
configuration for serial devices is for hardware handshaking to be
disabled. This setting allows “three wire” serial cables to work;
they don't support the necessary signals for hardware handshaking, and
if it were enabled by default, they'd be unable to transmit any
characters to change it.
Surprisingly, some serial communications programs don't enable
hardware handshaking, so if your modem supports hardware handshaking,
you should configure the modem to use it (check your modem manual for
what command to use), and also configure your serial device to use
it. The stty command has a
crtscts flag that enables hardware handshaking on a
device; you'll need to use this. The command is probably best issued
from the rc.serial file (or equivalent) at boot
time using commands like those shown in Example 4-3.
Example 4-3. Example rc.serial stty Commands
stty crtscts < /dev/ttyS0
stty crtscts < /dev/ttyS1
stty crtscts < /dev/ttyS2
stty crtscts < /dev/ttyS3
The stty command works on the current terminal by
default, but by using the input redirection (“<”)
feature of the shell, we can have stty manipulate
any tty device. It's a common mistake to forget whether you are
supposed to use “<” or “>”; modern
versions of the stty command have a much cleaner
syntax for doing this. To use the new syntax, we'd rewrite our sample
configuration to look like that shown in Example 4-4.
Example 4-4. Example rc.serial stty Commands Using Modern Syntax
stty crtscts -F /dev/ttyS0
stty crtscts -F /dev/ttyS1
stty crtscts -F /dev/ttyS2
stty crtscts -F /dev/ttyS3
We mentioned that the stty command can be used to display
the terminal configuration parameters of a tty device. To display all of
the active settings on a tty device, use:
The output of this command, shown in
Example 4-5, gives you the status of all flags
for that device; a flag shown with a preceding minus, as in
–crtscts, means that the flag has been
Example 4-5. Output of stty -a Command
speed 19200 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon
-ixoff -iuclc -ixany -imaxbel
-opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0
bs0 vt0 ff0
-isig -icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop
-echoprt echoctl echoke
A description of the most important of these flags is given in Table 4-2. Each of these flags is enabled
by supplying it to stty and disabled by supplying
it to stty with the – character in front of
it. Thus, to disable hardware handshaking on the
ttyS0 device, you would use:
$ stty -crtscts -F /dev/ttyS0
Table 4-2. stty Flags Most Relevant to Configuring Serial Devices
Set the line speed to N bits per second.
Enable/Disable hardware handshaking.
Enable/Disable XON/XOFF flow control.
Enable/Disable modem control signals such as DTR/DTS and DCD. This
is necessary if you are using a “three wire” serial cable because it does not supply these signals.
|cs5 cs6 cs7 cs8|
Set number of data bits to 5, 6, 7, or 8, respectively.
Enable odd parity. Disabling this flag enables even parity.
Enable parity checking. When this flag is negated, no parity is used.
Enable use of two stop bits per character. When this flag is
negated, one stop bit per character is used.
Enable/Disable echoing of received characters back to sender.
The next example combines some of these flags and sets the
ttyS0 device to 19,200 bps, 8 data bits, no
parity, and hardware handshaking with echo disabled:
$ stty 19200 cs8 -parenb crtscts -echo -F /dev/ttyS0