linux/Documentation/serial-console.txt
<<
>>
Prefs
   1                       Linux Serial Console
   2
   3To use a serial port as console you need to compile the support into your
   4kernel - by default it is not compiled in. For PC style serial ports
   5it's the config option next to "Standard/generic (dumb) serial support".
   6You must compile serial support into the kernel and not as a module.
   7
   8It is possible to specify multiple devices for console output. You can
   9define a new kernel command line option to select which device(s) to
  10use for console output.
  11
  12The format of this option is:
  13
  14        console=device,options
  15
  16        device:         tty0 for the foreground virtual console
  17                        ttyX for any other virtual console
  18                        ttySx for a serial port
  19                        lp0 for the first parallel port
  20                        ttyUSB0 for the first USB serial device
  21
  22        options:        depend on the driver. For the serial port this
  23                        defines the baudrate/parity/bits/flow control of
  24                        the port, in the format BBBBPNF, where BBBB is the
  25                        speed, P is parity (n/o/e), N is number of bits,
  26                        and F is flow control ('r' for RTS). Default is
  27                        9600n8. The maximum baudrate is 115200.
  28
  29You can specify multiple console= options on the kernel command line.
  30Output will appear on all of them. The last device will be used when
  31you open /dev/console. So, for example:
  32
  33        console=ttyS1,9600 console=tty0
  34
  35defines that opening /dev/console will get you the current foreground
  36virtual console, and kernel messages will appear on both the VGA
  37console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
  38
  39Note that you can only define one console per device type (serial, video).
  40
  41If no console device is specified, the first device found capable of
  42acting as a system console will be used. At this time, the system
  43first looks for a VGA card and then for a serial port. So if you don't
  44have a VGA card in your system the first serial port will automatically
  45become the console.
  46
  47You will need to create a new device to use /dev/console. The official
  48/dev/console is now character device 5,1.
  49
  50(You can also use a network device as a console.  See
  51Documentation/networking/netconsole.txt for information on that.)
  52
  53Here's an example that will use /dev/ttyS1 (COM2) as the console.
  54Replace the sample values as needed.
  55
  561. Create /dev/console (real console) and /dev/tty0 (master virtual
  57   console):
  58
  59   cd /dev
  60   rm -f console tty0
  61   mknod -m 622 console c 5 1
  62   mknod -m 622 tty0 c 4 0
  63
  642. LILO can also take input from a serial device. This is a very
  65   useful option. To tell LILO to use the serial port:
  66   In lilo.conf (global section): 
  67
  68   serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
  69
  703. Adjust to kernel flags for the new kernel,
  71   again in lilo.conf (kernel section)
  72
  73   append = "console=ttyS1,9600" 
  74
  754. Make sure a getty runs on the serial port so that you can login to
  76   it once the system is done booting. This is done by adding a line
  77   like this to /etc/inittab (exact syntax depends on your getty):
  78
  79   S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
  80
  815. Init and /etc/ioctl.save
  82
  83   Sysvinit remembers its stty settings in a file in /etc, called
  84   `/etc/ioctl.save'. REMOVE THIS FILE before using the serial
  85   console for the first time, because otherwise init will probably
  86   set the baudrate to 38400 (baudrate of the virtual console).
  87
  886. /dev/console and X
  89   Programs that want to do something with the virtual console usually
  90   open /dev/console. If you have created the new /dev/console device,
  91   and your console is NOT the virtual console some programs will fail.
  92   Those are programs that want to access the VT interface, and use
  93   /dev/console instead of /dev/tty0. Some of those programs are:
  94
  95   Xfree86, svgalib, gpm, SVGATextMode
  96
  97   It should be fixed in modern versions of these programs though.
  98
  99   Note that if you boot without a console= option (or with
 100   console=/dev/tty0), /dev/console is the same as /dev/tty0. In that
 101   case everything will still work.
 102
 1037. Thanks
 104
 105   Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
 106   for porting the patches from 2.1.4x to 2.1.6x for taking care of
 107   the integration of these patches into m68k, ppc and alpha.
 108
 109Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
 110
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.