1.. _serial_console:
  2
  3Linux Serial Console
  4====================
  5
  6To use a serial port as console you need to compile the support into your
  7kernel - by default it is not compiled in. For PC style serial ports
  8it's the config option next to menu option:
  9
 10:menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
 11
 12You must compile serial support into the kernel and not as a module.
 13
 14It is possible to specify multiple devices for console output. You can
 15define a new kernel command line option to select which device(s) to
 16use for console output.
 17
 18The format of this option is::
 19
 20	console=device,options
 21
 22	device:		tty0 for the foreground virtual console
 23			ttyX for any other virtual console
 24			ttySx for a serial port
 25			lp0 for the first parallel port
 26			ttyUSB0 for the first USB serial device
 27
 28	options:	depend on the driver. For the serial port this
 29			defines the baudrate/parity/bits/flow control of
 30			the port, in the format BBBBPNF, where BBBB is the
 31			speed, P is parity (n/o/e), N is number of bits,
 32			and F is flow control ('r' for RTS). Default is
 33			9600n8. The maximum baudrate is 115200.
 34
 35You can specify multiple console= options on the kernel command line.
 36Output will appear on all of them. The last device will be used when
 37you open ``/dev/console``. So, for example::
 38
 39	console=ttyS1,9600 console=tty0
 40
 41defines that opening ``/dev/console`` will get you the current foreground
 42virtual console, and kernel messages will appear on both the VGA
 43console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
 44
 45Note that you can only define one console per device type (serial, video).
 46
 47If no console device is specified, the first device found capable of
 48acting as a system console will be used. At this time, the system
 49first looks for a VGA card and then for a serial port. So if you don't
 50have a VGA card in your system the first serial port will automatically
 51become the console.
 52
 53You will need to create a new device to use ``/dev/console``. The official
 54``/dev/console`` is now character device 5,1.
 55
 56(You can also use a network device as a console.  See
 57``Documentation/networking/netconsole.txt`` for information on that.)
 58
 59Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
 60Replace the sample values as needed.
 61
 621. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
 63   console)::
 64
 65     cd /dev
 66     rm -f console tty0
 67     mknod -m 622 console c 5 1
 68     mknod -m 622 tty0 c 4 0
 69
 702. LILO can also take input from a serial device. This is a very
 71   useful option. To tell LILO to use the serial port:
 72   In lilo.conf (global section)::
 73
 74     serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
 75
 763. Adjust to kernel flags for the new kernel,
 77   again in lilo.conf (kernel section)::
 78
 79     append = "console=ttyS1,9600"
 80
 814. Make sure a getty runs on the serial port so that you can login to
 82   it once the system is done booting. This is done by adding a line
 83   like this to ``/etc/inittab`` (exact syntax depends on your getty)::
 84
 85     S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
 86
 875. Init and ``/etc/ioctl.save``
 88
 89   Sysvinit remembers its stty settings in a file in ``/etc``, called
 90   ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
 91   console for the first time, because otherwise init will probably
 92   set the baudrate to 38400 (baudrate of the virtual console).
 93
 946. ``/dev/console`` and X
 95   Programs that want to do something with the virtual console usually
 96   open ``/dev/console``. If you have created the new ``/dev/console`` device,
 97   and your console is NOT the virtual console some programs will fail.
 98   Those are programs that want to access the VT interface, and use
 99   ``/dev/console instead of /dev/tty0``. Some of those programs are::
100
101     Xfree86, svgalib, gpm, SVGATextMode
102
103   It should be fixed in modern versions of these programs though.
104
105   Note that if you boot without a ``console=`` option (or with
106   ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
107   In that case everything will still work.
108
1097. Thanks
110
111   Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
112   for porting the patches from 2.1.4x to 2.1.6x for taking care of
113   the integration of these patches into m68k, ppc and alpha.
114
115Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000