.. _serial_console:
  
  Linux Serial Console
  ====================
  
  To use a serial port as console you need to compile the support into your
  kernel - by default it is not compiled in. For PC style serial ports
  it's the config option next to menu option:
  
  :menuselection:`Character devices --> Serial drivers --> 8250/16550 and compatible serial support --> Console on 8250/16550 and compatible serial port`
  
  You must compile serial support into the kernel and not as a module.
  
  It is possible to specify multiple devices for console output. You can
  define a new kernel command line option to select which device(s) to
  use for console output.
  
  The format of this option is::
  
  	console=device,options
  
  	device:		tty0 for the foreground virtual console
  			ttyX for any other virtual console
  			ttySx for a serial port
  			lp0 for the first parallel port
  			ttyUSB0 for the first USB serial device
  
  	options:	depend on the driver. For the serial port this
  			defines the baudrate/parity/bits/flow control of
  			the port, in the format BBBBPNF, where BBBB is the
  			speed, P is parity (n/o/e), N is number of bits,
  			and F is flow control ('r' for RTS). Default is
  			9600n8. The maximum baudrate is 115200.
  
  You can specify multiple console= options on the kernel command line.
  Output will appear on all of them. The last device will be used when
  you open ``/dev/console``. So, for example::
  
  	console=ttyS1,9600 console=tty0
  
  defines that opening ``/dev/console`` will get you the current foreground
  virtual console, and kernel messages will appear on both the VGA
  console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
  
  Note that you can only define one console per device type (serial, video).
  
  If no console device is specified, the first device found capable of
  acting as a system console will be used. At this time, the system
  first looks for a VGA card and then for a serial port. So if you don't
  have a VGA card in your system the first serial port will automatically
  become the console.
  
  You will need to create a new device to use ``/dev/console``. The official
  ``/dev/console`` is now character device 5,1.
  
  (You can also use a network device as a console.  See
  ``Documentation/networking/netconsole.txt`` for information on that.)
  
  Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
  Replace the sample values as needed.
  
  1. Create ``/dev/console`` (real console) and ``/dev/tty0`` (master virtual
     console)::
  
       cd /dev
       rm -f console tty0
       mknod -m 622 console c 5 1
       mknod -m 622 tty0 c 4 0
  
  2. LILO can also take input from a serial device. This is a very
     useful option. To tell LILO to use the serial port:
     In lilo.conf (global section)::
  
       serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
  
  3. Adjust to kernel flags for the new kernel,
     again in lilo.conf (kernel section)::
  
       append = "console=ttyS1,9600"
  
  4. Make sure a getty runs on the serial port so that you can login to
     it once the system is done booting. This is done by adding a line
     like this to ``/etc/inittab`` (exact syntax depends on your getty)::
  
       S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
  
  5. Init and ``/etc/ioctl.save``
  
     Sysvinit remembers its stty settings in a file in ``/etc``, called
     ``/etc/ioctl.save``. REMOVE THIS FILE before using the serial
     console for the first time, because otherwise init will probably
     set the baudrate to 38400 (baudrate of the virtual console).
  
  6. ``/dev/console`` and X
     Programs that want to do something with the virtual console usually
     open ``/dev/console``. If you have created the new ``/dev/console`` device,
     and your console is NOT the virtual console some programs will fail.
     Those are programs that want to access the VT interface, and use
     ``/dev/console instead of /dev/tty0``. Some of those programs are::
  
       Xfree86, svgalib, gpm, SVGATextMode
  
     It should be fixed in modern versions of these programs though.
  
     Note that if you boot without a ``console=`` option (or with
     ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
     In that case everything will still work.
  
  7. Thanks
  
     Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
     for porting the patches from 2.1.4x to 2.1.6x for taking care of
     the integration of these patches into m68k, ppc and alpha.
  
  Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000