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 21 options: depend on the driver. For the serial port this 22 defines the baudrate/parity/bits of the port, 23 in the format BBBBPN, where BBBB is the speed, 24 P is parity (n/o/e), and N is bits. Default is 25 9600n8. The maximum baudrate is 115200. 26 27You can specify multiple console= options on the kernel command line. 28Output will appear on all of them. The last device will be used when 29you open /dev/console. So, for example: 30 31 console=ttyS1,9600 console=tty0 32 33defines that opening /dev/console will get you the current foreground 34virtual console, and kernel messages will appear on both the VGA 35console and the 2nd serial port (ttyS1 or COM2) at 9600 baud. 36 37Note that you can only define one console per device type (serial, video). 38 39If no console device is specified, the first device found capable of 40acting as a system console will be used. At this time, the system 41first looks for a VGA card and then for a serial port. So if you don't 42have a VGA card in your system the first serial port will automatically 43become the console. 44 45You will need to create a new device to use /dev/console. The official 46/dev/console is now character device 5,1. 47 48Here's an example that will use /dev/ttyS1 (COM2) as the console. 49Replace the sample values as needed. 50 511. Create /dev/console (real console) and /dev/tty0 (master virtual 52 console): 53 54 cd /dev 55 rm -f console tty0 56 mknod -m 622 console c 5 1 57 mknod -m 622 tty0 c 4 0 58 592. LILO can also take input from a serial device. This is a very 60 useful option. To tell LILO to use the serial port: 61 In lilo.conf (global section): 62 63 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits) 64 653. Adjust to kernel flags for the new kernel, 66 again in lilo.conf (kernel section) 67 68 append = "console=ttyS1,9600" 69 704. Make sure a getty runs on the serial port so that you can login to 71 it once the system is done booting. This is done by adding a line 72 like this to /etc/inittab (exact syntax depends on your getty): 73 74 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100 75 765. Init and /etc/ioctl.save 77 78 Sysvinit remembers its stty settings in a file in /etc, called 79 `/etc/ioctl.save'. REMOVE THIS FILE before using the serial 80 console for the first time, because otherwise init will probably 81 set the baudrate to 38400 (baudrate of the virtual console). 82 836. /dev/console and X 84 Programs that want to do something with the virtual console usually 85 open /dev/console. If you have created the new /dev/console device, 86 and your console is NOT the virtual console some programs will fail. 87 Those are programs that want to access the VT interface, and use 88 /dev/console instead of /dev/tty0. Some of those programs are: 89 90 Xfree86, svgalib, gpm, SVGATextMode 91 92 It should be fixed in modern versions of these programs though. 93 94 Note that if you boot without a console= option (or with 95 console=/dev/tty0), /dev/console is the same as /dev/tty0. In that 96 case everything will still work. 97 987. Thanks 99 100 Thanks to Geert Uytterhoeven <geert@linux-m68k.org> 101 for porting the patches from 2.1.4x to 2.1.6x for taking care of 102 the integration of these patches into m68k, ppc and alpha. 103 104Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000 105