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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	.. _serial_console:
2	
3	Linux Serial Console
4	====================
5	
6	To use a serial port as console you need to compile the support into your
7	kernel - by default it is not compiled in. For PC style serial ports
8	it'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	
12	You must compile serial support into the kernel and not as a module.
13	
14	It is possible to specify multiple devices for console output. You can
15	define a new kernel command line option to select which device(s) to
16	use for console output.
17	
18	The 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	
35	You can specify multiple console= options on the kernel command line.
36	Output will appear on all of them. The last device will be used when
37	you open ``/dev/console``. So, for example::
38	
39		console=ttyS1,9600 console=tty0
40	
41	defines that opening ``/dev/console`` will get you the current foreground
42	virtual console, and kernel messages will appear on both the VGA
43	console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
44	
45	Note that you can only define one console per device type (serial, video).
46	
47	If no console device is specified, the first device found capable of
48	acting as a system console will be used. At this time, the system
49	first looks for a VGA card and then for a serial port. So if you don't
50	have a VGA card in your system the first serial port will automatically
51	become the console.
52	
53	You 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	
59	Here's an example that will use ``/dev/ttyS1`` (COM2) as the console.
60	Replace the sample values as needed.
61	
62	1. 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	
70	2. 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	
76	3. Adjust to kernel flags for the new kernel,
77	   again in lilo.conf (kernel section)::
78	
79	     append = "console=ttyS1,9600"
80	
81	4. 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	
87	5. 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	
94	6. ``/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	
109	7. 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	
115	Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
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