SERIAL CONSOLE USAGE NOTES
Written by
Bill Paul <wpaul@ctr.columbia.edu>
Revised for 3.0-CURRENT by
Kazutaka Yokota <yokota@zodiac.mech.utsunomiya-u.ac.jp>
The FreeBSD boot block can now be used to boot FreeBSD on a system with
only a dumb terminal on a serial port as a console. This feature
is provided for the benefit of people who wish to install FreeBSD on
dedicated file/compute/terminal server machines that have no keyboard
(or monitor) attached, just as is possible with Sun workstations and
servers. People who don't need this extra functionality shouldn't notice
the changes at all (unless I've screwed something up horribly).
To boot FreeBSD in serial console mode, you must do the following:
- UNPLUG YOUR KEYBOARD. Most PC systems probe for the keyboard during the
Power-On Self-Test (POST) and will generate an error if the keyboard
isn't detected. Some BIOSes may be set to ignore this error. Many other
machines will pause the boot process and wait for you to reattach the
keyboard and press a key before proceeding any further. Consult the manual
of your motherboard to see how it responds to the error and how to
set the BIOS to ignore the error. If your computer complains about
the lack of a keyboard but boots anyway, then you don't have to do
anything special. (One machine with a PHOENIX BIOS that I have here
merely says 'Keyboard failed' then continues to boot normally.) If your
machine complains loudly about the lack of a keyboard and won't continue
to boot until you plug it back in, you'll have to go into your CMOS
configuration menu and change the 'Keyboard' setting to 'Not installed'
in order to bypass the keyboard probe.
NOTE #1:
Setting the keyboard to 'Not installed' in the CMOS configuration
does *NOT* mean that you won't be able to use your keyboard. All this
does is tell the BIOS not to probe for a keyboard at power-on so that
it won't bitch and moan if the keyboard isn't plugged in. You can leave the
keyboard plugged in even with this flag set to 'Not installed' and the
keyboard will still work. I repeat: changing the CMOS 'keyboard' setting
to 'Not installed' only disables the BIOS's keyboard probe; it does
*NOT* actually disable the keyboard.
NOTE #2:
If your system has a PS/2 mouse, chances are very good that you will
need to unplug your mouse as well as your keyboard. This is because
PS/2 mice share some hardware with the keyboard, and leaving the mouse
plugged in can fool the keyboard probe into thinking the keyboard is
still there. I have access to a Gateway 2000 Pentium 90Mhz system with
an AMI BIOS that behaves this way. In general this is not a problem
since the mouse isn't much good without the keyboard anyway.
- PLUG A DUMB TERMINAL INTO COM1. If you don't have a dumb terminal, you
can use an old PC/XT with a modem program, or the serial port on
another UNIX box. If you don't have a COM1, get one. At this time,
there is no way to select a port other than COM1 for the boot blocks
without recompiling the boot blocks. If you're already using COM1 for
another device, you'll have to temporarily remove that device and
install a new boot block and kernel once you get FreeBSD up and running.
(It is assumed that COM1 will be available on a file/compute/terminal
server anyway; if you really need COM1 for something else (and you can't
switch that something else to COM2), then you probably shouldn't even
be bothering with all this in the first place.)
NOTE #1:
The serial port settings are hardcoded to 9600 baud, 8 bits, no parity,
1 stop bit, by default. If you wish to change the speed, you need to
recompile at least the boot blocks. Add the following line to
/etc/make.conf and compile new boot blocks:
BOOT_COMCONSOLE_SPEED=115200
If the serial console is configured in some other way than by booting with
-h, or if the serial console used by the kernel is different from the one
used by the boot blocks, then you must also add the following option to the
kernel configuration file and compile a new kernel:
options CONSPEED=115200
NOTE #2:
In addition to a serial cable, you will need a null modem adapter
in order to connect the terminal to the PC's serial port. If you don't
have one, go to Radio Shack and buy one: they're cheap.
NOTE #3:
If you wish to drop into the kernel debugger from the serial console
(useful for remote diagnostics, but also dangerous if you generate a
spurious BREAK on the serial port!) then you should compile your kernel
with the following options:
options BREAK_TO_DEBUGGER
options DDB
- MAKE SURE THE CONFIGURATION FILE OF YOUR KERNEL HAS APPROPRIATE FLAGS
SET FOR COM1.
Relevant `flags' are:
0x10 enable console support for this unit. The other console flags
are ignored unless this is set. Currently, at most one unit can
have console support; the first one (in config file order) with
this flag set is preferred. This option alone will not make
the serial port the console. Set the following flag or use the -h
option described below, together with this flag.
0x20 force this unit to be the console (unless there is another
higher priority console). This replaces the COMCONSOLE option.
Example:
device sio0 at isa? port "IO_COM1" tty flags 0x10 irq 4
If the flags were not set, you need to run UserConfig (on a different
console) or recompile the kernel.
- CREATE THE FILE "boot.config" IN THE ROOT DIRECTORY of the `a' partition
on the boot drive. Put the kernel name and options in one line. The
kernel name is optional. Options are subset of those described in boot(8):
-h toggle internal and serial consoles. You can use this to switch
console devices. For instance, if you boot from the VGA console,
you can use -h to force the kernel to use the serial port as its
console device. Alternatively, if you boot from the serial port,
you can use the -h to force the kernel to use the VGA display
as the console instead. (Can you say 'toggle' boys and girls?
I knew you could. :)
-D toggle single and dual console configurations. In the single
configuration the console will be either the VGA display (internal
console) or the serial port, depending on the state of the -h option
above. In the dual console configuration, both the VGA display
and the serial port will become the console at the same time,
regardless of the state of the -h option.
However, the dual console configuration takes effect only during
the boot prompt. Once the kernel is loaded, the console specified
by the -h option becomes the only console.
-P probe the keyboard. If no keyboard is found, the -D and -h options
are automatically set.
Use either the -P option to select the console automatically, or the -h
option to force the serial console.
- BOOT THE MACHINE. You'll see a prompt appear on the terminal that looks
something like this:
No keyboard found
>> FreeBSD BOOT @ 0x10000: 640/7168 k of memory, serial console
Boot default: 0:wd(0,a)kernel
boot:
This is identical to the prompt that normally appears on the VGA console,
except for the 'No keyboard found' message that indicates a keyboard
couldn't be detected, if the -P option is in `boot.config'.
From here you can boot the system (or let it autoboot by itself) just
like you can from the VGA console and the kernel will automatically
use COM1 as the console device. This is done by passing a special flag
to the kernel in the 'boothowto' word. (The curious can refer to
<sys/reboot.h> and the sio driver sources for details.)
CAVEATS:
- The idea here is to allow people to set up dedicated servers that require
no graphics hardware or attached keyboards. Unfortunately, while (most?)
every system will let you boot without a keyboard, there are quite a few
that will not let you boot without a graphics adapter. Machines with
AMI BIOSes can be configured to boot with no graphics adapter installed
simply by changing the 'graphics adapter' setting in the CMOS configuration
to 'Not installed.' However, many machines do not support this option
and will refuse to boot if you have no display hardware in the system. With
these machines, you'll have to leave some kind of graphics card plugged in,
(even if it's just a junky mono board) although you won't have to attach
a monitor into it. You might also try installing an AMI BIOS. :)
- Using a port other than COM1 as the console requires some recompiling.
Again, it's usually assumed that COM1 will be available for use as a
console device on a dedicated file/compute/terminal server, so hopefully
you'll never need to do this. But if you feel you must change the console
to a different port, here's how:
o Get the kernel source package.
o Edit /etc/make.conf and set BOOT_COMCONSOLE_PORT to the
address of the port you want to use (0x3F8, 0x2F8, 0x3E8 or
0x2E8). Only COM1 through COM4 can be used; multiport serial
cards will not work. No interrupt setting is needed.
o Create a custom kernel configuration file and add appropriate
`flags' for the serial port you want to use. For example, if you
want to make COM2 the console:
device sio1 at isa? port "IO_COM2" tty flags 0x10 irq 3
The console flags for the other serial ports should not be set.
o Recompile both the boot blocks and the kernel.
o Install the boot blocks with the disklabel command and boot
from the new kernel.
$FreeBSD$
