Multiple serial consoles via null modem cable

[Michael Loftis]

--On Friday, January 22, 2010 10:05 PM +0100 Nicolas Rachinsky 
<fbsd-stable-0 at ml.turing-complete.org> wrote:

> * Jeremy Chadwick <freebsd at jdc.parodius.com> [2010-01-19 23:46 -0800]:
>> You cannot do something like where box1 COM1 is wired to box2 COM1, and
>> depending on what box you're on doing the "cu -l ttyu0" from, get a
>> login prompt on the other.  It doesn't work like that.  :-)
>
> Isn't the reason for different dial-in and dial-out devices that this
> should work? Or does that only work with modem?

You can't with two directly connected machines.  When the two are 
physically wired together, and getty is configured (via ttys) to fire up on 
the port it takes over the port.  If you connect two machines via a null 
modem cable, both with getty on the same port, the getty's will be chatting 
with each other.  The locking mechanism will "break" the chat loop when you 
try to use the dialout device on one end or the other but you may have to 
wait some time before the other end restarts getty (because it previously 
would have been dieing very rapidly due to login failures)

A NULL modem connection is ALWAYS active.  A regular modem, is NOT.  It has 
a state of 'inactive' or 'waiting for ring' if you will.

The correct way to do what you want is as others have suggested, two serial 
null modem cables, and two com ports on each machine.