Subject: Re: This is my console and this is my other console
To: der Mouse <mouse@Holo.Rodents.Montreal.QC.CA>
From: Rick Kelly <>
List: tech-kern
Date: 01/21/1997 22:55:52
der Mouse said:
>> Some machines have more than one console.  (Pyramid, and, uh, oh
>> yeah, Sequent spring to mind)  Typically the 2nd console is connected
>> to a modem and when there is a problem you enable the "remote"
>> console, Service dials in and assists.  Both ports accept keyboard
>> input and output goes to both screens.
>Not that I wish to represent it as a general solution, but if both
>consoles are serial lines (as in the modem-and-dumb-terminal case), you
>can fake it with three connectors and some wires.  (To do it right you
>really need two diodes and at least one pullup resistor, but I've had
>it work just wiring the lines together.)  Of course, if both the
>transmitters try to send characters timed such that they overlap on the
>wire, you will a framing error or corrupted data.  (Assuming a
>reasonably high baud rate and human typing speeds, the duty cycle will
>be low enough that this is fairly unlikely to be a problem, even when
>both humans are typing at the same time.  At 9600 baud characters are
>a shade over 1ms wide, and even fast typists rarely generate much more
>than 10cps sustained (120wpm is a good clip); if we round these off to
>1ms and 10cps, the duty cycle works out to 1%.)

In the case of the Sequent Symmetry 2000/xx machines the console could
be tty0 or tty1, but the toggle had to be done from tty0.

The ICL 25, which was a 68020 machine, had two physical consoles.  One
to boot a strange ICL kernel and the other to boot SVR2 on top of it.

Somewhere out there is a CRDS UNIVERSE 68 running CRDS UNOS with a double,
togglable console.  Mostly done in software.
Rick Kelly