Re: [COFF] DEC terminal line driver chips?

[Brad Spencer <brad@anduin.eldar.org>]

Tom Ivar Helbekkmo via COFF <coff@minnie.tuhs.org> writes:

> I've got a number of DEC terminals, ranging from the VT220 to the VT520
> (sadly, I got rid of my VT100 and VT102 many years ago, before I started
> collecting DEC equipment instead of just using it), and some of them
> have one or more burned out serial ports.  Before I start taking them
> apart to find out what chips were used, I figured I'd check if any of
> you folks happen to know.  I'd like to order a stash of replacements,
> and it would be nice to have them handy before I clear the work bench to
> start dismantling terminals...
>
> Oh, and for the record: the Q-bus PDP-11/23 uses 9636ACP and 9637ACP for
> output and input, respectively, while the VAX-11/630 substitutes a
> 9639ATC optocoupler for the 9637ACP differential receiver.  (I have a
> couple of spare CPU boards with damaged ports, as well, so these are all
> on my shopping list already.)
>
> -tih

I will not speak specifically about DEC serial terminals, but rather an
experience I had many years ago with Data General terminals at the
University I was at that may or may not be helpful.

I spent part of a summer fixing the DG terminals.  The problems mostly
in that case were the last set of driver chips after the CPU that blew.
The University was running a current loop set up with very long runs all
over campus and lightening would tend to take out a serial terminal from
time to time.  We later build our own lightening suppressors, but that
is another story.  Only in one case I remember did we blow a CPU.  In
the case of the DG, the CPU was an 8085 or some variant that had a
serial port on it.  This then was sent to a set of driver level chips
that did the 5v TTL serial to whatever you needed conversion, be it
regular RS-232 or the current loop specification that was used.  The
terminal took both and it was this last set of drivers that would often
burnout.  We usually found out what was really wrong by just replacing
the drivers and if that didn't work, replace the CPU.  We also saw many
cases where a terminal would transmit but not receive, or vise versa.
We tested for this too with a cross over cable from another known
working terminal.

You did not mention the age of these terminals, but I would also suspect
caps going bad somewhere.


In any case, good luck.  If you have a cheap scope, available for just a
small bit of $ on Amazon, you may be able to determine where the problem
actually is in a much simpler way if you can't get the schematics.
Serial ports are pretty slow and pretty simple to see with a scope.  Of
course, if you can find schematics it will be a whole lot simpler.




-- 
Brad Spencer - brad@anduin.eldar.org - KC8VKS - http://anduin.eldar.org
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