One more try - Can you ID this S-100 Serial board?

tony duell ard at
Thu Oct 1 07:30:19 CDT 2015

> >
> > I'm fairly certain this is a serial board - mostly because the PO told
> > me so, and when I received it, it had a 25-pin male connector on a
> > three-wire cable carelessly soldered to the pads behind one of the
> > cable headers on the top edge. But the cable was removed as a matter
> > of course when I was prepping the machine for a rebuild. I mistakenly
> > assumed that the docs would be trivial to locate - so no need to
> > record the original wiring connections.
> >
> no disputing that.  Glad Tony made his point, I was curious if it was
> early enough to not have had what we are accustomed to now days.

Does 'now days' include RS232 interfaces :-)

But before the 1488 (or at least before it was common), many manufacturers
used op-amps as RS232 drivers. Look at the HP11205 and 11206 interfaces
for the HP9830, for example.

For input I could believe taking the signal through a resistor to a zener diode
to ground. Remember a zener is reverse-biased in 'breakdown' mode so
that circuit will clamp +ve voltages to the zener voltages and -ve voltages (where
the zener is not the normal way round and therefore acts as a normal silicon
diode) to -0.6V. Or indeed resistor +diode clamps to +5V and ground. Then into
a TTL gate.

Given that the original connection was 3 wires, it's a good bet they were TxD, RxD and
ground. Ground should be trivial to find. The trace the SO output of the UART to an
op-amp and then to the header. Similarly trace SI back through a TTL gate (which has
been mentioned already) to a header pin. I can't believe this is a complicated circuit....

OK, I've been doing this sort of thing for years, but I wouldn't be surprised if 
I could trace those connections in less time that it's taken to try to look for the


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