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

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 docs... -tony