Hmmm, that table also does not specifically mention the serial port, just the parallel ports and a few other things. I suppose the safest thing is to assume it will be applying a lethal voltage and that I need to moderate it.


Looking at the circuit diagram, the TXD and RXD pins on the chip run out to a small socket on the board. They do not appear connected to anything else, and in operation, nothing is plugged into that socket.

I can see in the program that there is some code involving timer1 and SBUF, but it all seems to be commented out. I infer from all this that whatever used to be connected to the TXD and RXD is legacy.

Unfortunately, there is nobody left here that knows anything about this hardware, and the documentation that exists, is sparse, and vague. I had to do some research to find out what the software language was as I'd never seen it before, (turns out it is something called PL/M).