I had more time to play with this today. It had been working fine but then today it failed again. Seemed to depend on the humidity! To check my PCs serial port I wired up a separate circuit on a breadboard using the same MAX232 chip and the tantalum caps. That worked and I was able to confirm the serial port works fine (both the onboard and the USB to serial).

So I carefully checked my board for shorts. I couldn't see anything obvious but I gave it a good clean and moved and insulated any wires that might be possible candidates (coating them with clear nail varnish to ensure the enamel wasn't damaged).

Now I get a -7 volts on the pin 8 receive input when it's not sending anything on the USB serial port and -10.5 volts on the PCs real serial port. The both swing to the opposite positive voltage when data is being sent.

I really need to do a proper board for this machine. Veroboard and tons of wires is better than breadboard and tons of wire but not by much!

Simon

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My 6502 related blog: http://www.asciimation.co.nz/bb/category/6502-computer