John Allain wrote:
There's a big Toshiba TMP80C49P that looks like a 'keyboard'
MCU to me. Some minor things like Motorola SN74LS(05N,138N).
Nearest the SIO port are just a few resistors and two
transistors. At least one leg of the SIO goes directly to
the toshiba. Looks non-RS232 then... Whatsit 5V instead of 15?
Not necessarily: you can convert the voltage levels with just a few
discrete components. Mostly in such cases the receiving voltages are
used to charge a capacitor that delivers the negative voltage for
sending data. In that case there must be a diode somewhere in the
neighbourhood as well.