Quoted from wdennie:... The switch matrix runs off 5 volts which comes from the mpu.
Quoted from pintechev:Thank you so much for confirming that. I thought it was 5, but didn't want to give bad forum advice
NO !
The logic on the CPU board is indeed 5 volts powered : the Security Chip and the output latch (U23) are TTL. But this command an ULN2803 driver, that switch ground signal. There is then pull-up resistor connected to +12 volts. So there is either ground (active strobe) or +12v (inactive strobe) on the matrix switches.
On input, a diode filter only grounded signals - and there is LM339 comparators using again pull-up resistors to +12 and this voltage as reference. Note that, by this way, the pulluped 12 volts goes also to the matrix through the diode (when no switch establish connection to ground strobe).
Sending a high voltage on the matrix will usually not burn the input stage. As a diode protect the LM339 (U18/U19) inputs. But it can burn the output stage, as the ULN2803 (U20) may drive at the same time, a ground signal.