Ok what happens is when a switch is closed, it comes into the driver board through connector 2J3. Signal goes through 1 of 2 buffer chips ( ic 15 &16), then into the PIA chip at IC11, up through the 40 pin interconnect and into MPU.

In diagnostics, the driver board is ignored as its the MPU itself that strobes all the digits.

Check IC 15 & 16. These are designed to protect the PIA. They are inverters, so if a high signal is coming in ( 4-5V) then a low is outputed.
Or if low in then high out etc,

any other readings and these chips are gone.

If one of these chips is bad then we have to test PIA also at IC11 - youll need to test this anyway, but always best to start with the simpler stuff first