[SiriusHardware]

If you feel your weak link was the connections then I fear you may have the same problems with that, but, nothing ventured, nothing gained. My level shifters have arrived as well, so I'll put some of those together tomorrow, work allowing.

I'll read the ICs using my device reader / programmer first but if I have time afterwards I'll also try recreating your efforts with the Mega to see what needs to be done to make that work. (Definitely should work).

Your having the 'Mega' offers the possibility of another technique which is to use it as a diagnostic replacement for the CPU (essentially, wire its pins to a header which can be plugged into the CPU socket, you can then use it to run all sorts of tests by generating fixed signal levels or doing repeated reads from or writes to single addresses or generating ascending addresses... (much like a NOP test)). Probably all a bit too advanced just now, but it is all there as a future line of attack.