CPU Incompatibility problem ?!

[NotTooBright]

Hello Forum,
In re-designing an old game-system, based upon the Rockwell 'R65c02' CPU to run at a higher clock speed, i have run into a very curious problem : Replacing the CPU with the 'wdc65co2, i could not get output via the I/O interface board. Amongst other standard TTL 74LS chips on the interface board are 2-off 74LS174 present on the data-bus. If i replace these with DM74LS174 chips, the system works fine. But the same chips by ANY other manufacturer and the system will not give an output via these chips !? I have also tried replacing all of the LS with HCT, HC, F etc. etc. But to no avail ! Can anybody explain why this is ? Because i would like to understand this problem and also to use the wdc processor !

 

[moffy]

I would look at any discrepancies with the BE(bus enable) pin. Perhaps there is an undesired side effect. Only guessing though.

 

[OlPhart]

Unfortunately, it comes down to timing issues. Modern logic is SO much faster, you really should have the whole schematic to be certain critical signals aren't arriving too soon and goofing up the state machine.

Fortunate upside: clock speeds were ~2MHz so a decent DSO will freeze anything. Downside: you may not know what to look at, and/or its' relevance.

Good Hunting, this isn't trivial... I hope it's worth it. <<<)))

P.S. a direct replacement for the 6502 at its' proper clock speed IS available (maybe from me...) but makes restoration a LOT more feasible.
Modern builds of 6502 replacements aren't the same: fanout, signal timing, etc. If in question: Carefully compare specs between original and replacement, especially timing diagram relations.