There are a number of different hardware variants in the 6502 family, each with slightly different pinouts and auxiliary functionality - most famously the cut-down 28-pin 6504 and 6507, but also some full-fat 40-pin versions. The modern W65C02S is *mostly* a superset of these, but…
Consider the humble BBC Micro. It uses an NMOS 6502, right?
Correct. The later BBC Master uses a 65C02, right?
Wrong.Attachment:
Acorn-Master-AIV-internal.jpg [ 1009.29 KiB | Viewed 3711 times ]
Examining the above photo of an expanded Master Turbo, you should be able to find two CPUs. The one on the actual motherboard belongs to the Master proper - it's a 65SC12, lacking both the Rockwell instructions, a RDY input, and a Phi1 clock output - but unlike a 6502, it does have a Data Bus Enable pin, which might save you an octal transceiver in some applications. And the one on the daughterboard front and centre is a 65C102, which take a quadrupled clock and produces a quadrature clock for you, convenient for high-performance DRAM interfacing, as this 4MHz internal Second Processor card does. As proof, there's a 16MHz crystal visible.
In the NMOS family, there's also the 6512 which requires Phi1 as an
input. Normally it's produced as an output (and many SBCs happily ignore it).
For a quick diagnosis, the good old NOP generator might be a good idea. Refer to the WDC datasheet, and make sure all inputs for the WDC part are correctly provided. Your chips should all do something recognisable with that. Then look for signals *not* provided by your SBC but required by the WDC part. Finally, check the cheat sheet below to see if there's a chip that lacks a signal your SBC relies on, or handles so much differently that things may be going very wrong.
Attachment:
Screenshot 2020-02-25 04.47.50.png [ 97.57 KiB | Viewed 3711 times ]