So, what with the recent chat about
fake parts and discovering that my so-called "R65C02P4" is really probably an NMOS part (albeit working well enough so far that I still think it was worth the $2), I'm thinking that I'd like to cons up a simple board to make it easier to test and play with random supposedly-6502 CPUs.
One thing that would be nice would be to easily be able to do the simple "wire data bus to NOP" test, so you can just drop in a CPU and start scoping it out, without worrying about what's in your ROM (or whether you even have working ROM or RAM). I'm thinking the easiest way to do this would be to put a couple of 8-gang DIP switches on the bus, with one turning on 4K7 pull-downs to ground and the other 4K7 pull-ups to Vcc. Leave them all off and you have a normally functioning machine, or turn on the ones you need to pull the data bus to $EA or whatever your preferred NOP is. (I don't know if any other opcodes would be useful—maybe the ones that are NOP on some processors but not others.)
Is that a sensible way to do this?
You'd also want jumpers to handle the various pinout differences, such as one to connect pin 1 to ground or leave it floating, and so on.
For debugging after that, you'd probably want a basic SBC setup: RAM in the lower 32K, ROM in the upper 16K, and a PIO and ACIA mapped between them. (You should be able to decode all of that with a 74LS138 and a 74LS00, or similar.) Throw some jumpers on the chip selects to allow you to hold them high instead of using the decoding inputs (à la the
RC6502 Apple I SBC (schematic in the export/ subdir) and you're set.
(And yeah, it wouldn't be a very "safe" board to use, since it would be easy to mess up the jumpers and switches, but it's designed for hardware hackers, not beginners.)