"No internal ROM" might mean that it relies on a bunch of custom PLA chips which would not be an improvement. With a ROM there is at least a chance you can dump it and run a disassembler on the contents to figure out what it is trying to do.
Nope, I've got complete schematics, and a dump of the EPROM. It's going to take a while to figure out how the heck why it's hallucinating a power failure (the signals going into the CPU are appropriate).
Oh, it has a ROM, just on an external chip. I misunderstood your original post. I thought they had somehow programmed the device without using ROM at all, which made it either very old school or very custom.
Doesn't a PLA just boil down to a truth table that you can dump the same way? Or am I thinking of the other kind of programmable logic chip used for this purpose? GAL, I think?
If it’s only combinatorial logic, without any flip-flops or anything else you can consider “state”, it depends on how many inputs it has.
16 inputs is 65536 combinations. Even at a very low clock it takes you no time to just try all of them. 32 inputs is 4 billion. Now you’re getting somewhere, but even then, at somewhat reasonable clock speeds it should take you a day at the very most…
