3 ms·
Or imagine a Drake R8A receiver, described as "guaranteed NOT to work" that a friend picked up. It can be tricked into working, but it gives a "PWRLOS" displa
by mikewarot 2y ago
Or imagine a Drake R8A receiver, described as "guaranteed NOT to work" that a friend picked up. It can be tricked into working, but it gives a "PWRLOS" display most of the time, and there is no discernable cause. Thanks to the obscure NEC uPD78213 cpu actually being available and documented, and having no internal rom... it's time to write a disassembler[1] (or later find out that MAME has one[2] thats pretty good), look at all the schematics, and figure out how the firmware works.
*Still working on the disassembler, because I can eventually make mine interactive, add labels, comments, etc.
[1] https://github.com/mikewarot/Res78213 https://github.com/mikewarot/Res78213
[2] https://docs.mamedev.org/debugger/memory.html#debugger-command-dasm https://docs.mamedev.org/debugger/memory.html#debugger-comma...
- lnsru 2y agoOne must love Drake R8A receiver! I would say, that having no internal ROM is an advantage in this situation.
- jandrese 2y ago"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.
- lnsru 2y agoLooks like there is regular EPROM in this case. Nothing too ugly. Except very old microprocessor.
- mikewarot 2y agoNope, 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).
- jandrese 2y agoOh, 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.
- evoke4908 2y agoDoesn'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?
- jandrese 2y agoIt is a truth table, but normally there isn't a way to dump them.
- anyfoo 2y agoIf 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…