Deep IIci rejuvenation, possible ROM select problem — #17

Post #17 by Dennis Nedry

Source Forum	68kMLA
Category	Hardware
Post Date	Mon, 1 Mar 2010 - 19:47
Original URL	https://68kmla.org/bb/threads/deep-iici-rejuvenation-possible-rom-select-problem.19657/

Post

That sounds like a hack to allow programming an EEPROM or Flash module in place in the machine. Is your IIfx SIMM built out of ROM chips or EEPROM or Flash chips. Hmmm. Must be one of the latter two, as ROMs don't have WE pins. Even if it's ROM chips, I guess it's possible they designed the circuit board for a proto-type reprogrammable module and then used that same circuit board for some of the production ROM SIMMs.
My guess is that when they want to reprogram the module, they'd write to the ROM SIMM and every address would be the normal address plus the A19 bit, which would give them WE high. Wait, that doesn't make any sense. They'd have to hold A19 low to enable WE. Hmmm. So any time they read from the ROM without asserting A19 the chips would treat the read as a write operation? That's not good. Weird. Maybe there was an inverter on the module?

This is a real quandary.

It could be that on this particular ROM, that pin is different. Gamba's OS 8 on SE/30 page says that these ROMs are based on M5M27C101JK, which he said is EPROM. I'm not seeing a datasheet for that, or any that are similar enough, so just I'm assuming that this pin is a /WE. Most modern 32-pin ROMs have the same pinouts. I guess it's possible that this pin is NOT a /WE. These ROMs are ~20 years old. There is also another pin connected to a +5V supply that usually is NC. MAYBE this is actually /WE. I'm not sure.

I have 12 new Atmel AT29C020 ROMs coming, so whenever I can source some cheap DIP32 to PLCC32 adapters and get them from china, I'll give them a go onboard, with A19 patched in via wires. It'll be a cool experiment! The goal is a sampled startup sound from a Mac IIci.