I'll throw what I know into this discussion
If you're thinking about replacing the ROMs with new chips, this might be helpful:
Since the 68030 has a 32-bit data bus, I'd suspect that the two-chip variant of the motherboard is using a couple of 16-bit chips and interleaving them. It appears that the Classic II has a 512 KB ROM, so each chip is going to be 256 KB, or 2 megabits. So a valid replacement is going to be a 40-pin, 128K x 16-bit chip. I would assume that they used chips with a standard pinout, but we can't really know for sure unless you trace them like techknight said. At the very least, it would be worth it to figure out which pins are connected to VCC and ground on the motherboard -- if they match up with the standard pinout, you're probably good to go. When I did my IIci hacking with the DIP ROMs, I discovered that the four chips on the IIci motherboard use a standard JEDEC pinout. I'd be surprised if the Classic II chips don't.
The JEDEC pinout for 40-pin, 128K x 16 EPROMs is:
- Code: Select all
1 = VPP (programming voltage, probably not connected on a mask ROM)
2 = chip enable
3-10 = D15-D8
11 = ground
12-19 = D7-D0
20 = output enable
21-29 = A0-A8
30 = ground
31-38 = A9-A16
39 = program strobe (probably not connected on a mask ROM)
40 = VCC
Assuming it is a standard pinout chip, I'm thinking the chip you will be looking for is a 27C2048, such as the AT27C2048. I'm having a hard time finding any place that still stocks the 40-pin DIP version of that chip.
You might be able to get away with using a pair of 256K x 16 EPROMs instead. They are twice as big, but you could ignore the upper half of each chip. The only pinout difference is that pin 39 is now A17 instead of program strobe, so you could tie that pin to ground to force it to always address the lower half of the chip when reading. Digi-Key stocks the AT27C4096
(although it ain't cheap! Almost $9 per chip) Also, beware, they are one time programmable...you get one try and then you have to buy more blank chips if you did something wrong. Edit: Mouser has it for a better price.
Programming the replacement chips might be tricky. My Willem programmer only works with 8-bit chips. I'm sure there are 16-bit programmers out there somewhere...
If you do this, you should also make sure that the EPROM's VPP pin is tied to ground or VCC -- in the case of the AT27C4096, either one is ok. That just makes sure the chip is in a defined state. It's possible that they already wired it up that way on the motherboard--if they didn't, you will have to do it manually.
I can help you figure out the ROM interleaving if you do this...for every 4 bytes in the ROM, 2 bytes will go into one chip, and the next 2 bytes will go into the other chip.
All in all, as long as it's a standard JEDEC pinout, I'd say it's definitely doable if you can find a programmer device that can burn 16-bit EPROMs!