I did discover that the module I thought was a ROM-only was actually a cache module for a different PowerMac. I made the mistake because the cache chips from some Macs are the same size and pin number as the ROM chips on this Mac. I was looking up the specs on each of the chips and found that each side of the module I want to modify seems to be self-contained. The cache size contains 8 chips which I have confirmed are 16K high speed memory chips as well as a another one which turned out to be a "Fast CMOS 1-of-8 decoder" presumably to unify the 8 cache chips.
I did find that the ROM chips on this logic board are on the underside. They are actually directly below the L2/ROM slot and it looks like they share the connectors that also go to the L2 slot? At first it seemed a bit odd to me that the connectors they soldered the ROMs to would continue to go to the L2 slot rather than stop them at the ROM chips ... but then again it might have been more work for them to rework the slot. It does explain why the Mac won't boot with the ROM/L2 cache chip in place. When it tries to boot and call the ROM there are two conflicting replies from the two sets of chips.
Luckily I do have the hardware to remove the entire chip so I don't expect it to be a big deal. Hopefully when I get some time I can experiment with this soon and make my module and see what happens.
The real question is how much of a risk am I running by playing with my 'franken-module'? Should I look for a 'test' 5260 logic board for when I try this?
It seems fairly safe as long as there are no solder bridges but I don't want to overlook anything.
I'm having trouble finding the pinouts.
I did find that the ROM chips on this logic board are on the underside. They are actually directly below the L2/ROM slot and it looks like they share the connectors that also go to the L2 slot? At first it seemed a bit odd to me that the connectors they soldered the ROMs to would continue to go to the L2 slot rather than stop them at the ROM chips ... but then again it might have been more work for them to rework the slot. It does explain why the Mac won't boot with the ROM/L2 cache chip in place. When it tries to boot and call the ROM there are two conflicting replies from the two sets of chips.Luckily I do have the hardware to remove the entire chip so I don't expect it to be a big deal. Hopefully when I get some time I can experiment with this soon and make my module and see what happens.
The real question is how much of a risk am I running by playing with my 'franken-module'? Should I look for a 'test' 5260 logic board for when I try this?
It seems fairly safe as long as there are no solder bridges but I don't want to overlook anything.