Calphool
New member
Hello,
I'm working on a Pacman repair. I've attached a screen shot. The CPU is in a reset loop, so it's having trouble kicking the watchdog, that much is plain.
I have a Teensy based Z80 emulator that lets you do stuff kind of like a Fluke 9100A. Using that I can see that I can read all the ROMs okay and their checksums are good. However, when I write anything to RAM, either vram or cpu ram, it always returns as 0xFF when I read it back. So it *seems* like that would be easy to sort out, but after replacing the 2114 video ram, it's still the same.
So, I put the scope to the bus sync board and started trying to figure out if it's in there. There is a 74LS244 in there which seems like it would be a likely candidate, but nothing changes when I replace it. I can see the chip select line for the bus sync board toggling when I read or write to ram at 0x4000, so I know that much is getting to that bus sync board.
Can anybody explain how the bus sync board on pacman actually works? It's got a line going to the wait line on the cpu, and it's got lines that loop back into itself. It's not exactly obvious what it's trying to do. Clearly *part* of what it does is just normal data line buffering, but what is all that other stuff it's got going on?
If anybody has any ideas on how to proceed more quickly than tracing every single line from the CPU to the RAM I would appreciate the benefit of someone's experience/wisdom. I haven't worked on Pacman before.
Thanks!
I'm working on a Pacman repair. I've attached a screen shot. The CPU is in a reset loop, so it's having trouble kicking the watchdog, that much is plain.
I have a Teensy based Z80 emulator that lets you do stuff kind of like a Fluke 9100A. Using that I can see that I can read all the ROMs okay and their checksums are good. However, when I write anything to RAM, either vram or cpu ram, it always returns as 0xFF when I read it back. So it *seems* like that would be easy to sort out, but after replacing the 2114 video ram, it's still the same.
So, I put the scope to the bus sync board and started trying to figure out if it's in there. There is a 74LS244 in there which seems like it would be a likely candidate, but nothing changes when I replace it. I can see the chip select line for the bus sync board toggling when I read or write to ram at 0x4000, so I know that much is getting to that bus sync board.
Can anybody explain how the bus sync board on pacman actually works? It's got a line going to the wait line on the cpu, and it's got lines that loop back into itself. It's not exactly obvious what it's trying to do. Clearly *part* of what it does is just normal data line buffering, but what is all that other stuff it's got going on?
If anybody has any ideas on how to proceed more quickly than tracing every single line from the CPU to the RAM I would appreciate the benefit of someone's experience/wisdom. I haven't worked on Pacman before.
Thanks!







