Hi again everyone, hope you all had a good Christmas. Sorry to dig up an old thread, hope no one minds, but I thought it would be good to not leave it without any resolution.
Not fixed my beeb yet, it's been in the back of my mind for all this time but not made any progress. However, I did buy another BBC Model B, which is working. This has provided me with that all important yard stick to compare things to. I've also upgraded my oscilloscope to a nice Rigol DHO814, which is helping.
I've had success now with burning the ROMs - it helps to have a working system to validate that I have done that part correctly. The key is all in the addressing, which is different depending on the size of the ROM you use. Always the CPU will start at 0x7ffc,0x7ffd and that address translates differently depending on if you're using 8, 16 or 32KB ROM chips.
Any test ROMs I've burned are working well on the good machine... but still nothing on my old broken one.
I've been probing pins between the two doing a comparison of all the waveforms and not found anything wrong so far.
Finally today I was brave enough to risk breaking the working machine by carefully pulling the 6502A from it so I could test the one from the broken machine. That took a lot of patience as I don't think it had been moved for 40+ years!
Aaaand, it looks like it's the 6502 in the old machine that's dead - it's not working in the good machine. So now I'm going to source a replacement CPU from somewhere, trying to avoid the dodgy Chinese knockoffs! I'll update further when I can.
Thanks very much again to you guys for helping with this - I found this thread very useful to remind myself where I'd got to in the troubleshooting.
Not fixed my beeb yet, it's been in the back of my mind for all this time but not made any progress. However, I did buy another BBC Model B, which is working. This has provided me with that all important yard stick to compare things to. I've also upgraded my oscilloscope to a nice Rigol DHO814, which is helping.
I've had success now with burning the ROMs - it helps to have a working system to validate that I have done that part correctly. The key is all in the addressing, which is different depending on the size of the ROM you use. Always the CPU will start at 0x7ffc,0x7ffd and that address translates differently depending on if you're using 8, 16 or 32KB ROM chips.
Code:
ROMAddressValue-----------------------------------------------------------8KB ROM 0x1ffc-d0xE000(1110000000000000)16KB ROM0x3ffc-d0xC000(1100000000000000)32KB ROM0x7ffc-d0X8000(1000000000000000)I've been probing pins between the two doing a comparison of all the waveforms and not found anything wrong so far.
Finally today I was brave enough to risk breaking the working machine by carefully pulling the 6502A from it so I could test the one from the broken machine. That took a lot of patience as I don't think it had been moved for 40+ years!
Aaaand, it looks like it's the 6502 in the old machine that's dead - it's not working in the good machine. So now I'm going to source a replacement CPU from somewhere, trying to avoid the dodgy Chinese knockoffs! I'll update further when I can.
Thanks very much again to you guys for helping with this - I found this thread very useful to remind myself where I'd got to in the troubleshooting.
Statistics: Posted by JamesPearson — Fri Dec 27, 2024 4:21 pm