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How I troubleshot my dead 6821

PostPosted: Jan Fri 13, 2012 7:53 pm
by Dav1dr4y
I have been putting together a Replica I kit received as a Christmas gift. It didn't work right away. Like others I got the @ prompt but nothing happened when I pressed reset. I spent sever al days trying to diagnose what was going on. I thought I would note down what I ended up doing that seemed to help the most.

I took out the eeprom and the memory inserted into the eeprom socket a NOP generator. This essentially an eeprom emulator that returns the NOP instruction no matter what. I got the idea from Lee Davison here, I made it like Lee shows on a peice of strip board but you could probably make a temp on with a breadboard,

With this inserted the 6502 only sees NOPs when it fetches instructions. The 6502 then walks through the entire address space fetching NOPs. This gives you something to look at with an oscilloscope. I could see each address line toggling as expected (each higher line toggling half as fast) except A1. A1 was a solid high (or solid low... I forget which). I saw this with the o'scope but then it was easier to pursue with a logic probe. I figured out if I took 6821 out the A1 line acted normally. I bent the 6821 RS1 pin (which is tied to A1) out and put it back in and A1 still behaved as expected. I figured the 6821 was the fault, got another and, yay!, fixed.

I just thought I'd note this down because I thought Lee Davison's suggestion was so helpful.