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Old 23rd Feb 2021, 10:34 pm   #632
Mark1960
Octode
 
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,265
Default Re: Non-working Commodore PET 3016

I think setting up a NOP test would be a good next step. As Colin already has the databus disconnected from the 6502 it just needs resistors added to pull the data lines up or down to select the NOP instruction code. EA hex. I know some people tie the data lines to 5v or 0v but I think its safer to use resistors. Around 4.7k but not critical. D7, D6, D5, D3, D1 via resistors to 5v, D4, D2, D0 via resistors to 0v. Separate resistor for each data pin.

This should give nice regular pattern on the address lines and also repeating pattern on the address decoders chip select to the proms.

It might also be possible to see valid logic levels on the data bus while the prom chip selects are active.
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