On Fri, Oct 26, 2018 at 3:49 PM <silverdr@wfmh.org.pl> wrote: > > Now, we know those cases when the characters are garbled (not like garbage character but the character definitions are wrong). If not the CHAROM and not the CIA then the next goto chip is the U2 CIA. Checking the bits on its Port A is often enough to find that the lines read wrong combination, effectively preventing VIC from accessing the CHAROM. Now to the interesting case. I've got a CIA that passes all the diagnostic tests, including both ports. Moreover works all well everywhere (C64 U1, 1571) as long as it is not the U2 position in C64. There it causes the garbled characters symptom. Anyone willing to take a stab at possible explanation? did you try to look at the actual signals with a scope? I've found often that a failure mode of MOS Technology/CSG chips is a very weak pullup output capability on some output drivers. It may work up to a reasonable logic level (2.5 - 3V) when driving few inputs or no inputs at all, and fall down to 2V or less when driving more stiff inputs or more inputs together. These outputs will usually (but not always) completely fail in a few days/weeks of use. FrankReceived on 2018-10-26 16:02:20
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