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Old 22nd Feb 2021, 10:46 pm   #616
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 7,339
Default Re: Non-working Commodore PET 3016

Possibly there is a mixup between the probe x 1 / x10 setting and the scope 'x1 / x10' setting. Certainly on one of the captures the size of the waveform seems unfeasibly high if the sensitivity really is 10V per division.

There are a couple of factors adding to the difficulties of interpreting these results:

One, quite a few of the system devices including the PROMs pass their data to and from the CPU directly, not through the databus buffers. Specifically, the devices on circuit sheets 2, 3, and 4.

Sheet 2: The 6520 which handles the IEEE interface.

Sheet 3: The 6522 and 6520 which handle various I/O functions, in particular the user port and keyboard scanning.

Sheet 4: The PROMs.

The other factor is that we don't know, when looking at these traces, when and whether the databus buffers are enabled and in which direction. We can only reasonably expect to see the buffered side mirror the unbuffered side when the buffers are either enabled going from buffered data bus to CPU, or enabled going from CPU to buffered data bus.

The devices I mentioned above are mostly in sockets. My suggestion before doing any more hardware work or chip replacement is to try reinstating the CPU data bus pins (which will stop the CPU from running, we know) and then try removing UC6 (6520), UC5 (6522) and UC7 (6520) one after the other and then all together to see if the CPU will run with one or more of them removed. By 'run', I mean continuous activity on the address lines, especially A0 (CPU pin 9).

Edit: With regard to the document posts looking a bit scrambled, they've always looked that way to me with the headings offset down the side somewhere but nobody else ever mentioned it so I thought it was just me. The PDF version looks much better so if it's as easy for you to do them that way, I admit that would be better for me in future.

Last edited by SiriusHardware; 22nd Feb 2021 at 10:52 pm.
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