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Old 26th Feb 2021, 4:28 pm   #681
Mark1960
Heptode
 
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 677
Default Re: Non-working Commodore PET 3016

At 20ms those solid green areas on the data trace are where the sample rate and horizontal sweep are not fast enough to separate the different logic states of the data line, though it does look better for the chip select but we need to see what the data line is doing when chip select is active.

Could you show the options available for the trigger settings on your scope? On my scope, a different type, the default is to show the trigger point at the centre of the screen. It looks like yours shows trigger point at the extreme left, so when you trigger on rising edge the screen shows only the data line after the chip enable is disabled. Trigger on falling edge might be better, or move the display of the trigger point to the right, depending on your scope options.

There is also my misunderstanding of the chip select behaviour on the PET. I expected a separate chip select pulse for each memory access, but thats not the case. There must be a separate output enable control on the prom that will pulse for each memory access.

In the data on the 20us sweep you can start to see the logic level at different times. It shows clear high and low logic levels, but also quite a few times appears stuck at an intermediate level. This might just be times when the data line is floating, but also possible two separate devices trying to drive the data in different directions, one high and one low, which would be a problem.

If you could check the data lines again after trying to change the trigger settings, we can look to see if those intermediate levels on the data are only when one prom chip select is low, or when any one prom chip select is low.
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