Non-CRTC PET video circuitry...

Josh Dersch derschjo at
Fri Nov 14 02:50:45 CST 2014

On 11/13/2014 10:44 PM, tony duell wrote:
>> t's only drawing a single line of characters (garbage with some noise
>> due to a very terrible socket holding the character ROM), centered
>> vertically.  I initially thought this was a VRAM addressing issue (a
>> stuck counter or whatnot) but it turns out the VSync signal is much too
> Since the vertical timing signal is triggered by the counters getting to end of frame, are
> you sure it's not a counter problem? Are D6 and D7 counting correctly?

Thank you -- I knew that the counters fed back into the monitor control 
logic but after I had initially taken a look at the counters (at D6 and 
D7) I saw that the Load and Clear inputs were getting triggered rapidly 
(at 64us intervals) and I assumed (incorrectly) that this was the actual 
problem -- that this load / clear behavior was incorrect.

And after debugging that for a few hours last night I *completely* 
spaced on that counter-controls-the-monitor thing and after confirming 
that the 64us interval was actually correct never went back to look at 
the counters.

Well, I just did.  D6 is fine.  D7 has all of its outputs stuck high.  
Replaced it and I now have a full display (of random garbage).

Now to get the CPU to reset properly :).

Thanks for pointing me in the right direction, as usual...

  - Josh

>> fast -- it's only about 1.2ms long (it's supposed to be about 16ms).
>> I'm actually surprised the monitor can deal with that without damage, so
>> I've been leaving the monitor disconnected while debugging.  The HSync
>> period is correct (63.8us or so), and most of the addressing / character
>> display logic (which is driven by earlier portions of the video timing
>> chain) seems to be running correctly, it's just only getting a chance to
>> draw one line of characters before the next frame starts.
>> I'm not having much luck tracing down the fault; I have a schematic
>> (
>> but it's not making it easy to work out exactly how everything ties
>> together.  I can't find anything resembling a service manual for this
>> model, but perhaps I haven't looked hard enough yet.
> I agree, that's  not a nice schematic. It's not obvious if an input (e.g. a reset input) is actually a logic
> signal or just pulled high. You have to trace it all over the diagram.
> Anyway, I think I would start at C6 pin 13. Is it stuck high, or totally the wrong period?
> Then check the gates ay the bottom next to the copyright notice, E8, etc.
> -tony

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