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Old 10th Jul 2019, 9:23 pm   #152
john_newcombe
Hexode
 
Join Date: Apr 2019
Location: Skipton, North Yorkshire, UK.
Posts: 252
Default Re: Gemini 80-Bus System

This is starting to get a tad embarassing...

I have retested the IVC card with the 'address everything' test ROM and I am seeing slightly different results than I saw previously. I have photographed the images and posted them here https://glasstty.com/wiki/index.php/IVC_Scope_Images.

The short of it is that I see activity in all places I expected except /IORQ where the condition is a permanent high on pin 20 of the CPU and at the i/p of IC31 (pin 12). The output of IC31 (pin 8, connected to CRTC E) is permanently low.

The behaviour is different from that experienced during the previous tests with this ROM in that the signals were generally much narrower negative and positive going spikes and the same signal did seem to appear on the /IORQ. Clearly I have been doing something wrong/different between the two tests sessions, however, I am at a loss to explain what is different.

Just to be safe I checked the ROM's contents just to make sure I hadn't labeled it incorrectly or mixed it up with another. And so I am left thinking that perhaps I had a bad ground connection on the scope or some other probing issue. Or perhaps I am doing something wrong now, like I said, it's all a tad embarrassing .

I also checked the dot clock select again and I am now happy that this is working correctly with the Xtal clock being selected each time the 'clock test' ROM is used. This is also the case when the original IVC Monitor ROM is used.

At this point in this particular test session, the issue seems to be that with the 'address everything' test ROM (and the original ROM come to that), there is no signal present on IORQ despite the IN statement being in the loop.

Just for completeness I swapped the CPU for a known good CPU (different to any tried before) without any change in behaviour. I also removed IC35 (socketed) and the CRTC in order to leave the IORQ line isolated and there was no still signal on either /IORQ pin 20 CPU or pin 12 of IC35 socket.

So if all of this is correct and I haven't gone mad, what could be preventing the IORQ from strobing IC31 when there is a perfectly good IN statement in the code?
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