Ha. At the moment, I don't mind complicating my life for the sake of learning. My electronics hobby wants to be square with my vintage computing hobby, and I see this as an excellent opportunity to learn more about how this stuff works by using a real life example from when I was quite a bit younger. The fact I am the original owner as of ~1994 also adds to the desire.
Tests were with ISA VGA, VLB I/O (apologies), and auto config for timing (CPUCLOCK / 4. Mostly showing as 8.33MHz).
Some scoping - Hmm. The first signal I checked was the Address Line 00. The following video shows signal probe measurement as the hard drive loads up Doom and starts the demo level. I believe the start of it was the previous demo finishing, the command prompt appearing to show the next execution in the batch file (I set it up to loop about 12 times), the loading of Doom (you can hear the hard drive going with the volume up), and then the footage of Doom. You can see noise/garbage (appropriate term?) at the bottom of the signal during major graphics (previous demo finishing and then next demo loaded and running) that fills up a 1V division. The pulse rise/fall has extra junk that causes it to reach down and extra 1/2 volt. Ideally, shouldn't these be nice, clean 5V pulses? (Hmm... Perhaps the "noise" I am seeing is just the more frequent rises happening thanks to high volume of activity and a squirrelly start pulse)
Video of Address 00:
Address 00 (A31) during Doom finish/launch/startOSC (B30) signal - something that should be a representation of that smooth (at the crystal) 14MHz signal from my earlier post, I believe:
And then this next signal seems rather odd. I assume the bounce up/down from 0 in the following video is because my probe wasn't sitting properly in the outside pocket of the ISA signal I was probing, but this IRQ 4 or 3 (scoped both. Forgot which in this video) - Basically either COM1 or COM2 serial ports seem to be showing a signal (20+ second mark) that is in sync with the hard drive read access you hear in the video. Neither of the two COM ports has anything plugged into it.
Why would hard drive access manifest itself visually in an IRQ signal for COM port 1 (IRQ 4) or COM port 2 (IRQ 3)?
Video of either COM1 or COM2:
COM port monitorEDIT: For additional testing, I changed the I/O card for an ISA one - the original one in the system - and monitored several of the signals again. I also monitored the AT clock signal at B 20. The clock signal remained at 8.33 with an occasional quick dip to 4.XX. I just assume that is my alligator grounding style.
However, I did notice the AT Clock signal illustrating hard disk reads during a loading screen. I don't know what acceptable "noise" would be, but it seems like whatever could be in the clock signal should be within a specified tolerance and NOT allow any other signal activity in.
Is it worth (a subjective word, I know) replacing all the tantalums?