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Re: Not functional; Blinkenlights static

Posted: 25 Aug 2022, 11:23
by bmwtcu
download/file.php?id=1451 shows the PC incrementing from 0x1FF to 0x200, so no load is occurring. You have confirmed the PC is reset and at least in this waveform incrementing correctly. What I perhaps erroneously assumed was that your PC[15:8] was being loaded with the wrong value. You should trigger on PH_B going low to prove that loads from Y are correct. U5 and U6 only have CLK1/RST_B/PL_B/PH_B/Y[7:0] as inputs. If it's only ever incrementing to 0x200, you don't have a load issue.

Re: Not functional; Blinkenlights static

Posted: 01 Sep 2022, 09:05
by River-of-Dust

Huh, I see. Well, I tried setting it up to trigger on PH_B transitioning to Low, but it just triggers right as soon as I plug it in. Perhaps this is an artefact of plugging it in? Is there a way to bodge in an actual RESET button?

I'm not sure what to do at this point, as it seems that some part is defective or at least marginal in a way I haven't detected yet. This is very puzzling. I'm considering disassembling the whole thing, re-testing every component in isolation, then rebuilding it piece by piece. Still, I would very much like to learn how to tease out *exactly* what is really wrong with my Gigatron as part of the whole learning experience. Hmm...

I set my logic analyzer up to trigger on RST, with lines 0-7 connected to Y0-7, lines 8-11 still showing PC[8:11], and the rest showing RST, CLK 1, CLK 2, and PH_B. I then ran several tests in this configuration, with some screenshots of these tests below. Aside from noting that PH_B appears to pretty much NEVER transition to Low, the only other conclusion I come to is that the system is not consistent. Something is changing, such that otherwise identical runs yield different results, and the only thing I can think of for that is that some part is marginal, but which one? It's not just because the logic analyzer is plugged in, either, because, while the blinkenlights *usually* freeze with all four on, they also frequently freeze in other patterns, even with the logic analyzer not connected at all.

Any thoughts on how I should best proceed? Should I resolder EVERYTHING, in the hope of finding a cold solder joint somewhere that I have missed? I have already resoldered some parts that seemed likely, but that produced no change (and they all look fine visually, so no red flags for me to jump at there).