Paganini wrote:
Next I think I will have another try at making a more permanent keyboard circuit on my perf-board. I ordered some better wire-wrap wire (I hope!). The wire I was using has PVC insulation that won't fit through my wire-wrap tool the way
it does in Garth's primer. I ordered a spool of Kynar wire; hopefully that will work better!
Well, I did that. I tried to make a better ground / vcc network, and be more conscientious about making every wrap be a good quality one. I also rebuilt the whole board, including the LCD part that already worked. Here it is:
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Once again, it only kinda works. Just like before, the LCD part works fine, but the keyboard circuit is wonky, as though the RC circuit managing the clock has bad timing, or there is noise on the data line. So, to troubleshoot, I reverse engineered the wire-wrap board. Going point by point with my multimeter, I reconstructed it on a breadboard:
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Interestingly, this breadboard version has exactly the same problems as my wire-wrap version: lots of repeated keys, and intermittent incorrect scancodes.
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You can see that there is something conspicuously missing from the breadboard reconstruction, though - bypass caps! My wirewrap board does have a bypass cap for each IC, and I did put them on the breadboard during reverse engineering, but I took them back off again for dramatic effect. They don't seem to make any difference. But this one does:
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I can put that cap anywhere on that side of the breadboard, and everything immediately starts working. If I put it on the other set of power rails (which is where I initially had all the per-IC decoupling caps) it has no effect.
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If I only I can figure out the equivalent "magic spot" on my wire-wrap board and add a capacitor there...