Post #1428381
2026-04-11 16:15 UTC
POV: #PERQ hardware debugging, as usual. The #Z80 (serving as an I/O coprocessor) is behaving strangely: fetching instruction bytes without toggling the M1 pin, jumping to random locations at around 47uS after coming out of reset; what's going on here?
Replies (3)
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@stepleton@oldbytes.space 2026-04-11 19:39
@skeezicsb yes of course 😎 For all the fixes I've had to put in to the EIO board, I wish I'd had it printed much sooner...
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@skeezicsb@oldbytes.space 2026-04-11 18:15
@stepleton@oldbytes.space Hardcopy of the schematics and code listing, *spiral bound*? 🔥
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@skeezicsb@oldbytes.space 2026-04-11 19:05
@stepleton@oldbytes.space Hmm. I wonder if this is an actual manifestation of the symptom noted in the microcode source, where "the hardware has a glitch when first starting up the Writable Control Store RAMs, the power surge can screw up the Z80". It would be interesting to watch the +5V rail during bootup to see how pronounced that surge actually is. DDS 170 at least means that maybe your DIB is okay and you can skip desoldering and testing every 74S225 in sight. For now. :-)