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GanjaTron · (This post was last modified: 02-11-2023 09:09 PM by GanjaTron.)

Following up on this, here's a snap of what's going on in the IA line (top) and the IS line (bottom), with the scope externally triggered by Φ1:

Note that this pattern never changes, even immediately after powerup. Compared to the traces shown in Tony's Classic Notes and Jacques Laporte's website, that doesn't look right at all! The ROM address on the IA line (apparently all 1's here??) should alternate with the fetched instruction on the IS line below. My understanding is that IA and IS should never be active at the same time. But instead, IS is constantly oscillating, just like Φ. Infact, it actually appears to be shorted to the latter, presumably inside one of the ROMs or the A&R chip which buffers and interprets IS. That's my theory anyway.

So from this, and the fact that I've run out of ideas, I conclude this calc's logic board indeed buggered itself beyond repair after just 15 minutes of normal operation. So I may get in touch with Harold -- thanks for the hint!

--Roland