OK, that's also interesting.
You'd expect no DSP at all to yield all FFs there. The fact you're getting something, albeit nonsense, is at least a data point.
I've had all sorts of grief getting the DSP to accept writes (see other threads), but I've never had trouble getting it to read that wasn't timing related.
The fact there is anything other than FF or 00 on the last four bytes leads me to think it's not the DSP responding, but something else (or the DSP is truly broken!).
Depending how deep down the rabbit hole you want to go, U44 is worth looking at. The DSP selection and response is entirely co-ordinated by the GALs, COMBEL doesn't get involved. The interaction between them is complicated, though. The absolute final piece in the jigsaw is pin 15 of U44 -- DSP_CS. This tells the DSP to either write or accept data on the bus. You *could* try removing U44, bending that pin up a little and re-inserting without it. I would expect you to then see eight FFs in DSPDUMP.
Unfortunately pulling U44 completely will, I think, result in a hung boot.
Have any of the old mods had anything to do around the four GALs at the front of the board and U44 tucked up by the floppy?
BW
Screenshot 2021-10-28 at 19.52.17.png
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