The TX81Z was actually a run-up to doing a PSR-60 layout.
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Keys are hooked up (thanks to RB for already figuring out that part), but the copious buttons and LEDs will require emulating the little driver multiplexer chip. Shouldn't be too hard, I think.
The goal is to be able to switch instruments so I can see how the OPQ is handling things other than "organ", which seems to be the default.