So I spent hours and hours... well... days... months, actually, tracing the seven logic boards in the calculator. I have the schematics done, the component placements, and many of the nets labeled. I started on a higher-level diagram, also.
Pics and link to KiCAD+PDF schematics.
I used
this neat A4-sized flat light-panel to shine light through the PCB for easy tracing.
There are four card-edge slots in the calculator, and three of the boards are, conceptually, double-length PCBs where they essentially cut the board in half and folded it with wires across.
I'm taking a break now. I'm building a tester for the boards, which is where the
5/12v logic shifter project came in. Each slot has 88 pins on its two edge connectors. I determined which were power, logic input, and logic output. Lined them all up, and calculated how many shifters I would need. Some positions are always input only, some are always output only, and some are both (so need direction switching when changing boards). If I just make things uniform and only use directable shifters, I will need 83 shifting circuits. At 2 circuits per board, that's 42 boards.
The idea is that the tester would have the two card-edge connectors on it. They can slide because the distance between edge connectors is different for each board. That sliding motion could select which card you've put in, and control the directions of the shifters. Alternatively, there could be some detection about where the power and ground pins are (they're slightly different for each board) and then you could just have loose connectors. I'm not sure I like that solution since it relies on electrical detection on a possibly faulty board, and with loose connectors you might even flip the board around since the connectors aren't polarized.