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Time for something new! The VFD HP-25 is cool and all, but I've lusted after an HP-97 most of my life, plus I love nixie tubes! Starting to think about a nixie-display HP-97 emulator. Probably no card reader, but a small printer is certainly doable. Inspiration from Oliver De Smet (archived elswhere in this repository just in case):
I gave a talk at the HPCC 2020 conference on these calculators. You can see my slides here: http://www.hpcc.org/conferences/HPCC2020/HPCC2020_04_Retro_25_revC.pdf
Also, recommend to cut the trace to nWE on the EEPROM (U4, pin 27) and wire from pin 27 to pin 28. This disables writing to the EEPROM. Not essential, but I recommend programming the EEPROM separately in a programmer. Rogue code can easily wear out the EEPROM (as I found the hard way!) if writing is enabled. You have been warned.
A bit-banged serial port using an FTDI USB adapter is provided as bit 7 of ports 00 and 80. Note that these ports share keyboard scanning functions, so the code must be carefully written if both functions are used simultaneously.
