Does anyone have the datasheet for an MM57109?
If memory serves, the full datasheet is in the 1980 National
Semiconductor MOS Databook.
How hard would it be to make a replacement, either
with an FPGA or a
microcontroller or something?
It depends. Do you want it to be exactly as slow and inaccurate as the
real thing? That might be hard.
The original *was* a microcontroller, albeit a very slow 4-bit PMOS
microcontroller, so obviously there's an existence proof that it can
be done with a microcontroller.
Eric