Broken calculator

[blackjetrock]

I think your main problem will be the speed of the processor. the Rockwell has a clock speed of 200kHz and the Blue pill runs at 48MHz and it just has time to capture the bus. On the 850 I suspect the bus speed is a lot higher and you will need a processor clock speed much higher. I have used an STM32 with a clock speed of 400MHz or so, but even then you may not be able to sample an 8MHz bus,m for example.
If there's an external ROM chip then desoldering it is an option. Also, if the processor has a halt or bus request line then you could take over the bus and read the ROM contents that way. Another way to snoop the bus is to use a logic analyser, I've done that too, but you do need a logic analyser. An FPGA is another way, that is much faster.

Regards
Andrew