That's "so dim", not "SO-DIMM". This board predates such things!

I see a Dallas "Speed it μP" DS80C320, which is a 80C32 (like an 8051 with no ROM) - one on each board. I also see a removable PROM on each board, with a paper label leftover from manufacture. Could be the paper is covering up the UV port - this is old enough that EPROMs weren't taken for granted! I'll cover it up with some extra tape to be sure I don't erase anything with sunlight. If worse comes to worse, I'll have to dump these.

But before we jump into ROM dumps and reverse engineering, it might make more sense to attack this from another route. Replacing the microcontroller is always an option, since the LED drivers are all discrete chips. I'll keep that one in my back pocket as well.

For now, I'm interested in making the same noise as the MCU as it performs legitimate communication.