Quoted from Miguel351:
Thanks for the input compy and pkiefert. I can say from playing an original many times over the years that I'm not sure that the slot machine opto stuff was ever fully nailed down from the get go. On the one I've played, when it's the girl asking you to "Shoot for the Cherries, Kid" and you get back to the hotel saucer, there are times when the reels spin a few full rotations before it finally stops and the game reacts, with DMD animations and everything. There were even times where the reels would stop on a different symbol than what the DMD animation and callouts would display.
If the hardware syncing of the reels is that much of a problem, maybe forget about it. Maybe, instead just have the code talk to an "emulated" wheel, feeding back to the game generated wheel encodings. That same board could in turn drive a small display "emulating" the slot reels that matched the encoding (so both always in sync).
Plus, free screen in Hotel Lex for other cool things