Interested to see how this goes. Glad to hear you're bit-banging, though as the 328 is inline with the axis pulses to the stepper drivers it has to keep up at all time (not just homing) and I'd be curious if you can hold the signalling whilst I2Cing the display - interesting challenge.

I did realise after writing that the optos were for homing switches/reflection into the DDCS, so, yeah, will be fine - I'll withdraw that.