I scoped it last night with 2 channels in differential mode and the output from the 485 were sweet (common mode noise all gone), particularly compared against the single ended measure (full of noise). Also, last night, changed the kernel frequency of the uc300 to 50khz to increase the pulse width to 10us (at 100khz kernel the pw is a rather narrow 5us).

The thing that gets me is that this was working fine. Going single ended feels an inferior move than differential driven)... unless one of the 485 outputs is starting to fail hi-z.

Equivalent input circuit is as you describe, opto, reverse diode and series resistor -330R