You might want to look at common mode chokes and feed-through capacitors in addition to the typical bead inductors where power and control signals enter the digital section.

Use a high speed optocoupler (1 MHz is not a problem with the newer optoisolators) and run a split ground plane under that optocoupler.

For "industrial applications, I believe differential current loop interfaces are preferred because they handle voltage drops over long wiring distances and are far more immune to noise.

You might want to look at common mode chokes and feed-through capacitors in addition to the typical bead inductors where power and control signals enter the digital section.

Use a high speed optocoupler (1 MHz is not a problem with the high speed optoisolators - I've seen spec sheets going up to 100 Mbps!) and run a split ground plane under that optocoupler.

For "industrial applications, I believe differential current loop interfaces are preferred because they handle voltage drops over long wiring distances and are far more immune to noise.