I’ll do what I can to help, though I’ll have to do a bit of revision as I don’t remember a lot of it.

The one component that I added to ensure separate grounds was a small reed relay: using lpt2 pin 14 for a voltage supply and grounding it using pin 1 let the relay output switch the vfd on & off. Pin 18 sends the variable voltage to the vfd for the variable speed.

How far have you got yourself?

Gavin