panacea....m,
Been following your thread some, and still confused by it.
I don't know if you have one problem or two problems, namely computer or noise / rf or even a combination of the two.
1. The ss should not be loosing communications with PC when VFD / motor and other influences
are removed from the picture. That has to be achieved, got to start somewhere.
I would recomend divide and conquer. If that's in place then you can in a very disciplined way go about identifying
another culprit if there is one.
2. Noise / rf can be a real bitch. VFD and motors are prime sources of it. Shielding is meant to keep a noise out or inside the protecting conduit. You can for example, inductively pick it up and makes sure it stays mixed by keeping it inside a shield. Or you can provide a path for the noise \ rf along the shield and get it ito the grounding. Bitch is you can't see it, you can measure it somewhat to get a mental picture of intensity, and then use some logic or "black magic" to cure it. So sometimes easy, or logical, or complex, or black magic .....just a bitch.
You change one parameter , like opening the VFD cabinet, and you have a whole set of new possible cause and effect conditions. The motor starts and creates an atomic bomb of noise, or the noise just gradualy builds from it and evently affects something. Sometimes the only way to get rid of a electric field is shut it down so it resets back to zero.
Divide and conquer,
RICH