[mactec54]

Mactec: re the EMI power filter, that goes on the mains line into the project box? I could add it, but my suspicion was the the control electronics was getting EMI from being too close to the PSU. Maybe I am wrong but I thought the EMI went through the air between the two units ( radio wave EMI?) I didn't think the EMI went through the wiring from one unit to the other.

But now you made me think maybe the wiring from the G540 to the steppers is too close to the USB cable for the UC100. I also had trouble with losing the USB connection. I thought I took care in using shielded and grounded wires for the steppers and avoiding ground loops.

No that is not how it works each suspect device needs to have there own EMI Power Filter mounted close to there input power supply connection for that device

If you did not Ground the shields correctly then the shield will not do it's job it is very hard to create a Ground loop in a system that is only using ( 1 ) mains Power source

You can also shield the electronics with a simple aluminum cover

[cncuser1]

mactec: Does EMI Power Filter get connected DC input of the Gecko G540 or only mains voltage input of the PSU?

"You can also shield the electronics with a simple aluminum cover" For the cover, are you referring to PSU or control electronics?

[mactec54]

mactec: Does EMI Power Filter get connected DC input of the Gecko G540 or only mains voltage input of the PSU?

"You can also shield the electronics with a simple aluminum cover" For the cover, are you referring to PSU or control electronics?

Control Electronics

Mains Voltage only Ac

The EMI Power Filters you would only use on the Ac inputs to the main power supply that powers the device you want to reduce the EMI, they need to be mounted close to the device