PMDX 424 USB header shield ground

[Mike Abbott]

I've got a PMDX 424 with the USB header mounted to my controller enclosure which is made of AL.  It seems that if the usb plug touches the enclosure which is earth grounded, when I start Mach4 it seems to detect the 424 but then says it's lost communication after mach4 has fully come up.  Not the normal warning you'd get if the 424 was simply powered off during mach4 startup.  Any ideas?

Mike

[Bob at PMDX]

Connecting the USB shield to earth gnd on your chassis creates what is called a "ground loop" and allows current to flow through the shield, including any electrical noise from your motors/spindle.  This in turn can induce noise onto the USB signals inside the cable, or into the isolated side of the USB interface on the PMDX-424.  So my answer is "don't do that".

It *is* interesting that apparently the PMDX-424 can communicate with the PC at least briefly.

Bob