[jbar]

I have been banging my head against a brick wall with this for the last week or so, have done a lot of googling and learnt a hell of a lot about RS485 and modbus at the same time.

And a big lesson learned - don't beleive everything you read! Make you own measurements and checks.

Today was the turning point when I borrowed a USB osilloscope and actually started seeing the data transmission. And the simple solution - SWAP TWO WIRES!!!

The labelling of the RS- and RS+ terminals on the drive and the A and B terminals on the USB converter are not as expected.

Matching the potential of the terminals (relative to ground) was the key, ignoring the labelling. A [+5V] --> RS+ [+5V], B [0V] --> RS- [0V] and Ground -> ACM. The comms still work with the 120ohm termination resistor as well. And SpindleTalker also works as it is no longer receiving the 00 response (which was not coming out of the drive but the converter itself)

In the attached picture I see what appears to be a ground wire coming from the ACM terminal on the VFD, but can't tell if it goes to ground on your USB board? Does it? Zooming in I see your USB board has a ground pin. My USB board just has + and -, no ground. My motor comes on for a few seconds (5 - 20 seconds, longer at first then progressively shorter, as if somethings heating up.) then goes off. When it goes off I get Linuxcnc joint errors on my 3 axis, X, Y, Z. So I was thinking it was because I don't have a ground.