Hitachi X200 sends bad ModBus responses when spindle is spinning?

I modified the Hitachi WJ200 driver to work with the X200 VFD. The setup commands work ok (setting run, reset, trip, etc), and I can set the frequency. But after setting the frequency, the VFD stops sending valid responses (bad CRC, bad response length, etc etc). I can't read or write coils or registers reliably -- maybe 1 in 20 tries gets through.

That driver uses libmodbus. I also tried writing a python script (using minimalmodbus) and the same thing happens.

What's happening?

Sounds like a noise problem, Check grounding. Are you using shielded cables? Are the cable shields properly grounded? Are your com cables routed away from the power cables? Are your modbus cables properly terminated?

I am also struggling with modbus communication errors on a machine (not Hitachi drives). However mine seem to be unrelated to the running state of the spindles, and I've all but given up trying to improve it. It works mostly good enough.