hy_vfd ttyUSB0 disconnects & reconnects as ttyUSB1

I'm trying to use the hy_vfd component to control my Huanyang  VFD using a waveshare usb-to-RS485  device.

I can connect to the VFD, start it and set it to the right speed (e.g. using halcmd & in AXIS GUI)  but then ttyUSB0 disconnects and I lose control over it. When I check the /dev/ folder ttyUSB0 is gone and now the usb-to-RS485 device is at ttyUSB1.

I've tried making a udev rule so that "ttyUSB_RS485" always links to the usb-to-RS485 deviceIt connects correctly at program start up but it does not solve the problem as the ttyUSB still disconnects mid-program.
I've tried a different USB-to-RS485 a dapter but had the same problem.

 

Some more information.
This is the sequence of halrun commands I used:The spindle then starts and accelerates to 6000 - but then I get the following error message 

Have you checked dmesg to see what is causing the disconnects ?
Have you tried other USB ports on your machine ?
Does this happen outside of the Linuxcnc environment?
Eg if you tried to do a loop back test using a serial terminal program ?
It does sound like a frustrating issue.

It sounds like it may be a EMI issue
I would try adding a common mode choke (giant ferrite bead)
around the RS-485 cable near the USB converter to avoid
stray HF ground currents in the USB cable.

Thanks both - I've ordered a ferrite bead & I'll also try the troubleshooting suggested by BeagleBrainz .

This problem is now fixed - it was an EMI issue as PCW suggested.

I checked dmesg - a couple of lines pointed to the EMI issue:Also the fact that the connection dropped when the VFD (& its electrical noise) starts up is another clue

I tried adding a clip-on ferrite bead - but this did not solve the problem.
The main culprit I'm sure was the cable - I was using an unshielded twisted pair cat5 cable. I changed this for a shielded cat7 (plus the ferrite bead) and connected the shield to the ground terminal on the waveshare USB-RS485 convertor.

Thanks again for the help!