RS485 Modbus voltage levels

[kucharzb]

Hello,

I am still "figting" to get stable Modbus connection to my VFD. I am using USB Waveshare Industrial Isolated RS485 converter.

Problems:
1) the connection is not "constant" - the loops are going ok for some time, lets say 5-10 seconds then it suddenly stops for w while and goes on further - and this pattern repeats, but the pauses are rather random. This happens less when VFD is not running (just turned on) and more frequently when the VFD is running. It also regulary "stops" the communication when the "STOP" command is being sent and the spindle slows down. Then it restarts (the communication). I am not sure if the modbus parameters like "modbus delay" in VFD or speed can make it better.. at the moment I use 38400 and 6ms delay which somehow seems to be most stable, but maybe somebody has other recommendations?

2) there is clearly interference from VFD (you can see some spikes on the oscilloscope, as soon as the spindle is running. The RS485 interface was installed with super short USB cable near PC and then 2m shielded cable to VFD and otherway round - 2m USB and 10cm shielded cable to the VFD - seems to not have influence on those "pauses" in the communication.

Today I connected oscilloscope to my Modbus line and one thing struck me - line A+ has different voltage than B- - is this normal? What could be the issue? Should I worry?