How looks the RS485 section of these AC drivers? GND of RS485 available? Also optically isolated? Any connections from enclosure or even GND (internal signal ground) to protective earth?

Kiran, in a case like your's I would open all this stuff and look how the individual signal grounds, enclosure grounds and protective earths are managed. By this you can easily see, whether undesired ground loops are formed or if filtering is omitted.

Keeping everything in "safety class II", means avoiding any connection between GNDs and protective earth can be helpful, in order to avoid hum loops for instance. But when having any SMPS or even phase angle controlled AC drives in the application, then connections to protective ground, even if it's only for mains line filters, can be very helpful to shunt noise to protective earth. Without this measure noise can now be injected directly into the RS485 bus!! How, if everything is isolated by optos? Via the unavoidable stray capacitance, yes even via the input to output stray capacitance of optos...

Once I had a problem with an galvanically isolated RS485 transciever. A switch mode power supply (tiny DC/DC converter) was used to supply the isolated, means RS485 bus connected section. But switching noise was so high, that an awful common mode noise was introduced, making the application to fail the CE radiation test. In this case it was extremely helpful to connect a cap between the isolated signal ground (RS485 signal ground) and the internal signal ground, right at the DC/DC converter and with shortest wires of course.

So, even if you do everything right, you can still fail, only because the used modules are crap and generate unsane noise. In such a case it can be very difficult to find the mistake and get the stuff properly running.

Kai


PS.: Give the twisted pair a chance...