Thanks for Feedback everyone.

Can't Shut down VFD # 1, to answer your question, but did disconnect drive # 2 (can't shut it down either at the moment)from RS485 network(2-wire). Next grounded -ground pin of 15 pin connector on PLC to same point as PLC ground and grounded shield at drive end and not PLC end. Now Drive # 1 is perfect. Good news is they were hard wired already so this was only to start out as a monitor drive parameters. Later maybe control if all works well.

Drive #2 has different cable (cheap crap cable)that is going to be replaced with a true quality braided wire shielded cable, hopefully that will solve the issue.

Much prefer ethernet but this is built into Yaskawa drives so why not.

We do Inductive Heat Treat(high power 800hz/800 volts kinda thing so yes definitely noise is in the equation, when you stand by substation and these high frequency machines run you can hear buzz at transformer, bad stuff. VFD's need line side reactors or they regularly trip on OV.

Making progress so thanks again for your thoughts and idea's everyone. Will update when shielded cable has been replace with our standard. We have lots of old RS485 and modbus for main SCADA and some place in the plant a large roll of good quality cable we ran for all the other stuff.

Thanks everyone.

Well it turned out OkiePC nailed it, I was always taught ground one end of shield and I always did opposite of what they say and grounded at sensor end or Drive end seem to work better for me.

Ran new shielded cable to second VFD same problem, connected shield to second drive at drive end to IG ground assuming internal ground Yaskawa's words. Connected shield to other end (Daisy chained two drives) to other drives IG terminal zero errors. Shield cable to first drive only grounded at drive end, also no errors with this drive running, only difference between two drives is the one causing errors has line side reactors??? Also when second drive it running it is creating the problem but with OkiePC suggestion second drive running no errors. Awesome, still getting about 750 reads per minute so very happy with this setup and now to do some control not just monitoring.

Thanks again everyone.

"only difference between two drives is the one causing errors has line side reactors???"

Is the serial cable anywhere close to the line reactor? Any time you have a coil of wire with current flowing through it you create a magnetic field. A Line Reactor has copper wire wound around (usually) ferrite cores, and this increases the inductance and is what helps smooth out the power flow of spikes. However, this magnetic field can cause problems in other devices like communications lines or other objects with magnets or coils.

A shield that is properly grounded will of course help protect against this magnetic field and stray radio frequency waves in the air nearby the wires from getting through to your serial data lines.

Excellent point Pouches. The cable is about 18" from line side reactor, reactor is totally enclosed in metal box by itself. So I don't know if this would cause a problem, I just know when that drive runs with shield grounded on one end only lots of errors like 60-70% of the time. To much work to disconnect or bypass to confirm.

I was thinking something screwy with drive but again not sure how to prove it, today no induction heat treating machines are running, so once they start up on Monday hopefully everything is still ok. Again definitely drive running is when noise shows up.

Good catch though and thanks for the thought.