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RS 485 connection to any Unitonics PLC


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So I am sure that some of you may know factoid 2 already but I decided I would share the benefit of lots of testing with everyone. In our products we use the V570, V350 and the JAZZ units to talk to variable speed drives on a regular basis. Most drives come standard with 485 Modbus as the only communications type unless you spend extra money getting optional com cards for the VFD. Two factoids relative to noise.

There was a conversation that happened accidentally in the "I have a new project" topic where someone said that Unitronics was very susceptible to noise. Well VFDs make a large amount of noise and it is not always radio noise. A fair amount of the time the noise is actually contained on the local earth to where if you ground the shield of shielded cable you make your noise problem much much worse than in you do not ground your shield. The old conversation about sometimes you have to ground a shield on one end or the other or not at all must be practiced regularly.

Factoid 1:

I generally would not recommend one brand of product over another but 20 years of use on pressure transducers have shown conclusively that there are definite brands of transducers that work dramatically better than others in noisy conditions. If the noise is radio noise pretty much every transducer is as good as any other. If the noise is carried on the local earth ground of the machinery then really bad things happen on every brand of transducer I have ever used (list to follow) except 1.

We have used:



Pepperl & Fuchs




Dwyer (all private labeled so there is one that works)

and in 100% of the cases when there is a site that has noise infecting the ground because of VFD usage there is a horrible amount of noise created (it has nothing to do with Unitronics) and there is only one cure (separate the sensor from the metal pipe is it screwed into to isolate the sensor from the grounded pipe. If you ever have a site that has what appears to have an incurable noise issue and a VFD is involved unground the sensor and see what happens (pressure sensor just unscrew it and leave is unconnected to anything metal and if your noise issue goes away your grounded piece of equipment is causing your noise issue.

So what is the one sensor that seems to be impervious to noise on the grounded pipe? It is specific to exactly one model and one product type only. The SETRA 209 series pressure sensor works perfectly where nothing else will (without lots of noise smoothing). AST is second best (but not as good as the 209 series SETRA) and everything else is just bad. Unfortunately, the 209 series is not NEMA 4 rated and is only suitable to outdoor use with the help of a cover to make a 3R cover. No other model of SETRA transducer works like the 209.

Just as a note (if you are not needing Class 1 Division 2) there are companies that make electrically isolated pipe union fittings (for cathodic protection) and we have used these from time to time to help customers through ground noise problems.

Factoid 2:

It is in the Unitronics documentation but who reads everything (or remembers it over enough time). When connecting to RS485 it is not just important but absolutely imperative that you do not connect wires to pins 2,3,4 and 5. Even if the opposite end is connected to nothing at all (except 1 and 6). What testing has shown is that if any of the other 4 wires touch each other (because you cut them short) they will make an otherwise quite communication line unbelievably noisy. Also even if the ends are not touching the wires themselves act as little antennas and noise becomes worse. How do we know and not guess at this? Because sometimes we try to do what is right and it ends up being horribly wrong. One of our techs went out an sourced a high speed modem cable (shielded with shield all the way up to a metal sheath on the plug). It was not an inexpensive cable and sounded like a really good choice. We were buying 15 foot cables and then cutting this in half to make 2 cables (other devices have screw terminals for their connections and not an RJ12 (RJ12 for RS485 to get all 6 connections) so we would cut away all cables except the two making connection to 1 and 6 at the Unitronics port. We had 70 or so with these cables out and a high percentage of the build had special drain wire considerations that had to be made to make the communication work ok.

New project with a different brand of VFD has come along and we build our first three units and zero of them would communicate over 485 communication connection. (we tested everything before build and it worked) WHY?

The cable we were using had 4 sets of twisted pairs inside the shield (so two wires were not connected on either end). Testing on a V350 showed that as long as we had a computer connected to port 1 communication between the PLC and the VFD was fast and excellent TX/RX accuracy, but as soon as we disconnect the computer from port 1 communications would fail to a rate of about 75% packet loss (at slowest communication speeds). We tried different grounding of the shield to no effect, we replaced cables (didn't expect that one to work) to no effect, we took the test VFD from our desk and replaced that VFD with one in the constructed panels (no effect), so then just because there was nothing else to try we took one of the connectors that comes in the box for the V100-17-RS4X Modbus adder card and just twisted that to a standard 3 wire Beldon (really poor connection) and communication success went back up to about 99%. So we soldered 3 of the connectors from the V100-17-RS4X and shrink wrapped it all so that we could ship the 3 panels to their end customers.

Then began the process of figuring out the whys and wherefores as to what the heck was going on. Taking new (expensive shielded) cable we stripped multiples back all the way to the metal cased plug and found that the manufacturer had done a good job on construction. We removed just the two wires that were not connected to anything on either end and found that removal of the two unconnected wires help com success rise to the 60% level. Then made sure that no wires were touching out of the remaining 4 unconnected wires and coms rose to a 90% success rate. Then we cut the 4 remaining wires to be about 1 inch long and made sure they did not touch anything and coms rose to about 96%. Then made our own cable with an RJ12 connector to have only 2 (totally unshielded wires with low twists per foot) wires and comes were great.

So now we will be buying specialty cables that have only the two required conductors inside them.


Thank for reading and I hope our pain can save you some.


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