Connecting a DCS to an RTU

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Thread Starter

Kitimat

I've got an interesting application where I'm connecting a DCS to and RTU at a distance of 500 meters. I'm using RS485, Modbus 2 Wire for the communications.

As I understand how RS485 - 2 wire works, it sends signals back and forth using differential voltage on the 2 wires. However there is a ground connection (a third wire). This is so the differential voltage signals have a reference point. From what I’ve read, what can occur is the ground potential on both ends can be different, which will offset the grounded DC power supplies used to send the RS485 signals. In my case both the RTU and DCS have a connection for GROUND.

I'm concerned connecting the Grounds will cause a ground loop between the two devices. Is this a possibility? The DCS RS485 port on the serial card is "optically isolated" which I believe should prevent a ground loop from occurring. The RTU serial ports do not have optically isolated RS485 ports. Will Optically Isolated RS485 ports prevent ground loops?

Another interesting point is that some end devices I've connected to (Heat Trace, Flow Meters, etc) do not have a ground connection, only DATA+ and DATA-. I've never had a problem connecting to these devices but that is probably because they are in close proximity to the controller and they probably are connected to the same ground as the controller.
 
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Santosh Kubasad

RS485 is indeed a 2-wire only standard and the transceivers pick up this signal by taking a differential of the two lines (Data+ and Data-). The ground line is not required and in most properly designed networks, is not used. Connecting the grounds of the two nodes does create the possibility of a ground loop, especially if the nodes are far apart. As you pointed out, using an optically isolated output stage is one solution to avoid the ground loops. We recommend you use only two wires, a good quality twisted pair cable and proper termination and you should have a trouble free RS485 line setup for you without using the ground connection (and hence avoiding potential ground loops).

Santosh Kubasad
Modbus Protocol Engineer
Colways Solutions http://www.colwaysolutions.com)
 
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Lynn August Linse

My three decades of experience mean I do not agree with the last poster.

True 2-wire (with no ground) only works reliably when both devices are from the same vendor (or in indoor non-isolated products). It is always safest to assume it won't work, than that it will.

RS-485 chips source a tiny current on BOTH wires when they transmit; they sink a tiny current when they receives. This is the nature of forcing the voltage differential, or sensing it. So where does that current go? It's basic electrical theory - you need a return path.

So 2-wire RS-485 only works when both devices lack ground isolation, as that allows a 'bicycle' style circuit to be used, with the return path through the common ground.
 
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