K
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.
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.