RS-485 - The third wire

RS-485 is differential signal standard with 2 wires for Data and with the third wire being used as reference ground.
Some of our suppliers suggest never use the third wire when connecting Modbus RTU devices in a RS-485 bus.
Some people say multiple ground connections cause ground loops.
I prefer using the third wire and shielded twisted pair cable with shielded on one end.
Never had problems implementing Modbus connections using just two wires probably my Modbus networks are minimal (controller to HMI connection) and share common ground.

What is your take on this?

I have always understood that the third ground terminal is not for connecting a third wire, it is for connecting the cable shield that must be connected at only one end

See links..

https://www.google.com/search?q=RS-...=chrome..69i57j69i61&sourceid=chrome&ie=UTF-8

RS-485 is differential driver A minus driver B (or vice versa, I can't recall which off the top of my head), but the circuitry is not differential.

The circuit is driver-line-A-to-IC-ground-pin and driver-line-B-to-IC-ground-pin. Those two driver-line-to-IC-ground-pins measurements are subtracted for the 'differential'.

Driver lines A and B use a common ground. Sometimes the IC ground pin is grounded to chassis, sometimes it floats.

When IC ground is connected to chassis ground, the circuit can develop ground loops and saturate drivers from common mode.

When IC ground floats and is not connected to ground the connecting the 3rd wire signal grounds provides a very robust RS-485 network.

Here's what Robust Data Comm says about the "2 wire" myth, grounding and 3rd wire signal ground:








My take:
Vendors are negligent when the IC ground is a hard chassis or power supply ground.

Within a panel, RS-485 usually works OK. Probably a common ground point.

That's why you see a third wire in the trunk cable for SLC-500 Data Highway 485 networks; they use an isolated common conductor instead of ground or shield.

I like that RobustDC document; I've cited it a few times when folks argue about how RS485 circuits work.