Twisted Triad vs. Twisted Pair for RS-485

Question

I'm looking for information regarding twisted triad vs twisted pair cable use with RS-485, especially in applications with long distances outdoors.

With twisted pair like Belden 3105DB, the drain is connected to the shield, and this is the shared ground reference between RS-485 transceivers. Or do I have a misunderstanding of twisted-pair RS-485, and the drain should not be used as the reference ground?

Given that the purpose of the cable shielding is to reduce EMF coupling, would it not be better to isolate the reference ground with a twisted-triad, like Belden 3106A? Are there any downsides to using a twisted-triad over a twisted pair?

I've seen recommendations for using using a double twisted pair, but is this a recommendation over a twisted triad? I don't understand why this would be—is it to avoid capacitance between the reference ground and the data lines?

Answer 1

Belden is definitely an authority in cables. As it happens, you've misunderstood the specification of 3106A Cable. It is not a twisted triad, it is one twisted pair plus single ground conductor, which is basically #1 choice for RS-485 wiring.

If you check wiring guides (like this one from TI), they always have ground wire separate from shield and not twisted with a pair. See Figure 11-1c and 11-2 in the guide above.

It also answers your questions regarding isolated transceivers. You either have common ground separated from earth/shield potential by resistors, or you bind one end of common ground to earth and use isolated transceivers for other nodes.

I suspect the reason for separate untwisted ground conductor is to have slightly lower resistance and to reduce capacitive coupling.

As for using shield as ground, yes, it is possible. But this will waste the shielding function of it, so you might as well use any unshielded cable with enough conductors. For example 2-pair UTP with one pair as ground conductor.

However for 500m run I would strongly suggest using shielded twisted pair + ground. You can ground each shield segment individually at one side to local node ground, or you can connect them all together and ground only once.

Answer 2

For any differential data transmission cable, the shield should be used only as a shield, not a (potentially) current carrying conductor for potential equalization between transceivers.

And as even the RS-485 standars says, there must be common ground reference between the transceivers. The standard also mentions that in addition the twisted pair may have a shield, and the shield could be connected at one end only or on both ends, whatever suits best for the application.

These cables do have a drain wire that connects to the shield, and the drain/shield even has much lower resistance than the wires inside it.

So either cable should be fine, if you use the two wire cable then you must use the drain wire of the shield as ground between devices, and if you use the three wire cable, you can use the third wire as the ground wire between devices, and use the shield purely asa shield, possibly connecting the shield on one end only.

If you use an isolating transceiver, the bus side of the isolating transceiver still requires common ground for the bus, but it allows to isolate it from the device ground.