Why are there two wires connected to each terminal of the RS485 port in energy meters

Question

I want to connect the energy meters of our clients to a gateway so that the gateway can communicate with the meters using Modbus. I have visited several electrical rooms that have gateways installed and inspected the wiring in each energy meter. Every energy meter that support RS485 have two wires connected to each A(+) and B(-) terminals of the RS485 port (See images below). The gateway to which these meters are connected though have only one wire connected to each of their RS485 terminal.
My question is why are two wires connected to each terminal of the meters' RS485 port? Isn't one enough?

Answer 1

There's two wires at each terminal because it made sense to whoever designed the system at a time. It's probably because the nature of the RS-485 standard that you'd want two wires going to each connection.

You'd need to show the wiring diagram of the whole system for anyone to be sure, but RS-485 is usually used for multi-drop systems.

So a typical system that uses RS-485 to communicate will be hooked up as shown, with any number of devices that are all communicating on one shared pair of wires.

^{simulate this circuit – Schematic created using CircuitLab}

Your pictures with two wires to each RS-485 port indicate that the system wiring is designed to reflect that diagram.

Running two wires to each terminal like that is probably done because it makes it easier to wire, and because it makes the wiring easy to inspect.

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Note that there's more that needs to be built on top of this: usually the communications is asynchronous serial, with a protocol built on top of that so that there's only ever one box that's trying to transmit at a time while still insuring that each box gets a chance to communicate when it needs to.

Answer 2

One differential pair is enough between two RS485 devices, they probably want the ability to run the pigtail to two different devices.