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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?

enter image description here enter image description here

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    \$\begingroup\$ Could it be an RS485 bus? \$\endgroup\$
    – Andrew Morton
    Commented Mar 8 at 18:43
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    \$\begingroup\$ Where do the wires go? Does one set go to a device and one set to a terminating resistor? \$\endgroup\$
    – Justme
    Commented Mar 8 at 19:59
  • \$\begingroup\$ @AndrewMorton It might be, not sure though. \$\endgroup\$
    – Somanna
    Commented Mar 9 at 6:45

2 Answers 2

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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.

schematic

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.

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.

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    \$\begingroup\$ Nota bene: each device connects to a bus in two directions, or to a bus and a terminator; two wires at each I/O terminal.. \$\endgroup\$
    – Whit3rd
    Commented Mar 9 at 1:25
  • \$\begingroup\$ Thanks for your answer, but it seems you haven't answered the actual question. - Why are two wires connected to each terminal instead of just one? \$\endgroup\$
    – Somanna
    Commented Mar 9 at 6:39
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    \$\begingroup\$ @Somanna It is answered. It is a bus. Each device has wire going to next and previous device. If there is no next or previous device, then there is a termination resistor instead, at both ends of the bus. \$\endgroup\$
    – Justme
    Commented Mar 9 at 15:59
  • \$\begingroup\$ @Justme Oooooooooooooooh right. Now I get it. Thanks. \$\endgroup\$
    – Somanna
    Commented Mar 10 at 17:42
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One differential pair is enough between two RS485 devices, they probably want the ability to run the pigtail to two different devices.

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    \$\begingroup\$ Minor addition; one differential pair between devices is enough, if the devices share a ground reference, otherwise a third wire is required for ground reference between devices. \$\endgroup\$
    – Justme
    Commented Mar 8 at 19:58
  • \$\begingroup\$ Yeah, but that probably wouldn't come out of the RX/TX port and those units are probably chassis grounded or grounded between units. \$\endgroup\$
    – Voltage Spike ♦
    Commented Mar 8 at 21:49

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