RS485 Port Wiring

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Two-wire RS485 Port Wiring Between SIXNET Products

Note: The information in this topic applies to all SIXNET RS485 ports, except for any legacy VersaTRAK RTU or SixTRAK gateway "Main Port" configured for 2-wire RS485.

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When interconnecting SIXNET RS485 ports, connect all "G" terminals, "+" terminals and "-" terminals to one another as shown in the diagram above. At the two end units only ("A" and "C" in this example), jumper "-" to "T" or jumper "T1" to "T2" to terminate the RS485 network. See "RS485 Termination" below for more information.

Notes:

When inserting multiple wires under a screw terminal, always use the same size and type of wire.
If you have existing wiring that has only two conductors and a shield, you can use the shield to connect the signal grounds between stations. This is not optimal (especially for long cable runs) but it should work in most situations.

RS485 cable distance

Some SIXNET stations with an RS485 port can communicate over distances of 4,000 feet or more. The maximum distance increases with wire size and a corresponding reduction of baud rate. Here are the limits:

1 km (approx. 4,000 feet) using 24 AWG wire & maximum 57,600 baud
2 km (approx. 8,000 feet) using 20 AWG wire & maximum 38,400 baud
3.5 km (approx. 14,000 feet) using 18 AWG wire & maximum 19,200 baud

RS485 Termination

All SIXNET RS485 ports provide RS485 termination components (120 ohm resistor and a 0.1 mF capacitor connected in series). To terminate your RS485 network just jumper the two appropriate screw terminals together. Only the end stations of your RS485 network should be terminated. Always avoid terminating more than two stations on any RS485 network. When terminating a 3rd party device, please refer to the device’s user manual for termination instructions.

Bias Resistors

On a RS485 2-wire network, a pair of bias resistors (1K ohm typically) acting upon the transmit/receive wires may be required. If bias resistors are not present, the receive inputs on some RS485 devices may react to noise on the floating wires. The bias resistors will force the transmit/receive wires to a known (non-floating) state when none of the RS485 devices are transmitting data. Some RS485 devices have bias resistors built-in, and are enabled through DIP-switch or jumper settings. Make sure there is only one pair of bias resistors acting upon the network.

Note: If your RS485 network is made up exclusively of SIXNET devices then these bias resistors are not necessary.