Kamstrup RS-485 Installation and User Guide

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RS 485 Guide to successful communication

55122588-RS-485 Guide OSK 2021.06.23

Whenever using Modbus RTU or BACnet® MS/TP the electrical communication runs via RS-485.

RS 485 is a standard differential bus, which allows for high speed communication up to 400m.

To work well, 3 demands must be fulfilled.

1) Cable

RS-485 uses 3 signals, A B and GND, typically a twisted pair with a screen used for GND.

All 3 wires must be connected to all bus devices. Do not omit the screen.

Ask your local supplier for a “RS-485” cable

2) Wire topology

Reccomended: Line topology

Not reccomended:

Star, BUS, Tree or Ring topology

3) Line termination

Line termination is extremely important, to keep the bus operating at its best performance.

One 120 Ohm resistor connected between the A and B wires – in both ends.

( Check if your equipment already have a built in line termination resistor )

For Modbus: All Modbus devices must be set-up with identical communication parameters: Baudrate, parity

and stop bits.

For Modbus and BACnet: Always check for conflicting device-addresses.