E2. RS485 Wiring

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1. Introduction

Installing an RS485 network requires in-depth knowledge! Please leave the installation to professional contractors.

Not following the specifications can result in incorrect communications and equipment damage!

2. RS485 set-up

RS485 is a differential balanced line over twisted pair. It can span a relatively large distance up to 1200m | 4000 feet. However, you will have to be aware of the voltage drop: The power at the end of the line is not the same as at the beginning. It depends on the specific cable resistance and the power consumption of each sensor (Chapter E1).

The wires should be connected in a point-to-point configuration, called a daisy chain.Modbus wiring schematic
Do not install a star or ring network. This may cause reflections of the signals.Icon start network
Icon ring network

2.1 Setting up the network

In the following figure, you see an example of a properly wired Modbus network in the correct daisy chain format.


  • The main cable is called the “trunk line” and goes from the Master to all Junction boxes. It contains at least three wires: the twisted pair wires (A and B) which carry the digital signal and the GND or “common” wire, which is absolutely essential for providing a proper reference to the Modbus transceivers. When our standard junction boxes are used, this will be the 0 Volt line.
  • The main cable is shielded. Ideally, the shield is separated from the 0 Volt line, but it may be combined when you are absolutely sure that the shield is clean of voltage fluctuations.
  • The junction boxes are used to make T-junctions to drop down to all the devices (officially called slaves, in the picture you see flow meters, but this can be any other Modbus RS485 device).
  • Cable length between the device and the junction box should be kept as short as possible.
  • The shield has to be continuous, as any gap in the shield will allow RF interference to enter the cable network.
  • Required cable quality: depends on total cable distance, number of nodes (T-junctions) and environmental influences. Contact a local cable contractor to select the correct cable for the specific situation.

3. Electrical Scheme

Electrical scheme RS485

4. Next Step

Now you are able to start measuring your system. This will result in data that you will be able to readout. The next step is to look at this data and analyze it

Next Step: E3. Analog output wiring

Updated on May 4, 2020

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