9 rules for correct cabling of the Modbus RS485 communication systems

Modbus RS485 cabling rules

The cabling of the industrial communication systems (Modbus RS485) is different in some ways from the cabling used for power cabling and the electrician may experience some difficulties if he is not an expert in Modbus communication networks.

A Modbus RS485 connects a Master device to one or more Slave devices. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices.

The main rules are set out below that must be followed for the cabling of this type of network:

1. Connection port
2. Connection between the devices
3. Maximum distance and maximum number of devices
4. Use of repeaters
5. Type of cable to use
6. Connecting to the terminals
7. Earth connection of the shield
8. Termination resistance
9. Connection to personal computer

1. Connection port

Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel.

All the ‘A’ terminals must be connected together and all the ‘B’ terminals must be connected together respectively.

In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B).

This makes it easier to identify cabling errors. The communication port on the Master device, whatever it is, has two terminals that correspond to A and B.

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2. Connection between the devices

Unlike what happens in many energy distribution systems, the manner in which the devices are connected in parallel is important. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible.

The branches must be no longer than 1200 m! Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data.

Figure 1 shows an example of a correct Bus connection.

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3. Maximum distance and maximum number of devices

The main cable must be no longer than 700 m! This distance does not include the branches (which must nevertheless be short). The maximum number of devices that can be connected to a main cable is 32, including the Master.

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4. Use of repeaters

In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other.

Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system.

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5. Type of cable to use

The cable to be used is a shielded twisted pair (telephone type). For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. The twin consists of two conductors that are twisted together.

The shielding may be braided (be formed by a mesh of thin conducting wires) or be a foil (consisting of a sheet of metal wound around the conductors): the two types are equivalent.

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6. Connecting to the terminals

In some countries, inserting two cables into the same screw terminal is permitted. In this case it is possible to connect the main inlet and outlet terminal directly to the terminals of an instrument without creating a branch.

If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected.

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7. Earth connection of the shield

The cable shield must be earthed only in one point. Normally, this connection is made at one end of the main cable.

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8. Termination resistance

In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. The end resistance must be used only at the ends of the main cable. If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable.

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9. Connection to personal computer

If the master used is a personal computer, in general an RS232/RS485 serial converter provides the connection to the bus.

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Reference // Practical guide to electrical measurements in low voltage switchboards by ABB