MODBUS RS-485 communication systems connecting rules

How to connect MODBUS RS-485 communication systems?

Cabling of industrial communication systems (Modbus RS485) differs in some parameters from cabling used for power cables. The technician may face some difficulties if he is not an expert in Modbus communication networks.

Modbus RS485 connects a master device to one or more slave devices. Later in the article, we will treat slave devices as serial communication meters, even if the cabling is the same for all Modbus devices.

1. Connection port

Each device has a communication port with two terminals, labeled A and B for convenience. A communication cable can be connected to these two terminals so that all devices involved in the communication are connected in parallel.

All "A" terminals must be connected together, as well as all "B" terminals must be connected together.

Inverting device connections "A" and "B" not only prevents the correct operation of the device and its communication, but disrupts the operation of the entire communication system due to incorrect forward (polarization) voltage detected at the terminals of the incorrectly connected device.

To avoid errors when connecting a large number of devices, use different colored cables for all connections to terminals A and B of different devices (for example, white for A and blue for B).

2. Communication between devices

Unlike many power distribution systems, the way devices are connected in parallel is important. The RS485 system used for Modbus communication provides the main cable (bus or backbone) to which all devices must be connected using the shortest spurs (also called stubs).

Cable spurs must NOT be longer than 1200 m! Longer cable spurs can cause signal reflections and noise, resulting in errors in data reception.

3. Maximum distance and maximum number of devices

The main cable must NOT be longer than 700 m! This distance does not include cable spurs (which should, however, be short). The maximum number of devices that can be connected to the main cable is 32, including the master device.

4. Use of repeaters

To increase the length of the Modbus network, you can use repeaters, as well as signal amplification and recovery devices equipped with two communication ports that transmit the signal from the previous device to all connected devices.

If the segment distance exceeds 1200m or the number of masters in the segment is more than 32, a repeater should be used to create the next segment. Moreover, each network segment must be connected to a terminator.

The repeater allows you to divide the main cable into different segments, each segment can be up to 700 m long and connect 32 devices (this number includes repeaters). A maximum of 3 repeaters can be connected in series. More repeaters result in excessive delays in the communication system.

5. What type of cable to use?

The cable used is shielded twisted pair (telephone type). For example, you can specify Belden 3105A cable, but you can use other cable types with equivalent specifications. This cable consists of two conductors twisted together.

The use of such a cable improves immunity to electromagnetic interference, since the cable forms a series of coils in series, each of which faces in the opposite direction to the next. Thus, any magnetic field in the environment travels through each pair of coils in opposite directions and its effect is minimal (the effect on each coil is directly opposite to the effect on the next, and thus the effect is cancelled).

Cable shielding can be braided (formed by a mesh of thin conductive wires) or foil (consisting of a sheet of metal wound around the conductors). These two types are equivalent.

6. Terminal connection

In some countries it is allowed to ins ert two cables in to one screw terminal. In this case, you can connect the main input and output terminals directly to the instrument terminals without creating a branch.

If, on the other hand, each terminal can only accept one cable, it is necessary to create a corresponding branch using three auxiliary terminals for each connected device.

7. Grounding the shielded cable cover

The shielding of the cable must only be earthed at one point. Usually, this connection is made at one end of the main cable.

8. Termination resistance

To avoid signal reflection, a 120 ohm terminating resistor is required at each end of the main cable. Termination resistance should only be used at the ends of the main cable. If the total length of the main cable is less than 50 m, terminating resistors at the ends of the main cable can be avoided.

9. Connecting to a personal computer

If a PC is used as the master, a serial RS232 / RS485 converter usually provides the bus connection.