Cabling / Wiring a Modbus device to 75F smart devices or CCU is unique in its own way, compared to other devices which use a normal communication protocol, the installer can experience some difficulties if he is not an expert and not well informed about the factors to be considered while wiring device in Modbus communication networks.
Below are a some of those factors.
Every Modbus device is provided with a 'A' and a 'B' indications in them. The wires in the communication cable (RS485 system/ twisted pair shielded cable) are connected to these two terminals. The connection or wiring is done in such a way, all the devices that are a part of the Modbus communication network are parallel to each other as below.
Inversing the ‘A’ and ‘B’ connections of a device does not only stop the communication between that devices, but also stop the communication in the entre network of system which is due to the incorrect direct (polarisation) voltage of the terminals of the incorrectly connected device. In order to prevent this from happening wires of the same colour is used for connection of a type of terminal (e.g. white for A and blue for B).
Connection between Devices
The RS485 system (twisted pair shielded cable) used which is used in Modbus communication network has a main cable (Bus or backbone), to which all the devices have to be connected at the possible shortest distance from the main cable.
The branches must be no longer than 1200m. Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data.
Maximum distance of Cable and Maximum number of Devices
The main cable must be not longer than 700m 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.
In order to increase the extent of the Modbus network, repeaters can be used, which are signal amplifying and regenerating devices provided with two communication ports that transfer what they receive from device at one end to the other and vice versa.
Type of Cables
The cable to be used is a shielded twisted pair (telephone type). A Belden 3105A cable can be specified, but different types of cable with a equivalent characteristics can be used.
The twin consists of two conductors that are twisted together. This arrangement improves immunity of the network, because the cable forms a series of successive coils, each of which faces in the opposite direction to the next one: in this manner any magnetic field in the environment traverses each pair of coils in opposite directions, and its effect is thus very reduced (theoretically, the effect on each coil is exactly the opposite of the effect on the next one and thus the effect is nullified).
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.
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), both the types are equivalent.
Connection at the terminal end
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.
The cable shield must be earthed only in one point. Normally, this connection is made at one end of the main cable.
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.