Modbus Line Termination and Polarization (RS485)

Overview

A multi-point Modbus must have line termination and polarization.

Equipment connectable to this bus are:

Other PLCs like M340, Premium, Quantum, Twido or Nano
Schneider Automation devices like Altivar, Security module XPS, SEPAM, XBT or Momentum
Other Modbus protocol compliant devices
Modem, Hub

An example of multi-point Modbus network including a BMX P34 2010 processor is presented in this manual.

NOTE: A point to point Modbus network can also be performed.

Electrical schema of line termination and polarization:

Line Termination

Line termination is made externally: it consists of two 120 Ω resistors and 1 nF capacitor placed at each end of the network (VW3 A8 306 RC or VW3 A8 306 DRC).

Don’t place line termination at the end of a derivation cable.

Line Polarization

On a Modbus line, polarization is needed for an RS485 network.

If the M340 CPU is used as a master, it is automatically driven by the system so there is no need of external polarization.
If the M340 CPU is used as a slave, the polarization must be implemented by two 450 to 650 Ω resistors (Rp) connected on the RS485 balanced pair:
- a pull-up resistor to a 5 V voltage on the D1 circuit,
- a pull-down resistor to the common circuit on D0 circuit.

Network

There are two meanings of the word "network".

In LD (Ladder Diagram): a network is a set of interconnected graphic elements. The scope of a network is local, concerning the organizational unit (section) of the program containing the network.
With expert communication modules: a network is a set of stations that intercommunicate. The term "network" is also used to define a group interconnected graphic elements. This group then makes up part of a program that may comprise a group of networks.