RAC18-IP - Modbus - Problem with Modbus RTU client communication

Issue

The RAC18-IP controller loses communication with devices connected to its RS485 port or cannot start the communication.

Possible causes and solutions

The OS or libraries version is outdated and contains known issues

In case of any problems with Modbus communication, first, check the version of the OS and libraries installed on the controller. Early and outdated versions of the OS and libraries contain known errors, which can be the reason to break Modbus communication and generally impede the work of the controller. It is recommended to install the latest available version. The earliest stable version is 1.4.1.7340.

Incorrect wiring of the RS485 network

The Modbus protocol documentation outlines the rules of proper RS485 network wiring. In the RAC18-IP Hardware User Manual section, it is described how to wire the RS485 network in compliance with the Modbus standard. It is especially important to connect biasing and termination resistors. The RAC18-IP controller has a built-in 3-position DIP switch enabling to switch on the termination resistor, termination and biasing resistors, or to switch off both.

Wrong parameters of the RS485 communication

In the Modbus RTU network, all devices need to have communication parameters configured in the same way. In the RAC18-IP, communication settings are defined in the SerialPort component located in Networks > Modbus > Interfaces.