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

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Issue Summary

The RAC18-IP controller loses communication with devices connected to its RS485 port or fails to start Modbus RTU client communication.

Symptoms

  • Devices on the RS485 network stop responding or are not detected.

  • Modbus points fail to update in the controller.

  • Communication errors appear in the iC Tool or Niagara environment.

Root Cause

Several factors can cause Modbus RTU communication issues:

  1. Outdated OS or libraries

    • Early versions contain known bugs that disrupt Modbus communication.

  2. Incorrect RS485 wiring

    • Missing or improperly connected biasing and termination resistors.

  3. Wrong RS485 communication parameters

    • Baud rate, parity, and stop bits not matching across all devices.

  4. Invalid ping register configuration

    • RAC18-IP requires a 16-bit input or holding register for ping.

When Does This Occur?

This issue typically occurs when:

  • The controller runs an OS version older than 1.4.1.7340.

  • RS485 wiring does not comply with Modbus standards.

  • Communication parameters differ between devices.

  • Ping register type is incorrect.

Solution

1. Update OS and Libraries

2. Verify RS485 Wiring

  • Follow Modbus protocol guidelines and RAC18-IP Hardware User Manual.

  • Ensure proper biasing and termination resistors are connected.

  • Use the built-in DIP switch to enable:

    • Termination only

    • Termination + biasing

    • Or disable both

3. Check Communication Parameters

  • All devices must share identical settings:

    • Baud rate

    • Parity

    • Stop bits

  • Configure these in SerialPort under:
    Networks > Modbus > Interfaces

4. Validate Ping Register

  • RAC18-IP does not allow changing the ping address.

  • Ensure the ping register is 16-bit input or holding type.

Prevention

  • Always update OS and libraries before commissioning Modbus networks.

  • Follow RS485 wiring best practices and enable termination/biasing as required.

  • Standardize communication parameters across all devices.

  • Confirm ping register type during device configuration.

Product: iSMA CONTROLLI RAC18-IP
Category:Troubleshooting
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