Modbus TCP Slave Support with Redundant CPUs (X20CP3686X)

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1

Hi everyone,

I’m working with a redundant PLC configuration using 2 × X20CP3686X.

I need to expose approximately 20 PLC variables to a third-party Distech controller via Modbus TCP.

However, when I enable “Use as Modbus Slave” on the Ethernet interface (IF2), Automation Studio displays the following error:

“Use as Modbus slave’ is not supported under a redundant configuration.”

I have a few questions:

  1. Is Modbus TCP Slave unsupported on redundant CPU configurations?
  2. Is there any supported workaround, such as binding the Modbus server to a specific CPU interface instead of the Cluster IP?
  3. I noticed that Modbus TCP Master does not generate the same error. Is Modbus Master fully supported with redundancy and switchover?
  4. If Modbus TCP Slave is not supported, what is the recommended method for exposing a small number of PLC variables to a third-party BMS in a redundant system? Is OPC UA Server the only officially supported solution?

Any clarification or references to the relevant documentation would be greatly appreciated.

Thank you

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2

Hello,

Regarding the limitations of Modbus TCP in redundant systems, check the help.

3

Hello,

As an alternative I know of a redundancy project that uses the MBusTCP library for both master and slave communication.
The function block will use the cluster IP address, but when a switchover occurs it is necessary to disable and reenable the function.