bject:
Modbus RTU Communication Failure After Power Cycle in S7-400H with ET 200SP and CM PtP Modules

Project Description:
This system uses a Siemens S7-400H redundant CPU connected via PROFINET to an ET 200SP remote I/O station. The ET 200SP includes six CM PtP modules (6ES7 137-6AA01-0BA0) configured as Modbus RTU masters to communicate with multiple slave devices.

Software Configuration:

• FB640 is used for the initial configuration of each CM PtP module.

• FB641 handles Modbus RTU communication.

• In OB100, the signal COM_RST := TRUE is activated to reset and initialize the CM PtP modules during startup.

Problem Description:
After a power cycle, Modbus RTU communication does not resume automatically. The CM PtP modules stop exchanging data with the connected Modbus slaves. Communication only resumes if the program is downloaded again to the CPU.

Request:
Please advise how to permanently resolve this issue so that Modbus RTU communication works correctly after each power cycle, without requiring a program download