I am experiencing same issues.

I have a CPU 1517H and modbus redundand with CP 1543-1.

PLC is server and VIM is client.

If i try to connect with generic modbus client (modscan), everything works fine server side.

When i try to connect VIM i have unexpected behavior.

If VIM can succesfully connect, whenever i try to switch the connection for redundancy test (unplug one of the 2 ethernet wire for modbus, or power off one of the siemens CPU) communication drop.

After communication break there is no way to restore it in auto.

I need to switch off and on again VIM, re-download config and pray to re-establish communication.

I am starting to think VIM is faulty or there is no compatibility between Siemens and Emerson VIM.

Was anyone out there succesfully integrate modbus Emerson VIM and S7-1500? 

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Split from Siemens Modbus TCP communication with DeltaV.