All, 

Just to share a solution here for my scnenario of intermittent communication lost. 

Software - PCS 7 V9.0 SP3 UC05

Modbus TCP PN Red

FB909 - MB_REDCL + Additional blocks JobList FB

CPU410-5H PN port = MB TCP Client, destination Prosoft TCP gateway = MB TCP Master. 

We encounter this intermittent communication fault from time to time.

We tried playing the "Cyclical time" in Job list block, Abort and Init MB_REDCL, the communication was able to reestablish / re-initializing for few days/hours , and the problem will come back. 

We also tried to reset the communication failure by plug/unplug the ethernet cable from the communication port or power cycle the prosoft gateway. The result is the same. 

The issue is resolved (based on 1 week results). We found the MB_REDCL block and Joblist block run sequence was not arranged properly.

Solution:

MB_REDCL FB909 run sequence must be arranged and placed in the first block and Job list block as the second block.