S7-400 Intermitted Profibus Fault

I am very new to Siemens PLC’s and have been asked to look into an intermitted problem with a S7-400 and it’s profibus network.

This is an existing network that has been running for about 7 yrs with no new nodes added. We have approx. 80 nodes on the profibus with repeaters where required and run between encoders, VFD’s, remote racks, and an OP27 station.



We get an intermitted communication loss stopping PLC functions and restarting almost right away. I noticed we get all 3 lights (intF, extF, busF) on the DP module of the PLC come on red. We do not need to restart the PLC or any remote I/O, it all comes back on its own and the indicator lights go out.



The PLC diagnostic buffer generates faults related to loss of communication with remote rack, mostly OB86 and OB85.



The only problem we had that may have led up to this was a failure of an encoder node address, the encoder and profibus termination were replaced but we had to relocate the profibus connection point and tie it in at a different point. End of line resistors were switched and it all worked ok. We have rechecked these terminations and others around it, all checked out ok.

Any help to point me in the right direction is much appreciated

I think you have checked it, but may be the connections. If everything works for 7 years, you do not change anything and after a few seconds everything is back online check the wiring.

What EXACTLY does the diagnosic buffer say ?

Has anything changed in the setup ? Or did the problems start by itself ?

Is the OP27 and any other masters than the S7-400 included in the STEP7 project ?