S7 DP/DP Coupler. Is 1st bit a "OK" status bit ?

[JesperMP]

Maybe this is obvious to someone more experienced with DP/DP couplers than I am.

We have setup a system with my PLC connected to a supervisory PLC via a DP/DP coupler.
Notice that there are OLM copper-to-fiber converters in between the two Profibus networks.

What puzzles me is that the very first input bit in both directions is always on. It cannot be controlled by the corresponding output bit in either of the PLC's.
For example, my inputs start at I40.0 and this bit is always on.
Is this normal ?
Is it kind of a status display that the connection is OK ?

Everything else works as expected.
One thing is that I have set the DP/DP Coupler to be "fail-safe", i.e. Inputs goes off if the connection is lost.

 

[JesperMP]

Ah, how I could have overlooked this in the manual I dont know.

It says clearly in the manual that if the DIA switch is on, then the 1st input bit behaves as I have observed.

 

[erfrabal]

Hi! Indeed, that is reserved for a DIAGNOSIS and data comparison between the 2 networks. I dont remember if a 0 is for data missmatch or a 1. If you are trying to send words or bytes in the first byte, try to set 1 byte spare in the first input asociated in each dp network and then continue with your I/O definitions

Greetings