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Profibus Line Redundancy Issue in FREELANCE

We have Freelance 2013 SP1 and redundant AC902F controllers and redundant CI930F .. Whenever I enable Profibus Line Redundancy both CI840 , CI930F and redundant controller start switch over continuously. Without line Redundancy System working fine through Line A. To ensure proper cabling of Line B I shifted line B cable to Line A and it's working fine.. Even I tried by configuration of Stand Alone controller also but results same.As I enable Profibus Line Redundancy Both Controller and CI start switching.
-Controller AC902F and CI930F Firmware updated already. ..
.. Please suggest something to resolve this issue. .. Thanks in advance. ..