cp5611 card failure

Hi,

I am Balakrishnan representing schwing stetter India. We are using CP5611 card 6gk1561-1aa00 to communicate with plc from wincc through MPI port. But often this CP5611 card fails with the error message "synchronization error, profibus network failed" but when I do the hardware test it says the hardware is ok. We have provided the proper earth connection to the computer as well the panel also even though this card fails. Please let me know the reason of failures and what is the corrective action to be taken in order to avoid this problem.