Bus(Profubus) Redundancy via Software redundancy with two CPU315-2DP

Dear All,

I just comissioned two redundant CPU315-2DP via software redundancy. Each CPU connect ET200M rack via its profibus thus haveing profibus redundncy. The CPU switchover redundancy is working fine. The problem occurs when bus redundancy is tested. When the active bus is diconnected from the CPU side what happens is that the digital status of Output bit change though the state of Inputbits didnot change. As soon as this bus is connected back again the output bits status switches back to the earlier state. Both the CPU have identical program for the output bit.

The system also contains redundant output. For it simplly we have to identical ET200M rack. Inside program based on a flag instead of changing state of one bit we change state of two bits of different racks. Soon bus switchover the output bit of both the racks change identically.

Apart from this there is a Y couppler cotaining a 300 CPU and two CP342. CP342 are also declared in hardware configuration of redundant system. They combine dual bus to single bus.

Can someone please help in suggesting possible point of error.