Hi Mujeeb,
I have had once a defect IM/IM-BusModule, what happens was that the last active channel stays active. This introduce a problem when the active channel is switched off because the CPU is going to STOP where this active channel is on. But you will never know what is happening 
If it is very important I/O behind the IMs then you may built up it twice as switched I/O. 
The system have the possibility to get the I/O thats ok is switched by the extra softwareblocks that are coming with the STEP7 software {FB450 RED_IN; FB451 RED_OUT}.
You may mix all kind of I/O, single, double, single Profibus and double Profibus.
So when you have switched double I/O then is the change that it falls out very low. 
I hope that the answer will help, if you have still questions, don't hesitate to ask me.
All the best, Late
Never say never, think in solutions, not in trouble......
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