State of controller when servers are down
In my plant the AS(Aspect server) and CS(Connectivity Server) are of 1oo2 configuration and the controller is PM891. We are using PROFIBUS DP for field network.
I would like to know whether the running sequence will go into HOLD or run normally (till the completion of running sequence) if suddenly all the servers are down due to power failure. Please note that the field network is woring fine at the same time. Also is there any possibility that the sequence will not cut off as required(e.g If i have started a sequnece of transferring material of 500 Kg, and after sometime the servers are down (but the controller and IO panels are UP) then will the phase will be completed after transferring 500 Kg or the complete control system will fail.)
Also suggest how to put the system in to Failsafe mode whenever there is any communication error between controller and CS.
Operation of Controller does not depend on connectivity to Aspect server or connectivity server.
You said your AS and CS are in 1oo2 configuration therefore both failing at same time is rare. also if you have redundancy at network level then chances of total network failure is also minimized.
No matter what happens to network, AS or CS , the controller will continue to execute the program except that system is running 'blind' towards operation.
Although possible to program the controller to failsafe when network connection breaks, it is not advisable to do so, because it is not necessary or it might not be good for safety. Rather make your system redundant at each level so that there in no single point of failure.