I'm trying to set up a three-state DC block that has a fairly complex interlock requirement and I like to get the groups opinion on the best way to implement it. What it is required to do is if the block in running in Active 2 and on the the required inputs fails it needs to go to Active 1 and prevent the operator from returning to active 2 without a reset button being pressed. The same for Active 1 if it fails it needs to change to Passive and not allow return until reset.
I know how to interlock to the passive state but I'm not sure the best way to implement the active 2 to active 1 interlock. Any suggestions would be appriciated.
Based on how the interlock function works I'd say that your best option is to have the move from Active 2 to Active 1 be handled by logic that sets the mode to CAS and set the CAS_IN_D = 1. You could then have the logic that is going to make the INTERLOCK_D = 0 put the mode back to AUTO though that's not required but you'll need for it to be in AUTO before you restart so the operator can choose the correct setpoint.