In addition to startups and shutdown of continuous systems,batch process are potential applications for computer controlled system.A stored control program is used to repeat all the events in an operation,step-by-step,throughout a production cycle. The program elements consist of on/off relay logic,time delay and elapsed-time periods,predetermined counts of pulses and pulse outputs for set-point control.
Elementary mathematical manipulations can be performed within a step of the logic program. Each of the stepped phases of all operational events is controlled by input contacts and feedback. These contacts inputs are from push buttons for manually initiated action or from limit switches for positional action.Also contact inputs can be derived from analog inputs for signal comparison or from other permissive interlock or inhibit action.