The system layer is the layer responsible for processing and controlling process signals. The process signals are binary values that represent the state of sensors or actuators. The system layer uses bit-controlled logic functions to link the process signals and control corresponding outputs. The system layer is divided into zones, each of which monitors or controls a part of the overall process.
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One possible subdivision of signals in an automated system is the distinction between Constantly Monitored Zone (CMZ) signals and process signals. CMZs include all signals that are relevant to the safety and protection of the plant, such as fuses, monitoring, emergency stop switches, etc. These signals are constantly monitored and evaluated to trigger a shutdown or a warning if necessary. Process signals, on the other hand, are all signals that control or regulate the actual process sequence, such as sensors, actuators, measured values, etc. The division of signals into CMZ and process signals helps to define and simplify the requirements for sensor technology, signal processing and communication.