simatic is similar to a digital computer which can store and run programs. the simatic also has some inputs and outputs. the running program on the simatic controls these inputs and outputs. both are connected by electrical wires with the machine. that makes a communication between the simatic and any machine possible. as the case may be the inputs and outputs are set on a “high” signal or on a “low” signal by the program. these are different electrical definitions. finally these signals are important if an action in the machine happens or not.