Combinational Circuits

Abstract

Combinational circuits realize logic functions by means of switching elements. There must be a physical carrier for the values 0 and 1. Low and high air pressure represent, for instance, these values in pneumatic circuits. If the air is substituted by liquid, again low and high pressure represent these values in hydraulic circuits. Both pneumatic and hydraulic combinational circuits are used in control systems, but mostly the physical carrier for the values 0 and 1 is the electrical voltage. A threshold divides the maximum voltage used for the circuit into two parts. Generally the lower interval of voltage is associated to the value 0, and voltage values higher than the fixed threshold represent the value 1. An electrical combinational circuit consists of switching elements that realize logical operations. These elements are called gates. The voltages at the inputs of a gate determine the voltage at its output. A wire connects the output of one gate to at least one input of another gate or to the output of the whole circuit. The output of a gate determines the voltage of the wire and controls the voltage of the connected inputs of other gates. Both the gates and the wired connections between them determine the behaviour of the combinational circuit. The outputs of a given circuit structure depend therefore only on the values (expressed by voltages) at the inputs of the circuit.