Review of Combinational-logic Techniques
L.F. Lind, John C. C. Nelson · 1977
Any logic device (or indeed, system) can be viewed as a processing unit. Certain inputs are applied, and certain outputs then appear. These inputs and outputs can take many different forms. For example, they can be voltages, currents, pressure, light, etc., or a mixture of these variables. In the majority of logic devices currently available, the inputs and outputs are voltages. For definiteness, all inputs and outputs in the ensuing discussion will be assumed to be voltages unless stated otherwise.