The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete control system is one designed to operate on Boolean (“on” or “off”) signals supplied by discrete sensors such as process switches.
A form of discrete control taught in every introductory course on digital electronics involves the use of circuits called logic gates. These circuits input one or more Boolean signals, and output a Boolean signal according to a simple rule such as “AND” or “OR”:
Industrial control systems rarely utilize logic gates in a direct fashion for discrete control systems, although the fundamental concepts of “AND,” “OR,” and other gate types are universally applied. Instead, control functions are either implemented using electromechanical relays and/or with programmable digital devices such as PLCs (Programmable Logic Controllers). This chapter focuses on the practical use of both technologies for industrial discrete control.
An “AND” function is equivalent to series-connected normally-open contacts in a relay control circuit, because the lamp will energize only if switch A and switch B are actuated:
An “OR” function is equivalent to parallel-connected normally-open contacts in a relay control circuit, because the lamp will energize if switch A or switch B is actuated:
The “NOT” function is equivalent to a single normally-closed contact in a relay control circuit, because the lamp will energize only if the switch is not actuated: