Storage Elements

Abstract

Logic elements alone—used in the straightforward manner shown in the previous chapter—are not sufficient to build a computer. It is necessary to have elements which perform the function of storage. This fundamental need can be illustrated by a basically simple example. Push-buttons with momentary contacts produce a certain output (opened or closed contacts) only as long as certain input conditions prevail (the button is pushed or not pushed). In this respect, they act like (and really are) logic elements. Using only such push-buttons, it will not be possible to design a circuit which turns a light on when a button is pressed, but leaves it on after the button is released. In order to accomplish this task, some storage element has to be incorporated into the circuit which stores (or “remembers”) the fact that the button had been pressed. The storage element may be a relay as in a push-button motor control, or a simple mechanical device, as in a toggle switch which keeps the switch in the position into which it had been set last.