Learning Event 4:

EXPLAIN HOW COMBINATIONS OF GATES CAN CHANGE FUNCTIONS

1.

Logic systems can be designed many different ways to produce a specific result. The circuit

arrangement selected usually depends on the equipment designer. There is seldom a "best" way of

designing any binary system. Your job as a technician is to analyze and understand systems that already

exist, not to design systems. Very often, however, you will find that a careful analysis of a system or

partial system is the only way to prove its true operation.

2.

Look at the system in Figure 2-5 for an example. At first glance it appears to perform an OR

function of several inverted signals. If however, you use Boolean algebra, the system can be simplified

and proved to be performing an entirely different function. Simplify the final expression and what

remains? A 3-input AND function of ABC. The function is not at all what it first seems to be; the

function, at least for the output we are considering, represents an ANDing of the inputs.

Figure 2-5. A small system

3.

A truth table confirms our analysis of the function. The only input combination that enables the

circuit to produce a high output is when A, B, and C are all true (high). Trace each combination listed in

the truth table through the gates to be sure you understand the results.

4.

Normally, this circuit would not have been designed this way if only the ABC output was to be

used; a single AND circuit would have been used instead. However, peculiar arrangements of basic

circuits such as this one often occur within large logic diagrams, usually because other combinations of

the same input signals are required to satisfy other gates.