Page 62 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Primitive Logic Functioms

Functions versus Gates
Simple functions such as BUF, NOT, AND, NAND, OR, NOR, XOR, and
XNOR are ofteri known as primitive gates, primitives, logic gates, or simply
Strictly speaking, the term logic function implies an abstract mathematical
relationship, while logic gate implies an underlying physical implementation. In
practice, however, these terms are often used interchangeably.

More complex functions can be constructed by combining primitive gates
in different ways. A complete design-say a computer-employs a great many

gates connected together to achieve the required result. When rhe time arrives
to translate the abstract representation into a particular physical implementation,
the logic symbols are converted into appropriate equivalents such as switches,
transistors, or pneumatic valves. Similarly, the FAL5E and TRUE logic values are
mapped into appropriate equivalents such as switch positions, voltage levels,
or air pressures. The majority of designs are translated into a single technology.
owever, one of the advantages of abstract representations is that they allow
designers to implement different portions of a single design in dissimilar tech-
nologies with relative ease+ Throughout the remainder of this book we will be
concentrating on electronic implementations.
Finally, if some of the above seems to be a little esoteric, consider a real-
world example from your home, such as two light switches mounted at opposite
ends of a hallway controlling the same light. If both of the switches are UP or
DOWN the light will be ON; for any other combination the light will be OFF.
Constructing a truth table reveals a classic example of an XNOR function.

* The reasoning behind using the term “gate” is that these functions serve t3 control electronic
signals in much thse same way that farmyard gates can be used to control animals.