Understanding Digital
Buffer,, Gate, and Logic
IC Circuits
by Ray Marston
PART 1: Digital Buffer IC Circuits
Ray Marston explains logic gate basics and takes a detailed look
at practical TTL and CMOS digital buffer IC circuits in this
opening installment of a five-part mini series.
Logic Gate Symbology
The eight best known types of digital
logic gates are the buffer and the NOT, OR,
NOR, AND, NAND, XOR (EX-OR), and
XNOR (EX-NOR) types. Many different
symbols can be used to represent each of
these eight basic logic gate elements. Figure
1 shows four different families of symbols
that are widely used in different parts of the
world today; of these, the American
MIL/ANSI symbols are by far the most popular, are instantly recognizable, are used by
most of the world’s practical digital engineers, and are used throughout this series.
Two useful variations of these
American symbols are also widely used
and are shown added to a standard
inverter symbol in Figure 2; the left-hand
symbol is internationally recognized and
indicates that the logic element has a
Schmitt trigger input action; the right-hand
symbol — which is widely used but is not
universally recognized — indicates that the
logic element has an open-drain (o.d.) or
open-collector (o.c.) output stage.
Logic Gate Functions
The functional action of any logic gate
can be described either in words or in a
tabular or symbolic way. The following list
describes the functions of all eight
basic types of logic
FIGURE 1. A selection of
widely used logic symbols.
72
April 2007
