Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Digital Electronics
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occur on the inputs. Notice that the truth table lists the four possible
input conditions; together with each of their resultant outputs for each
condition. As shown by the truth table, the only time that the output
goes “high” is when the A input “and” the B input are high.
Logic gates can have more than two inputs. Figure 13-2 illustrates the
truth table for a three-input AND gate. Common logic gates are available
with up to eight inputs.
Referring again to Fig. 13-1, note the OR gate and its associated truth
table in Fig. 13-2. As the name implies, its output goes high whenever a
high appears on the A input, or on the B input (or both).
In digital terminology, a not function means that a logical condition
is inverted, or reversed. A NAND gate (short for not AND) is an AND gate,
with the output inverted. Notice that the outputs in the truth tables for
the AND gate, and the NAND gate, are simply inverted. This same principle
holds true for the OR and NOR gates.
It is common for the output of one logic gate to provide inputs for
several other logic gates. The maximum number of inputs that can be
driven by a particular logic gate is specified as its fanout. Typical logic
gates have fanouts ranging from 5 to 20. If it becomes necessary to drive
a greater number of inputs than the fanout of a particular gate, a buffer
is used to increase the fanout capability. The symbol for a buffer is illustrated
in Fig. 13-1.
The need often arises to invert a logic signal. The symbol for an
inverter is shown in Fig. 13-1. An inverter is sometimes called a not gate.
Note that it has a small circle on its output just like the NOT AND (NAND)
and NOT OR (NOR) gates. Anytime a small circle appears on an input or
output of a logic device, it is symbolizing the inversion of the logic signals
(or data). Also notice the horizontal line above the A output of the
inverter. It is called a not symbol. Whenever a horizontal line is placed
above a logic expression, it means that it is inverted.
Another common type of logic gate is the exclusive OR gate. Refer to its
symbol and the associated truth table in Figs. 13-1 and 13-2. As the truth
table indicates, its output only goes high when its inputs are different
from each other. The exclusive NOR gate provides the same logic function
with an inverted output.
Combining Logic Gates
The most basic type of digital system is designed to provide a logical output
based on a set of input conditions. This is referred to as conditional