8. Digital Circuits. Many modern electronic systems rely heavily on
the use of digital circuits.
Digital circuits are essentially
circuits within a circuit that perform a specialized function.
Digital circuits make use of two distinct voltage levels, a high
level and a low level, to convey information and to control functions
within the circuit.
These two voltage levels are known as the
"logic" levels of the digital circuit.
In order to understand and
work with digital circuits, it is necessary to have an understanding
of the binary numbering system and Boolean algebra.
The details of
the binary system and Boolean algebra are beyond the scope of this
subcourse.
For a review of these principles, see FM 11-72, Digital
Computers.
The basic elements used in digital circuits are gates.
Gates are
special circuits that produce a high or a low output based on the
high or low levels of two or more inputs.
There are two primary
types of digital gates, with several variations.
a. AND Gates.
The AND gate is a digital circuit designed to
produce a high level output when all input levels are high. If any
of the input levels is low, the output of the gate will be low. AND
gates are represented by the symbol shown in figure 1-31A.
Figure 1-31.
Logic Circuit Gates.
b. OR Gates.
The OR gate is designed to produce a high level
output when any one of the input levels is a high level.
OR gates
are represented by the symbol shown in figure 1-31B.
To provide variations to the two basic gates, inverters are used at
either the input or the output of the gate. The inverter is used to
change the logic level of the input or output from a high to a low,
or from a low to a high. The symbol for a logic
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OD1725
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