التجارب العملية

Inverter Operation
Figure 3-2 shows the output of an inverter for a pulse input, where t l and t z indicate the cor-
responding points on the input and output pulse waveforms.
When the input is LOW, the output is HIGH; when the input is HIGH, the output
is LOW, thereby producing an inverted output pulse.
THE AND GATE
The AND gate is one of the basic gates that can be combined to form any logic function.
An AND gate can have two or more inputs and performs what is known as logical
multiplication.
After completing this section, you should be able to
- Identify an AND gate by its distinctive shape symbol or by its rectangular outline
symbol - Describe the operation of an AND gate - Generate the truth table for an
AND gate with any number of inputs - Produce a timing diagram for an AND gate
with any specified input waveforms - Write the logic expression for an AND gate with
any number of inputs - Discuss examples of AND gate applications
The term gate is used to describe a circuit that performs a basic logic operation. The
AND gate is composed of two or more inputs and a single output, as indicated by the stan-
dard logic symbols shown in Figure 3-8. Inputs are on the left, and the output is on the right
in each symbol. Gates with two inputs are shown; however, an AND gate can have any num-
ber of inputs greater than one. Although examples of both distinctive shape symbols and
rectangular outline symbols are shown, the distinctive shape symbol, shown in part (a), is
used predominantly in this book.
Operation of an AND Gate
An AND gate produces a HIGH output only when all of the inputs are HIGH. When any of
the inputs is LOW, the output is LOW. Therefore, the basic purpose of an AND gate is to
determine when certain conditions are simultaneously true, as indicated by HIGH levels on
all of its inputs, and to produce a HIGH on its output to indicate that all these conditions are
true. The inputs of the 2-input AND gate in Figure 3-8 are labeled A and B, and the output
is labeled X. The gate operation can be stated as follows:
For a 2-input AND gate, output X is HIGH only when inputs A and B are HIGH;
X is LOW when either A or B is LOW, or when both A and B are LOW.
THE OR GATE
An OR gate can have more than
two inputs.
The OR gate is another of the basic gates from which all logic functions are
constructed. An OR gate can have two or more inputs and performs what is known as
logical addition.
After completing this section. you should be able to
- Identify an OR gate by its distinctive shape symbol or by its rectangular outline
symbol - Describe the operation of an OR gate - Generate the truth table for an OR
gate with any number of inputs - Produce a timing diagram for an OR gate with any
specified input waveforms - Write the logic expression for an OR gate with any
number of inputs - Discuss examples of OR gate applications
An OR gate has two or more inputs and one output, as indicated by the standard logic
symbols in Figure 3-17, where OR gates with two inputs are illustrated. An OR gate can
have any number of inputs greater than one. Although both distinctive shape and rectan-
gular outline symbols are shown. the distinctive shape OR gate symbol is used in this
textbook.
Operation of an OR Gate
An OR gate produces a HIGH on the output when any of the inputs is HIGH. The output is
LOW only when all of the inputs are LOW. Therefore, an OR gate determines when one or
more of its inputs are HIGH and produces a HIGH on its output to indicate this condition.
The inputs of the 2-input OR gate in Figure 3-17 are labeled A and B. and the output is la-
beled X. The operation of the gate can be stated as follows:
For a 2-input OR gate, output X is HIGH when either input A or input B is HIGH,
or when both A and B are HIGH; X is LOW only when both A and B are LOW.
The HIGH level is the active or asseI1ed output level for the OR gate. Figure 3-18 illus-
trates the operation for a 2-input OR gate for all four possible input combinations.