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Unformatted text preview: mputer are carried out by means of combinations of signals
passing through standard blocks of builtin circuits that are known as logic gates. In other
words, a logic gate is an electronic circuit which operates on one or more input signals to
produce standard output signals. These logic gates are the building blocks of all the
circuits in a computer.
Computer circuits are built up using combinations of different types of logic gates to
perform the necessary operation. There are several types^of gates, but we shall consider
here only some of the most important ones. These are sufficient to introduce the concept
of circuit design using logic gates.
AND Gate
An AND gate is the physical realization of the logical multiplication (AND) operation.
That is, it is an electronic circuit that generates an output signal of 1 only if all input
signals are also 1.
To have a conceptual idea, let us consider the case of Figure 6.18. Here two switches A
and B are connected in series. It is obvious that the input current will reach the output
point only when both the switches are in the on(l) state. There will be no output (output =
0) if either one or both the switches are in the off(0) state. Hence, two or more switches
connected in series behave as an AND gate.
Input
Output
The behaviour of a logic gate, that is the state of its output signal depending on the
various combinations of input signals, is conveniently described by means of a truth
table. The truth table and the block diagram symbol for an AND gate for two input
signals are shown in Figure 6.19. Since there are only two inputs (A and B), only four
(22) combinations of inputs are possible. Also observe from the truth table that an output
of 1 is obtained only when both the inputs are in 1 state, otherwise it is 0. Inputs Output A B C = AB 0
0
1
1 0
1
0
1 0
0
0
1 Figure 6.19. Block diagram symbol and truth table for an AND gate.
OR Gate
An OR gate is the physical realization of the logical addition (OR) operation. That is, it is
an electronic circuit that generate...
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This document was uploaded on 04/07/2014.
 Spring '14

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