Page 67 - Digital Electronics by harish

4-to-1 Multiplexer

In a 4-to-1 multiplexer, there are 4 data input lines and one output line. We need at least 2
select lines to select any one of the 4 inputs to the output line. The logic diagram, truth table
and logic equation of 4-to-1 multiplexer are shown in figure.

Figure : Logic diagram of 4-to-1 multiplexer

Select lines Output
A B Y
0 0 D 0
0 1 D 1
1 0 D 2
1 1 D 3

Figure : Truth table of 4-to-1 multiplexer

Y = D 0 + B D 1 + A D 2 + A B D 3

When A = 0 and B = 0, the first AND gate is enabled. Hence, the input D 0 (0 or 1) will be
available at the output. When A = 0 and B = 1, the second AND gate is enabled and the
output Y will be D 1. When A = 1 and B = 0, we will get D 2 at the output. Similarly, Y = D 3
when A = 1 and B = 1.
IC 74153 is a dual 4-to-1 multiplexer. It has two sets of 4-to-1 multiplexer in a single
package. There are 16 pins in this IC.

8-to-1 Multiplexer

In a8-to-1 multiplexer, there are 8 data input lines and one output line. We need at least 3
select lines to select any one of the 8 inputs to the output line. The logic diagram, truth table
and logic equation of 8-to-1 multiplexer are shown in figure.

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