Page 66 - Digital Electronics by harish

BCD to 7-segment display decoder logic is available in IC form also. IC 7447 is used for
common anode displays and IC 7448 is used for common cathode displays.

2.12 Multiplexer (MUX)

Multiplex means „many to one‟. A multiplexer is a logic circuit with many inputs and only
one output. We can select from any one of the data inputs to the output by using the control
input signals. Multiplexer is also called „data selector‟ and the control inputs are called
„select‟ inputs.

m
m
If there are „m‟ select lines, we can have a maximum of 2 data input lines. Hence, n ≤ 2 .

2-to-1 Multiplexer

In a 2-to-1 multiplexer, there are 2 data input lines (D0, D1) and one output line (Y). We
need at least 1 select line (A) to select any one of the 2 inputs to the output line. The logic
diagram, truth table and logic equation of 2-to-1 multiplexer are shown in figure.

Figure : Logic diagram of 2-to-1 multiplexer

Select line Output
A Y
0 D 0
1 D 1

Figure : Truth table of 2-to-1 multiplexer

Y = D 0 + A D 1

When A = 0, the first AND gate is enabled. Hence, the input D 0 (0 or 1) will be available at
the output. When A = 1, the second AND gate is enabled and the output Y will be D 1.
IC 74157 is quad 2-to-1 multiplexer. It has four sets of 2-to-1 multiplexer in a single package.
There are 16 pins in this IC.

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