Page 68 - Digital Electronics by harish

Figure : Logic diagram of 8-to-1 multiplexer

Select lines Output
A B C Y
0 0 0 D 0
0 0 1 D 1
0 1 0 D 2
0 1 1 D 3
1 0 0 D 4
1 0 1 D 5
1 1 0 D 6
1 1 1 D 7

Figure : Truth table of 8-to-1 multiplexer

Y = D 0 + C D 1 + B D 2 + B C D 3 + A D 4 + A C D 5 + A B D 6 + A B C
D 7

When A = 0, B = 0 and C = 0, the first AND gate is enabled. Hence, the input D 0 (0 or 1) will
be available at the output. When A = 1, B = 1 and C = 1, the last (eighth) AND gate is
enabled and the output Y will be D 7. Similarly, the output is D2, D3, D4, D5 and D6 when
the inputs ABC are 001, 010, 011, 100, 101 and 110 respectively. IC 74151 is the 8-to-1
multiplexer. There are 16 pins in this IC.

68