Page 82 - Computer science 868 Class 12
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The block diagram and truth table of a 4 × 1 Multiplexer is given below:
Inputs Output
A
3
S 1 S 0 Y
A 2 4 × 1 0 0 A
Multiplier Y 0 1 A 0
A 1
1
1 0 A 2
A 0 1 1 A 3
Truth table of 4 x 1 MUX
S 1 S 0
To draw the logic circuit of a 4 × 1 Multiplexer we have to first draw the circuit of a 2 × 4 Decoder. Add four input lines
with each of the AND gates. Now, pass the outputs of the four AND gates to a single OR gate. The circuit will be as
follows:
S 1 S 0
A' 0
A' 1
A 2
A 3
Working Principle of 4 × 1 Multiplexer
From the truth table, the Boolean expression for output Y can be written as:
Y = S '.S '.A + S 'S .A + S .S '.A + S .S .A 3
0
0
1
2
0
1 0
1
0
1
1
Say the state of selection switch S = 1 and S = 0 then output Y will be:
0
1
Y = 0.1.A + 0.0.A + 1.1.A + 1.0.A 3
2
0
1
Or Y = 0 + 0 + A + 0
2
From the high signal produced in the OR gate, it is evident that A is selected in this example.
2
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