Page 101 - Computer science 868 Class 12
P. 101
33. State the application of a Half Adder. Draw the truth table and circuit diagram for a Half Adder. [ISC 2017]
Ans. The half adder is a combinational circuit that is designed to perform the addition of two bits and produce the two binary outputs
as sum (S) and carry (C) bits.
Truth Table
Input Output
A B Sum Carry
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1
We can thus derive the Boolean expression of the sum and carry term as: Sum term = A'.B + A.B' which is the Boolean expression
of two variable XOR gate. Carry term = A.B which can be represented using AND gate. The logic circuit diagram of the half adder
is shown below:
A Sum = A⊕B
B
Carry = A.B
34. What is & Multiplexer? How is it different from a decoder? Draw the circuit diagram for a 8 : 1 Multiplexer. [ISC 2017]
Ans. Multiplexer is a combinational circuit which selects a single output from a set of inputs. A multiplexer has 2n input lines and one
output line, where n is the number of selection lines.
Decoder is a combinational circuit that converts any binary number to its equivalent Octal, Decimal or Hexadecimal form. It has
‘n’ input lines and maximum 2n output lines
The logic circuit diagram of a 8 x 1 MUX is given below:
A 0
A 1
A 2
A 3
A
4
Y
A 5
A 6
A 7
S 2 S 1 S 0
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