Page 95 - Computer science 868 Class 12
P. 95
10. X (1)
Y (X+Y)
F
X' (2) ((X+Y)+(X'.Z)')'
(X'.Z)'
Z
With reference to the logic diagram given above, where X,X',Y and Z are inputs and F is the output, answer the following
questions: [ISC 2022]
(i) What is the expression at (1)?
(a) X.Y (b) X'+Y'
(c) X + Y (d) X+Y'
Ans. (c)
(ii) What is the expression at (2)? [ISC 2022]
(a) (X+Z)' (b) (X+Z')
(c) X’+Z (d) (X'.Z)'
Ans. (d)
(iii) What is the final expression F(X,Y,Z)? [ISC 2022]
(a) (XY + X'Z)' (b) ((X+Y) + (X'.Z)')'
(c) ((X+Y) .(X'.Z)')' (d) (X'+Z) + (XY)'
Ans. (b)
11. Draw the logic circuit of the XOR gate using the NOR gate only. [ISC 2020]
Ans. The POS expression of the XOR gate is (A'+B').(A+B)
A'
B' (A'.B')'
(A'+B').(A+B)
A
B (A+B)'
12. Draw the logic gate diagram for the reduced expression. Assume that the variables and their complements are available as
inputs. [ISC 2020]
Ans. B'
C'
A
B
13. Draw the logic circuit diagram for an octal to binary encoder and explain its working when a particular digit is pressed. Also,
state the difference between encoders and decoders. [ISC 2020]
Ans. The logic circuit diagram of an Octal to Binary Encoder is as 0
shown here.
1
Working Principle of Octal to Binary Encoder: Say the switch of 2
Octal number 2 is closed. 2 is connected only with OR gate B . 3
1
So a trigger applied makes B = 1 while B and B will retain their 4
1
2
0
default value of 0. Thus B = 0 B = 1 and B0 = 0 or 0 1 0 which is 5
2
1
the binary equivalent of 2 Similarly on pressing switch 5 which 6
is connected with B and B , we get B = 1 B = 0 and B = 1 or 7
1
0
2
0
2
1 0 1 which is the binary equivalent of 5.
An Encoder is a combinational circuit that convert decimal,
octal or hexadecimal numbers to its equivalent binary from. An
n
encoder has a maximum of 2 input lines and n output lines. B 2 B 1 B 0
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Computer Hardware 93

