GATE | GATE CS 1999 | Question 33
Consider the circuit shown below . In a certain steady state, the line Y is at ‘ 1 ‘. What are the possible values of A, B and C in this state?
a). A=0, B=0, C=1
b). A=0, B=1, C=1
c). A=1, B=0, C=1
d). A=1, B=1, C=1
(A) b only
(B) Both a and b
(C) Both a and c
(D) a, c and d
Explanation: Since f = 1,
2nd level AND gate must give 1 as a result for which C has to be 1 along with the lower first level NAND gate.
Thus, C = 1 is compulsory
Now NAND is 1 when any of the inputs are zero.
Let G be the NAND gate with input A and H be the NAND with input B.
Since, C =1 and H = 1 is needed so clearly we have following possibilities :
B = 0 , G = 1 ………(1) B = 0 , G = 0 ………(2) B = 1, G = 0 ……….(3)
Since G is also a NAND gate and f = 1
G = 1 means A = 0 and G = 0 means A = 1
using these in (1),(2) and (3) :
A = 0 , B = 0 , C = 1 A = 1 , B = 0 , C = 1 A = 1, B = 1 , C = 1
Option (D) is correct.
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