DIGITAL LOGIC DESIGN JNTU previous years question papers
Time: 3 hours Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
1. (a) Design a circuit with four inputs and one output where the output is 1 if the input is divisible by 3 or 7.
(b) A safe has 5 locks:v,w,x,y,all of which must be unlocked for the safe to open.The keys to the locks are distributed among _ve executives in the following manner: Mr.A has keys for locks v& x
Mr.B has keys for locks v& y
Mr.C has keys for locks w& y
Mr.D has keys for locks x& z
Mr.E has keys for locks v& z
i. Determine the minimal no. of executives required to open the safe.
ii. Find all the combinations of executives that can open the safe, write an expression f(A,B,C,D,E) which speci_es when the safe can be opened as a function of which executives are present
iii. Who is the `essential executive' without whom the safe cannot be opened.
2. Explain about 4-bit binary Ripple Counters? 
3. Explain about the following:
(a) latch excitation table
(b) Merging of ow tables. 
4. Explain about PLA in Detail? 
5. Design a Excess-3 to BCD code converter using minimum number of NAND gates.
6. Reduce the number of states in the following state table and tabulate the reduce stateTable 
Present state Next state output
X=0 X=1 X=0 X=1
a f g 0 0
b d c 0 0
c f e 0 0
d g a 1 0
e d c 0 0
f f b 1 1
g g h 0 1
h g a 1 0
7. (a) Show the weights of three di_erent 4 bit self complementing codes whose only negative weight is - 4 and write down number system from 0 to 9
(b) Decimal system became popular because we have 10 fingers. A rich person on earth has decided to distribute Rs.one lakh equally to the following persons from various planets. Find out the amount each one of them will get in their respective currencies:
A from planet VENUS possessing 8 fingers
B from planet MARS possessing 6 fingers
C from planet JUPITER possessing 14 fingers
D from planet MOON possessing 16 fingers [7+8]
8. A Communication system is designed to transmit just two code words A(x1,x2,x3,x4) = 0010 and B(x1,x2,x3,x4)=1101.However, due to noise in the system, the received word can have as many as two errors. Design a combinational circuit such that output f1will be equal to 1 if the received word is A; output f2 will be equal to 1 if the received word is B and output f3 will be equal to 1 if the word received none of these two.