August 2000
LD201 : LOGIC DESIGN

QUESTION 4

Total Marks: 15 Marks

• Question 1
• Question 2
• Question 3
• Question 5
• Cover Page

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SUGGESTED SOLUTIONS
for Question 4

(a) (i) Give the truth table for a 2 × 4 decoder. [2]
(ii) Draw a circuit implementing this behaviour. [2]
(iii) Show how to construct a 3 × 8 decoder, using two 2 × 4 decoders and
other suitable gates. [2]

(b) The circuit below implements a three-bit adder using XOR gates.

(i) Give the characterisation table for an XNOR gate. [1]
(ii) Redesign the three-bit adder to use XNOR gates in place of the XOR
gates. [3]

(c) Convert F(A,B,C,D) = (0,1,3,5,9,11,13,15) into the minterm form and
Implement the function with a multiplexer and other necessary logic gates.
Show the implementation table using A as input and B,C,D as the selectors.
You need not give the Truth table. [5]