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Digital Electronics Question Paper

Digital Electronics

Course:Bachelor Of Science In Information Technology

Institution: Kca University question papers

Exam Year:2009

UNIVERSITY EXAMINATIONS: 2008/2009
FIRST YEAR EXAMINATION FOR THE DEGREE OF BACHELOR OF
SCIENCE IN INFORMATION TECHNOLOGY
BIT 1202: DIGITAL ELECTRONICS
DATE: AUGUST 2009 TIME: 2 HOURS
INSTRUCTIONS: Answer question ONE and any other TWO questions
QUESTION ONE (COMPULSORY) (30 Marks)
a) Perform the following arithmetic on the number systems indicated:
i) 4 2 3 (5) + 2 4 3 (5)
ii) 4 8 F 7 E H + C A 5 F H
iii) 7 2 4 (8) – 5 6 7 (8)
iv) E 7 2 . 2 5 D H - F B 4 . A 8 C H ( 4 Marks )
b) Use 2’s complement to subtract B from A in the following :
i) A = 1 0 0 0 1 1 1 (2); B = 1 1 1 0 0 1 (2)
ii) A = 1 1 0 1 0 1 1 (2); B = 1 1 1 0 0 1 1 (2) ( 4 Marks )
c) Draw and label the logic symbol and truth table for each of the following 2–input logic gates:
i) AND ii) NOR iii) Exclusive–NOR (XNOR) ( 6 Marks )
d) Draw the logic circuit, with its corresponding truth table, for an R-S flip-flop using
i) 2-input NAND–gates only
ii) 2-input NOR-gates only. ( 4 Marks )
e) Copy and fill in appropriately the coding systems given in the table below ( 12 Marks )
2
DECIMAL BINARY OCTAL
(BASE -8)
HEXADECIMAL
(BASE- 16)
GRAY
CODE
EXCESS - 3 2421
BCD
5 6
B 9
1011001
11000111
QUESTION TWO (20 Marks)
a) Name what are referred to as universal gates and state any two advantages these gates have over
the basic gates. ( 4 Marks )
b) A function is given by
F = AB C + BC + A C
Copy and fill in the table below. (8 Marks)
Digits A B C
A B C B C C A F Minterms Maxterms
0
1
2
3
4
5
6
7
c) Using the filled table in part (b) write down the function F in terms of
i) standard Sum-of–Products (SOPs)
ii) standard Product-of–Sums (POSs)
3
iii) the equivalent digits of the minterms
iv) the equivalent digits of the maxterms ( 8 Marks )
QUESTION THREE (20 Marks)
a) Give the truth table for the following circuit ( 8 Marks )
Y
A B C
G4
G1
G3
G2 G5
b) A logic function is expressed as F(A, B,C) =
i) Derive the standard SOP expression of the function F.
ii) Draw the K-map for function F.
iii) Use the K-map and derive the simplified SOP expression of the function F.
iv) Hence or otherwise derive the simplified POS expression of the same function
( 8 Marks )
c) Convert;
i) The Gray-code 1 0 1 1 1 0 1 0 0 1 1 to its equivalent binary word.
ii) The binary word 1 1 0 0 1 0 1 1 1 0 1 to its equivalent Gray–code. ( 4 Marks )
QUESTION FOUR (20 Marks)
a) A 4–input digital system sounds an alarm when two or fewer inputs are logical 0. (The alarm
sounds if the output X is a logical 1)
i) draw the truth-table for the alarm system.
ii) develop a standard SOP expression from the truth-table.
iii) using the K-map, minimize the SOP expression. ( 10 Marks )
4
b) A digital system is given by the function
Express the function Y in terms of;
i) its minterms ii) its Maxterms ( 4 Marks )
c) Draw separate K-maps of the function Y in part (b) and then use them to determine;
i) the simplified POS form of Y
ii) the simplified SOP form of Y ( 6 Marks )
QUESTION FIVE (20 Marks)
a) i) Draw a well-labeled circuit diagram of a clocked D-type flip-flop using NAND–gates only.
ii) Give the truth-table of the flip-flop ( 6 Marks )
b) If a 1.6 KHz clock signal is divided using some binary flip–flops to produce a 50Hz output signal,
find the number of flip –flops required. ( 4 Marks )
c) The circuit diagram in fig. Q5 (c ) is for a ripple–up counter
Given that both the clocking and clearing operations occur on the negative-edges;
i) Draw waveforms (timing diagrams) to show the clock (CLK) , QC , QB , QA and clear (CLR)
signals in that order.
ii) Hence from the waveforms, draw the truth table and determine, with a brief explanation, the
modulus (Mod–N) of the counter. ( 10 Marks )
Fig Q5 (c )

Digital Electronics Question Paper

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