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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: DECEMBER 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) 5 3 4 (7) + 4 3 6 (7)
ii) F 5 A C H - 9 E 4 8 H
iii) -7 6 5 (8) + 5 2 7 (8)
iv) 2 7 E . D 5 H + 4 B F . C 8 H ( 6 Marks )
b) Use 2’s complement to subtract B from A in the following :
i) A = 1 1 0 0 0 1 (2); B = 1 0 0 1 1 (2)
ii) A = 1 0 1 0 1 1 (2); B = 1 1 0 0 1 1 (2) ( 5 Marks )
c) Draw and label the logic symbol and truth table for each of the following 2–input logic gates:
i) NAND ii) Exclusive–OR (XOR) ( 4 Marks )
d) Determine the inverse (complement) and dual of the following logic expressions. (Do not simplify)
i) X = AB + BC
ii) Y = (A + B) • (B + C) (4 Marks )
e) Compute the following
i) 10’s complement of 6 3 8 . 4 7
ii) 8’s complement of 5 2 7 (8) ( 3 Marks)
f) Copy and fill in appropriately the coding systems given in table Q1(f) ( 8 Marks )
Clearly show all your working.
2
DECIMAL BINARY OCTAL
(BASE-8)
HEXADECIMAL
(BASE-16)
GRAY CODE EXCESS-3 8421 BCD
5 6
1 1 0 0 1 1
1 0 1 1 0 1 0 1
Table Q1(f)
QUESTION TWO (20 Marks)
a) (i) What are universal gates?
(ii) Give the names of the two universal gates and state clearly how each is used. ( 4 Marks )
b) A function is given by F = A BC + AC + BC
Copy and fill in the table below. ( 8 Marks )
Digits A B C ABC A C BC F Minterms Maxterms
c) Using the filled table in part (b) write down the function F in terms of
i) standard Sum-of-Products (SOP)
ii) standard Product-of-Sums (POS)
iii) the equivalent digits of the minterms
iv) the equivalent digits of the maxterms ( 8 Marks )
3
QUESTION THREE (20 Marks)
a) For the circuit given in fig. Q3;
(i) express the output function Y as a product of three sum-terms,
(ii) derive the truth table of the function Y. ( 8 Marks )
Y
G2
G4
G1
G3
B C A
G5
Fig. Q3
b) A logic function is expressed as F(A,B,C) = ?M ( 0 , 2 , 3 , 7 )
i) Derive the standard POS expression of the function F.
ii) Draw the K-map for function F.
iii) Use the K-map and derive the simplified POS expression of the function F.
iv) Hence or otherwise derive the simplified SOP expression of the same function
(8 Marks)
c) Draw the circuit diagram of the following logic function using 2–input NOR, AND, OR gates only.
(Do not use NOT-gates)
X = A B + A C + B C (4 Marks)
4
QUESTION FOUR (20 Marks)
a) A 4–input digital system sounds an alarm when two or more inputs are logical 1. ( 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 )
b) A digital system is given by the function
Y(A, B,C,D) = ?M( 1,5,8,9,12,13,14,15 )
Draw the truth-table for the function and express the function Y in terms of;
i) its minterms ii) its Maxterms ( 4 Marks )
c) Draw two 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 3.2 KHz clock signal is divided using some binary flip–flops to produce a 50 Hz output signal,
find the number of flip–flops required. ( 4 Marks )
c) Draw the logic circuit diagram, with its corresponding truth-table, for an R-S flip-flop using 2-
input NOR gates only. (4 Marks)
d) A Mod-6 (modulo-6) counter is designed as a ripple-up counter using toggle flip-flops. Assume the
clock signal and clear signal are negative edged.
i) Draw the truth-table of the Mod-6 counter.
ii) Hence, with a brief explanation, draw and label the circuit diagram of the Mod-6 counter.
(6 Marks)
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