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Digital Circuit Design Question Paper
Digital Circuit Design
Course:Bachelor Of Computer Science (It Telecommunication)
Institution: Kabarak University question papers
Exam Year:2010
Page 1 of 5
KABARAK UNIVERSITY
UNIVERSITY EXAMINATIONS
2010/2011 ACADEMIC YEAR
FOR THE DEGREE OF BACHELOR OF COMPUTER SCIENCE
COURSE CODE: COMP 223
COURSE TITLE: DIGITAL CIRCUIT DESIGN
STREAM: Y2S2
DAY: THURSDAY
TIME 9.00 – 11.00 A.M
DATE: 09/12/2010
INSTRUCTIONS:
• Answer Question ONE and any other TWO Questions. Question One carries
30marks while each of the other Two Questions carry 20marks.
PLEASE TURNOVER
Page 2 of 5
QUESTION 1 (30 marks)
a) i) Perform the following arithmetic
I) CDFH + ABCH (2mks)
II) 00001000 – 00000011 (2mks)
ii) Convert (15.625)10 into binary (2mks)
b) i) State De-Morgan’s theorem of two variables (2mks)
ii) Consider the given logic circuit
If the inputs are A, B, C and D in that order from top to bottom and the output
is Y;
I) obtained the unsimplified output logic expression for the
above circuit (3mks)
II) Using De-Morgan’s and Boolean theorem’s, simplify the
output logic expression in (I) (3mks)
III) Draw a logic circuit of the simplified function in (II) (1mk)
c) i) What is a logic gate? (1mk)
ii) Show using diagrams how you can use a NAND gate to implement an AND
function and an OR function (4mks)
d) i) What is a truth table? (1mk)
ii) State two uses of truth tables (2mks)
iii) Negate the given logic function (2mks)
X Y X Y
- -
. + .
e) In a chemical processing plant, a liquid chemical is used in a manufacturing process.
The chemical is stored in three different tanks. A level sensor in each tank produces a
HIGH voltage when the level of chemical in the tank drops below a specified fixed point.
Page 3 of 5
Design and draw a circuit that monitors the chemical level in each tank and indicates
when the level in any two of the tanks drops below the specified point
(3mks)
f) Draw a combinational circuit to implement the given logic function (2mks)
W = XY(Z + Y )+ XZ
QUESTION 2 (20 marks)
a) i) Draw a logic symbol of a NOR gate (1mk)
ii) Manipulate the given logic function into a form which can be implemented using
NOR gates only (5mks)
- - -
Y = ABC+ AC + B
iii) Draw the logic diagram of the resulting manipulated function in (ii) above (3mks)
b) Simplify the following logic expressions and draw the logic circuits for the
simplified functions.
i)
- - -
W = X.Y + X .Y + X .Y (3mks)
ii)
______________________
Y = (A + C) × (B + D) (4mks)
f) Determine the output of the following logic circuits:
(4mks)
A
B
C
D
Page 4 of 5
QUESTION 3 (20 marks)
a) i) Design a two bit comparator circuit that will produce a logic 1 output when the two
input signals are identical (3mks)
ii) Manipulate the output logic expression of the two bits circuit in (i) into a form which
can be implemented using NAND gates only. (2mks)
iii) Draw the circuit diagram of the manipulated function in (ii) (2mks)
b) i) What is Karnaugh map? (1mk)
ii) Prepare K-map for the given function (3mks)
- - - - - -
W = ABCD + ABCD + AB D + ACD + AC D
iii) Use the prepared K-map to simplify the function, write down the simplified logic
function (2mks)
c) Explain the following logic circuits
i) Sequential logic circuit (2mks)
ii) Combinational logic circuit (2mks)
d) State and explain two classifications of sequential logic circuits (3mks)
QUESTION 4 (20 marks)
a) Differentiate between the following (4mks)
i). Programmable logic devices and fixed logic devices
ii). Serial input/output and parallel input/output register
b) i a) i) What is a counter? (1mk)
ii) State two uses of counters (2mks)
iii) Draw a 4-bit asynchronous binary counter using JK flip-flops (4mks)
iv) State two advantages of synchronous counters (2mks)
c) Connect the chips provided below to implement
Y A B BC
-
= . + , given that pin 7 and 14 on each IC represent GND and +Vss respectively.
(5mks)
d) State two uses of shift register (2mks)
Page 5 of 5
QUESTION 5 (20 marks)
a) Define the following terms as used with sequential circuits (4mks)
i). State
ii). State diagrams
iii). State tables
iv). Clock width
b) Consider the following sequential circuit
The circuit has one input X, one output Z and two state variables Q1 and Q2
i). Write the Boolean expressions which can be used to determine the behaviour of
the circuit (3mks)
ii). From the Boolean expressions in (i), develop the state table for this circuit.
Assume the circuit present state is 00 and input X = 0 (4mks)
iii). Use the state table to develop the state diagram for this circuit. (4mks)
c) i) What is a programmable array? (1mk)
ii) Draw a programmable array which can give (4mks)
W A.B 1
-
= ,
- -
W = A.B 2 ,
-
W = AB 3 , W A.B 4 =
S
D
CP
R
Q2
2
_
Q2
S
D1
CP
R
Q1
_
Q1
X
CLK
Z
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