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Computer Pioneers
Computer Ace
Computer Studies O level
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 2 of 22
This pamphlet incorporates the latest changes in the syllabus and should be used as
additional teaching material for the Computer Pioneers Computer Ace course.
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 3 of 22
1. System Analysis and Design (Chapter 6) ........................................................................... 4
1.1. Data Flow Diagrams (New Symbols) ........................................................................ 4
1.2. Action Plans ............................................................................................................ 5
PERT (Project Evaluation and Review Technique) ......................................................... 5
Gantt Charts ........................................................................................................... 6
Critical Path Analysis ............................................................................................... 7
1.3. Testing – (Chapter 6 – Section 3. Systems Implementation) ......................................... 8
2. Algorithm Design and Programming Concepts (Chapter 8) .................................................. 9
2.1. For – Next loop ....................................................................................................... 9
3. Video conferencing.........................................................................................................10
4. Logic Gates and Circuits .................................................................................................12
4.1. Introduction to Logic Gates ...................................................................................... 12
4.2. Truth Tables .......................................................................................................... 13
4.3. Combination of Logic Gates ..................................................................................... 16
4.4. Designing a Simple Logic Network ............................................................................ 19
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 4 of 22
1. System Analysis and Design (Chapter 6)
1.1. Data Flow Diagrams (New Symbols)
DFD is a pictorial representation that shows the flow of data in a system. It shows the
interaction between the entities and system.
The symbols used for DFD are:
This symbol is used to represent a process within
the system.
This symbol is used to represent an Entity that
interacts with the process
This symbol is used to represent a data store/file
used in the system.
Arrows show the flow of data in the system.
Process
Entity
Data Store
Data Flow
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V4.0 August 2011 Page 5 of 22
1.2. Action Plans
PERT (Project Evaluation and Review Technique)
PERT is a project management tool that is used to show and analyse the tasks involved in a
project. It shows the flow of dependent tasks that must be followed in order to complete the
project.
PERT charts are used to handle large projects where analysis of task and time allocation is
of great importance. They were originally developed in 1950s.
The steps used in PERT chart planning are:
1. Identification of activities involved in the project
2. Analysing the sequence of flow of dependencies
3. Developing a network diagram for the above identified activities
4. Estimating the time for each activity identified in Step 1
5. Determining the critical path for the set activities.
6. Updating the PERT chart as the project activities continue
Benefits of PERT
Ability to identify the expected project completion time
Shows the activity start and end dates
Helps to identify the probability of completion of tasks before a specified date
Shows the critical path activities that directly impact the completion time
Shows the activities that have slack time and could lend resources to critical path
activities in order to complete the project
Activity Symbol
PERT charts are created using series of boxes as shown above. Each box represents an
activity or a task.
Task name
Latest finish time
of an activity
Slack time
Duration of the activity
Earliest finish
time of an
activity
Latest Start time
of an activity
Earliest Start
time of an
activity
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 6 of 22
Example
In the above example, there are 2 activities, A and C.
Activity A must be completed before Activity C begins. Activity A requires 5 days and
Activity C requires 4 days. Therefore this part of the project will require 9 days in total.
Gantt Charts
A Gantt chart is a pictorial representation that illustrates a project schedule. It shows all the
tasks and resources needed to complete a project. It is a project management tool that
helps to show planned work targets.
Example:
A Gantt chart for a simple project is given below. The project has 5 tasks. Each task has
been allocated the number of days required to complete it.
Earliest Start
Duration
Earliest Finish
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V4.0 August 2011 Page 7 of 22
In this example, Task 1 (Analyse data) requires 5 days, Task 2 (Design the product)
requires 12 days and so on. Task 2 can start after 4 days of work of Task 1 and other
activities will follow according to the scheduled start dates.
Critical Path Analysis When planning a project, we want to know how long will the project take. When will we be
able to start a particular task? If this task is not completed on time, will the entire project
be delayed? Which tasks should we speed up in order to finish the project earlier?
Critical Path Analysis (CPA) is a pictorial representation for scheduling activities within a
project. It helps to determine the time required to complete a project and shows which
activities are critical and which are not.
The Critical Path is the route through the CPA, which has no idle time in any of the
activities. In other words, if there is any delay to any of the activities on the critical path,
the whole project will be delayed
The total time along this critical path is also the minimum time in which the whole project
can be completed.
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V4.0 August 2011 Page 8 of 22
Example
In this example, there is a clear sequence of events that have to happen in the right order.
If any of the events on the critical path is delayed, then the project cannot be completed on
time.
Task A-C-F takes 3 + 1 + 3 = 7 weeks
Task A-D takes 3 + 2 = 5 weeks
Task B-E takes 4 + 2 = 6 weeks
Task A-C-F is said to be the critical path as if it is delayed the entire project would be
delayed.
1.3. Testing – (Chapter 6 – Section 3. Systems Implementation)
There are three general categories of test data:
Normal data:
Valid data in a range that the system is able to handle.
Extreme data:
Valid data at the highest or lowest value that can be accepted for a variable.
Abnormal data:
Invalid data that should be rejected by the system.
Example 1: In the context of numbers within the range of 1 to 200:
1. Normal data: Any number between 1 and 200
2. Extreme data: Both 1 and 200, the maximum and the minimum values
3. Abnormal data: Any number outside the range 1 to 200, any negative number, any
alphabetic character or other symbol/character
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V4.0 August 2011 Page 9 of 22
Example 2: In the context of an alphabetic value:
1. Normal data: Any letter between „a‟ and „z‟
2. Extreme data: Both „a‟ and „z‟
3. Abnormal data: Any number or any other symbol/character
Example Test Case Field Data
type
Length Test Data Expected
Result
Actual Result
Customer
Number
Numeric 4 Normal : 24 System accepts System accepts
Abnormal : -2A System rejects System rejects
Extreme : 9999 System accepts System accepts
2. Algorithm Design and Programming Concepts
(Chapter 8 – Section 5. Pseudocodes - Repetition Control Structure)
As you have already learned, a repetition control structure has a repetition loop that will end
when the condition is false. In addition to the Repeat … Until and While … Do …
Endwhile structures, the For – Next loop is also a repetition structure.
2.1. For – Next loop
Example 1
Write an algorithm using pseudocode to input 5 numbers and display them. Use a For
Next loop.
For x = 1 to 5
Input x
Print x
Next x
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 10 of 22
Figure 7.8 Video conferencing
Example 2
Write an algorithm using pseudocode to print 5 “*” and display them. Use a For Next
loop.
For x = 1 to 5
Print “*”
Next x
Example 3
Write an algorithm using pseudocode to input 25 marks. If mark greater than 50, it
should display “Pass” or otherwise display “Fail”. Use a For Next Loop.
Start
Define variable x, mark
For x = 1 to 25
Input mark
If (mark > 50) then
Display “Pass”
Else
Display “” Fail”
End if
Next x
3. Video conferencing
Video conferencing is a communication technology which is also
known as video teleconferencing. This allows users to
communicate in two or more locations via video and audio
transmissions simultaneously (i.e. moving pictures and sound).
This facility can be used whenever a live conversation is needed
where the people involved cannot physically come to the same
location for the meeting. Another consideration resulting in the
use of video conferencing could be the expenses or the time that a person would have to
utilise to travel to a meeting.
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 11 of 22
Basic requirements needed for this facility:
Video camera or Webcam
Computer monitor or television
Microphones
Speakers, display device or telephone
Data transfer – telephone network, LAN or internet
Due to the availability of high speed internet connection with better images at a reasonable
cost, more people can afford to use this facility nowadays.
Reasons for the popularity of video conferencing:
Reduces cost - money is saved on travelling, accommodation, costs of lost time whilst
employees are out of the office, etc.).
Less impact on the environment (e.g. less traffic)
Flexibility – ability to call off a meeting at short notice, workers can work from home,
etc.
Allows staff around the world to see and interact with each other – more personal than
only hearing the voice
Applications of video conferencing:
Education – Distance learning programmes, virtual field trips
Business - Meetings on short notice
Media – Live press conferences
Health and Medicine – transmit data about a patient to another doctor for specialist
advice etc.
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4. Logic Gates and Circuits
4.1. Introduction to Logic Gates
A computer is an electronic device which can solve problems by accepting data, performing
various operations with the data, and supplying the desired output. The operations
performed by the computer can be divided into two categories; arithmetic operations and
logical operations. The central processing unit (CPU) in the system unit carries out all of
these operations. The central processing unit has a highly complex, extensive set of
electronic circuitry that executes stored program instructions. Have you ever wondered how
a computer carries out its operations? In order to find the answer to this question, you need
to understand something called Boolean algebra.
Algebra is a part of mathematics in which signs and letters represent numbers. Boolean
algebra is used as the basis of computer operations. This was developed by an English
mathematician called George Boole in the mid-nineteenth century.
Boolean algebra deals with binary variables and logic operations. Boolean variables can
have only two values, 0 or 1. Logic gates process signals which represent true or false.
Some common representations of logic operations are:
0 / 1
False / True
Off / On
Low / High
No / Yes
Open Switch / Close Switch
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 13 of 22
A X
NOT A X
Logic Gate Symbols
The following are the five main logical operators used in Boolean algebra:
Simple graphical
representations
MIL symbols used to
represent logic gates
4.2. Truth Tables
A truth table is used to show how the logic gate functions.
NOT Operator
This logical operator is different from the rest as it normally has only one operand/input.
Note that NOT simply reverses the truth-value of its operand.
A X (NOT A)
0 1
1 0
NOR
NAND
OR
AND
NOT
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AND Operator
Can you see that X (A and B) is true only when both A and B are True? It is false when at
least one of A and B is false.
OR Operator
Can you see that X (A OR B) becomes true when at least one of A and B is true? It is false
when both A and B are false.
A simple expression has only one operator. The examples above are simple Boolean
operators. Expressions can also have more than one operator and these are known as
compound expressions. Now we are going to derive truth tables for some compound
Boolean expressions.
A B X (A AND B)
0 0 0
0 1 0
1 0 0
1 1 1
A B X (A OR B)
0 0 0
0 1 1
1 0 1
1 1 1
A
B X
AND A X
X
A
B
OR A X
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NAND (AND & NOT) Operator
First values for A AND B were derived. Then the NOT operator was applied. You can
derive truth tables for compound Boolean expressions in stages as shown above.
NOR (OR and NOT) Operator
First apply the OR operator to variables A and B. Then apply the NOT operator to the
result from the OR operator.
A B A AND B NAND X
0 0 0 1
0 1 0 1
1 0 0 1
1 1 1 0
A B A OR B NOR X
0 0 0 1
0 1 1 0
1 0 1 0
1 1 1 0
X
A
B
NOR A X
A
B
X
NAND A X
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4.3. Combination of Logic Gates
Two or more logic gates can be combined in order to develop a more complex logic network.
Look at the following example. This contains two inputs (A, B) and two logic gates:
To draw the truth table for the above logic circuit, we need to identify the number of
combinations. There are two inputs (A, B). Therefore, number of combinations of 1s and 0s
would be 22=4. To derive the output of P we need to refer to the output of B.
A B P (NOT B) X (P*A)
1 1 0 0
1 0 1 1
0 1 0 0
0 0 1 0
Now let‟s consider three inputs (A, B, C) and three logic gates:
A
B
C
X
AND
X A
B
C P
Q
OR
NOT
AND
A
B
X
P NOT
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We begin by identifying the number of combinations. There are three inputs (A, B, C).
Therefore the number of combinations of 1s and 0s would be 23=8. Now we look at the
outputs produced at P and Q and construct the truth table.
A B C P Q (P * B)
1 1 1 0 0
1 1 0 1 1
1 0 1 0 0
1 0 0 1 0
0 1 1 0 0
0 1 0 1 1
0 0 1 0 0
0 0 0 1 0
Now the final output (i.e. Q OR A)
A Q X
1 0 1
1 1 1
1 0 1
1 0 1
0 0 0
0 1 1
0 0 0
0 0 0
Computer Ace – Computer Studies O Level NCC Education Computer Pioneers
V4.0 August 2011 Page 18 of 22
Exercise
Draw the truth tables for the following logic gate circuits.
1.
2.
3.
NOR
NOT
AND
A
B
X
AND
NOT
OR X NOT
AND
NOT
OR
A
B
X
A
B
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V4.0 August 2011 Page 19 of 22
4.4. Designing a Simple Logic Network
Consider the following statement:
“If A and B are on, then the lights will be on.”
In order to convert this on to a logic gate, let‟s consider 1=ON and 0=OFF.
If A=1 AND B=1 then X should be =1.
Now let‟s draw the logic network.
Once the logic circuit is drawn, we can then test it by drawing the truth table.
A B X (A*B)
1 1 1
1 0 0
0 1 0
0 0 0
Remember X should be ON only when A and B both are ON. The truth table shows us that
the above logic circuit is correct.
A
B
X
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V4.0 August 2011 Page 20 of 22
Now let‟s consider another problem.
“If A AND B are on AND if C is on, then the lights will be on.”
This means:
If (A=1 AND B=1) AND C=1 then X=1
To draw the logic circuit, let‟s first consider the part written in brackets (i.e. A=1 AND B=1)
which is one gate.
Then we move on to the next gate (i.e. P=1 AND C=1), which again is an AND gate.
Now let‟s combine both the gates, and draw the truth table.
A B P (A*B) C X (P*C)
1 1 1 1 1
1 1 1 0 0
1 0 0 1 0
1 0 0 0 0
0 1 0 1 0
0 1 0 0 0
0 0 0 1 0
0 0 0 0 0
A
B
C
X
P
C
X
A
B
P
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V4.0 August 2011 Page 21 of 22
Test and Review
Q1
Draw the truth tables for the following Logic gate circuits.
1.
2.
3.
AND
NAND
NOT
OR
NOR
A
B
C
X
NOR
NOR
NOR
A
B
C
D
X
NOR
NAND
A
B
C
X
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V4.0 August 2011 Page 22 of 22
Q2 Produce a simple logic network and the truth table for these statements.
1. If button A or button B are on and button C is off, then the alarm X is on.
2. A stadium has 3 entrances (A, B and C). A system is created which makes an alarm
X sound each time a door opens.
3. A nuclear plant gives out a warning signal (X = 1) when a process goes wrong. A
logic network is used to provide input and to decide whether X=1 or not. See the
table below.
A warning signal (X=1) will be generated if:
Either Temperature>=125C˚ (i.e. A=1) or
Temperature>=125C˚ (i.e. A=1) and Pressure< 160Pounds/Sq Inch and Chemical
Rate < 10 m3/s (i.e. B and C are both 0)
Input Binary Value Plant Status
A 1
0
Temperature>=125C˚
Temperature<125C˚
B 1
0
Pressure>= 160Pounds/Sq Inch
Pressure< 160Pounds/Sq Inch
C 1
0
Chemical Rate = 10 m3/s
Chemical Rate < 10 m3/s