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1. Basic Logic Functions
Module 3
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.
LCST – Logic Circuits and Switching Theory
Source: Digital Fundamentals by Floyd, T. 11th Edition

2. Introduction
 In its basic form, logic is the realm of human reasoning that
tells you a certain proposition (declarative statement) is true if
certain conditions are true.
 Propositions can be classified as true or false.
 Many situations and processes that you encounter in your daily
life can be expressed in the form of propositional, or logic,
functions.
 Since such functions are true/false or yes/no statements,
digital circuits with their two-state characteristics are
applicable.
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

3. Learning Objectives
 After completing this module, the learners
should be able to:
1. List three basic logic functions
2. Define the NOT function
3. Define the AND function
4. Define the OR function
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

4. Basic Logic Functions
 In the 1850s, the Irish logician and mathematician George
Boole developed a mathematical system for formulating logic
statements with symbols so that problems can be written and
solved in a manner similar to ordinary algebra.
 Boolean algebra, as it is known today, is applied in the design
and analysis of digital systems and will be covered in Chapter
4.
 The term logic is applied to digital circuits used to implement
logic functions.
 Several kinds of digital logic circuits are the basic elements
that form the building blocks for such complex digital systems
as the computer.
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

5. Basic Logic Functions
 Three basic logic functions (NOT, AND, and OR) are indicated
by standard distinctive shape symbols
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

6. NOT
 The NOT function changes one logic level to the opposite logic
level.
 The NOT function is implemented by a logic circuit known as an
inverter.
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

7. AND
 The AND function produces a HIGH output only when all the
inputs are HIGH.
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

8. OR
 The OR function produces a HIGH output when one or more
inputs are HIGH.
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Module 3 – Basic Logic Functions
Prepared by:; fscjr.

9. Mastery Exercises
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1. When does the NOT function produce a HIGH output?
 When the input is LOW
2. When does the AND function produce a HIGH output?
 When all inputs are HIGH
3. When does the OR function produce a HIGH output?
 When any or all inputs are HIGH
4. What is an inverter?
 An inverter is a NOT circuit.
5. What is a logic gate?
 A logic gate is a circuit that performs a logic operation
(AND, OR).
Module 3 – Basic Logic Functions
Prepared by:; fscjr.