This document discusses Boolean algebra concepts including logic gates, Boolean expressions, truth tables, and logic circuit design. It covers:
1) The definitions and truth tables of common logic gates like AND, OR, NAND, and NOR.
2) How to derive the Boolean expression of a logic circuit and construct a circuit from a given Boolean expression.
3) Converting between Sum of Products (SOP) and Product of Sums (POS) forms using techniques like standard SOP from a truth table.
This topic introduces the numbering systems: decimal, binary, octal and hexadecimal. The topic covers the conversion between numbering systems, binary arithmetic, one's complement, two's complement, signed number and coding system. This topic also covers the digital logic components.
This topic introduces the numbering systems: decimal, binary, octal and hexadecimal. The topic covers the conversion between numbering systems, binary arithmetic, one's complement, two's complement, signed number and coding system. This topic also covers the digital logic components.
1. 1
Further logic concepts
• NAND, NOR, XOR gates, utility of universal
logic functions,Boolean expressions of NAND
and NOR gate
• How to derive output Boolean expression of
given logic circuit.
• How to construct truth table given Boolean
expression
• How to draw logic circuit schematic given
Boolean expression.
• Obtain output Boolean Expressions in SOP and
POS form
2. 2
Variable, Literal (in Boolean Expressions)
• Variable is a symbol used to represent a logical
quantity
• Any variable can have a 1 or 0 value
• The complement of a variable A is indicated by
Ā or A’ (read as A bar)
• Literal is a variable or the complement of a
variable
3. 3
Boolean Expressions of Basic Gates
Input
A B
Output
X= A+B
0 0
0 1
1 0
1 1
0
1
1
1
Truth table for
OR gate
Input
A B
Output
X=A.B
0 0
0 1
1 0
1 1
0
0
0
1
Truth table for
AND gate
Input
A
Output
X=Ā
Can be written as
X= A’
0
1
1
0
Truth table for
NOT gate
4. 4
Boolean Addition
• Boolean Addition is equivalent to the OR
operation
• Basic rules for Boolean Addition:
1 + 1 = 1
1 + 0 = 1
0 + 1 = 1
0 + 0 = 0
• A sum term is equal to 1 when one or more
literals in the term are 1
• A sum term is equal to 0 only if each of the
literals is 0
5. 5
Boolean Multiplication
• Boolean Multiplication is equivalent to the AND
operation
• Basic rules for Boolean Addition:
1.1 = 1
1.0 = 0
0.1 = 0
0.0 = 0
• A product term is equal to 1 only if each of the
literals in the term is 1
• A product term is equal to 0 when one or more
literals are 0
6. 6
NAND Gate, NOR Gate
Input
A B
Output
Y=(A.B)’
0 0
0 1
1 0
1 1
1
1
1
0
Truth table for NAND gate
Input
A B
Output
Y=(A+B)’
0 0
0 1
1 0
1 1
1
0
0
0
Truth table for NOR gate
7. 7
Boolean Expression of a Logic Circuit
• To derive the Boolean Expression for a given
logic circuit, begin at the left-most inputs and
work towards the final output by writing the
expression for each gate
A
B
C
Y=
A.B
C
A.B.C
A.B.C + A
8. 8
Constructing Logic Circuit Given Boolean
Expression
• Identify the logic gates that you need from the
Boolean Expressions
• Connect the gates in such a manner to obtain
the desired output in the given Boolean
Expression
CBCACABy ++=The given equation is:
9. 9
Constructing Logic Circuit Given
Boolean Expression (Example)
• Draw the logic circuit schematic based on the
below Boolean expression.
).).(( ACBBAy ++=
`
A
B
C
y
).).(( ACBBAy ++=
)( BA+
).( ACB +
A
B
A
CB.(
B
C
10. Sum-of-Products (SOP)
• Two or more product terms summed by Boolean
Addition
• A single bar cannot extend over more than one
variable
DBACBAX .... +=DBACBAX .... += ✓
• Implementation
requires OR to
combine the outputs
of two or more AND
gates
11. Product-of-Sum (POS)
• Two or more sum terms multiplied
• A single bar cannot extend over more than one
variable
✓
• Implementation
requires AND to
combine the outputs
of two or more OR
gates
)).(( DBACBAX ++++=)).(( DBACBAX ++++=
12. Standard SOP from Truth Table
• List the binary values of the input variables for
which the output is 1
• Convert each binary value to the corresponding
product term by:
✓ Replacing 1 with corresponding variable
✓ Replacing 0 with corresponding variable complement
• These product terms which are composed of
every input variable or its complement ANDed
together are known as minterms
• Sum these minterms together
13. Standard POS from Truth Table
• List the binary values of the input variables for
which the output is 0
• Convert each binary value to the
corresponding sum term by:
✓ Replacing 0 with corresponding variable
✓ Replacing 1 with corresponding variable
complement
• These sum terms which are composed of
every input variable or its complement ORed
together are known as maxterms
• Multiply these maxterms together
14. 14
Constructing Truth Table from Boolean
Expressions (Example)
• Evaluate the Boolean expression for all possible
combinations of values for the input variables
Y = 1 when A=0, C=0, D=0, B=X (does not matter/don’t care)
OR when B=1, C=1, D=0, A=X (does not matter/don’t care)
OR when A=1, B=1, C=0, D=X (does not matter/don’t care)
A B C D Y
0 0 0 0 1
0 0 0 1 0
0 0 1 0 0
0 0 1 1 0
0 1 0 0 1
0 1 0 1 0
0 1 1 0 1
0 1 1 1 0
1 0 0 0 0
1 0 0 1 0
1 0 1 0 0
1 0 1 1 0
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 0
A
B
C
D
Y
DCA
DBC
CAB
CABDBCDCAY ++=