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Boolean Algebra

Introduction

Boolean Algebra is a mathematical way of representing combinational logic circuits made from logic gates. The identities and theorems of Boolean Algebra allow complex logic circuits to be simplified.

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Logic Operations

The AND function is represented by a DOT like the product symbol sometimes used in mathematics

AND

Q = A . B


The OR function is represented by a PLUS like the addition symbol used in mathematics

OR

Q = A + B


The NOT function is represented by a BAR above the symbol

NOT

Q = A


The NAND function is a combination of the DOT for the AND and a BAR for the NOT. The BAR is over the DOT and includes both inputs

NAND

Q = A . B


The NOR function is a combination of the PLUS for the OR and a BAR for the NOT. The BAR is over the PLUS and includes both inputs

NOR

Q = A + B


Using Boolean Algebra to describe logic circuits

Logic circuits can be described using Boolean algebra. The aim is to express Q in terms of A and B

Example 1

Boolean Logic

Q = A + B


X = A
Q = X + B

Therefore:

Q = A + B
This reads as Q equals (NOT A) OR B

Example 2

Boolean Logic

Q = (A . B) + C


X = A
Y = X . B
Y = A . B
Q = Y + C

Therefore:

Q = (A . B) + C
This reads as Q equals ((NOT A) AND B) OR C

Example 3

Boolean Logic

Q = (A . B) + (A + B)


X = A . B
Y = A + B
Q = X + Y

Therefore:

Q = (A . B) + (A + B)
This reads as Q equals (A AND B) OR (A OR B)

Example 4

Boolean Logic

Q = (A . B) + B


Example 5

Boolean Logic

Q = (A + B) . (B + C)


Example 6

Boolean Logic

Q = B . (A + B)



Using Boolean Algebra to describe truth tables

Truth tables can be represented using Boolean algebra. For each row where Q = 1 there is a Boolean expression. If more that one row has Q = 1 then the individual Boolean expressions are combined with the Logic OR function.

Example 1

Boolean Logic truth tableThere is only one row where Q = 1 which is when A = 0 and B = 1

Alternatively, there is only one row where Q = 1 which is when A = 1 and B = 1

This can be expressed as Q = 1 when A = 1 AND B = 1 which is written as Q = A . B


Q = A . B

Example 2

Boolean Logic truth tableThere are two rows where Q = 1 which are when A = 0 and B = 0 or when A = 1 and B = 0

Alternatively, there are two rows when Q = 1 which are when A = 1 and B = 1 or when A = 1 and B = 1

This can be expressed as Q = 1 when (A = 1 AND B = 1) OR (A = 1 AND B = 1) which is written as Q = (A . B) + (A . B)


Q = (A . B) + (A . B)

Example 3

Boolean Logic truth tableThere are two rows where Q = 1 which are when A = 0 and B = 0 or when A = 0 and B = 1

Alternatively, there are two rows when Q = 1 which are when A = 1 and B = 1 or when A = 1 and B = 1

This can be expressed as Q = 1 when (A = 1 AND B = 1) OR (A = 1 AND B = 1) which is written as Q = (A . B) + (A . B)


Q = (A . B) + (A . B)

Example 4

Boolean Logic truth tableBy considering each row where Q = 1 the truth table can be represented as:

Q = (A . B) + (A . B) + (A . B)


Example 5

Boolean Logic truth tableThe same principle applies when there are three inputs (or more). Consider each row where Q = 1 and combine the Boolean expressions with Logical OR functions (+)

By considering each row where Q = 1 the truth table can be represented as:

Q = (A . B . C) + (A . B . C) + (A . B. C)


Logic AND Identities

A . 0 = 0
A . 1 = A
A . A = A
A . A = 0

AND TTEach of the above identities can be shown to be correct by considering the truth table for the AND function. In the identities A is the variable and can be either Logic 0 or Logic 1

Consider the case when A = 0. The four identities can now be written as 0 . 0 = 0 and read as "zero AND zero equals zero", 0 . 1 = 0, 0 . 0 = 0 and 0 . 1 = 0 all of which agree with the truth table.

Consider the case when A = 1. The four identities can now be written as 1 . 0 = 0 and read as "one AND zero equals zero", 1 . 1 = 1, 1 . 1 = 1 and 1 . 0 = 0 all of which agree with the truth table.


Logic OR Identities

A + 0 = A
A + 1 = 1
A + A = A
A + A = 1

AND TTEach of the following identities can be shown to be correct by considering the truth table for the OR function. In the identities A is the variable and can be either Logic 0 or Logic 1

Consider the case when A = 0. The four identities can now be written as 0 + 0 = 0 and read as "zero OR zero equals zero", 0 + 1 = 1, 0 + 0 = 0 and 0 + 1 = 1 all of which agree with the truth table.

Consider the case when A = 1. The four identities can now be written as 1 + 0 = 1 and read as "one OR zero equals one", 1 + 1 = 1, 1 + 1 = 1 and 1 + 0 = 1 all of which agree with the truth table.


Logic NOT Identity

There is only one NOT identity. A NOT followed by another NOT has no effect. The symbol is a double bar above the character.

A = A

This reads as NOT NOT A equals A. Consider when A = 0 and so NOT A equals one, A = 1 and therefore NOT (NOT A) is zero A = 0 giving A = A. A similar argument applies when A = 1.

DeMorgan's Theorem

DeMorgan's Theorem(s) are particularly important because they relate the (N)OR function and the (N)AND function and allow one to be equated to the other.

A . B = A + B

This reads as (NOT A) AND (NOT B) is the same as A NOR B. It is easy to prove this relationship with a truth table.

A + B = A . B

This reads as (NOT A) OR (NOT B) is the same as A NAND B. Again, this can be verified with a truth table.