Section 3.1
Boolean Variables & Truth Tables

Boolean algebra differs in a major way from ordinary algebra in that boolean constants and variables are allowed to have only two possible values, 0 or 1.
See Section 1.4 to see how to define 0 and 1 values.

Boolean 0 and 1 do not represent actual numbers but instead represent the state of a voltage variable, or what is called its logic level.

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Some common representation of 0 and 1 is shown in the following diagram.

Logic 0	Logic 1
False	True
Off	On
Low	High
No	Yes
Open Switch	Close Switch

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In boolean algebra, there are three basic logic operations:
OR, AND and NOT.
These logic gates are digital circuits constructed from diodes, transistors, and resistors connected in such a way that the circuit output is the result of a basic logic operation (OR, AND, NOT) performed on the inputs.

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Truth Table

A truth table is a means for describing how a logic circuit's output depends on the logic levels present at the circuit's inputs.

In the following two-inputs logic circuit, the table lists all possible combinations of logic levels present at inputs A and B along with the corresponding output level X.

When either input A OR B is 1, the output X is 1. Therefore the "?" in the box is an OR gate. Go to next section to explore more on the OR gate.

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