Introduction

Logic states

True False

1 0

High Low

+Vs 0V

On Off

The fundamental building block of all logic circuits is the gate. Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms which are used for the true and false states are shown in the table on the right.

Gates are identified by their function: NOT, AND, NAND, OR, NOR and EX-OR. Capital letters are normally used to make it clear that the term refers to a logic gate.

Logic gates are not always required because simple logic functions can be performed with switches or diodes:

Inputs and outputs

Gates have two or more inputs, except a NOT gate which has only one input. All gates have only one output. Usually the letters A, B, C and so on are used to label inputs, and Q is used to label the output. On this page the inputs are shown on the left and the output on the right.

The inverting circle (o)

Some gate symbols have a circle on their output which means that their function includes inverting of the output. It is equivalent to feeding the output through a NOT gate. For example the NAND (Not AND) gate symbol shown on the right is the same as an AND gate symbol but with the addition of an inverting circle on the output.

Truth tables

A B C

0 0 0

0 1 0

1 0 0

1 1 1

A truth table is a good way to show the function of a logic gate. It shows the output states for every possible combination of input states. The symbols 0 (false) and 1 (true) are usually used in truth tables. In the table to the right A B are the inputs and C is the ouput.

NOT gate

A NOT gate can only have one input. A NOT gate is also called an inverter.

A B C

1 0 0 1

0 1 0 1

Symbol Truth Table

Assuming the AND gate is used, A B are the inputs, C is the output and C- is the NOT gate. Now C is inverted to give us C-

AND gate

The output C is high if input A AND input B are both high(if both inputs...