Basic Logic Gates

There are three basic logic gates each of which performs a basic logic function, they are called NOT, AND and OR. All other logic functions can ultimately be derived from combinations of these three. For each of the three basic logic gates a summary is given including the logic symbol , the corresponding truth table and the Boolean expression.

The NOT gate

The NOT gate is unique in that it only has one input. It looks like

The input to the NOT gate is inverted i.e the binary input state of 0 gives an output of 1 and the binary input state of 1 gives an output of 0.

 

is known as "NOT A" or alternatively as the complement of .

The truth table for the NOT gate appears as below

0	1
1	0

 

The AND gate

The AND gate has two or more inputs. The output from the AND gate is 1 if and only if all of the inputs are 1, otherwise the output from the gate is 0. The AND gate is drawn as follows

 

The output from the AND gate is written as (the dot can be written half way up the line as here or on the line. Note that some textbooks omit the dot completely).

The truth table for a two-input AND gate looks like

0	0	0
0	1	0
1	0	0
1	1	1

 

It is also possible to represent an AND gate with a simple analogue circuit, this is illustrated as an animation.

The OR gate

The OR gate has two or more inputs. The output from the OR gate is 1 if any of the inputs is 1. The gate output is 0 if and only if all inputs are 0. The OR gate is drawn as follows

 

The output from the OR gate is written as .

The truth table for a two-input OR gate looks like

0	0	0
0	1	1
1	0	1
1	1	1