LOGIC GATES: And, Or, Not

Revision Guide

Types of Gates;

The main gates are;

  • AND
  • OR
  • NOT

Each of these gates have a different purpose and do different things, each of them have a different way of inputting data and outputting it. In each of these descriptions try and imaging the switches and how a light bulb works.

First of all an 'AND' gate

The AND GATE is a basic digital logic gate- it behaves according to the truth table below. A HIGH output (1) results only if both the inputs to the AND gate are HIGH (1). If neither or only one input to the AND gate is HIGH, a LOW output results. So imagine this as some switches and light bulbs. If both switches are off there is no light, if one switch is one and the other is off there is still no light, the only way you can possibly get light is to make sure that both switches are on. So the only way to get an output from an AND GATE is to make sure that there are 2 firm inputs. If there is only one input and there is nothing else then you will receive no output.


Secondly a 'OR' gate

The OR GATE is a digital logic gate- it behaves according to the truth table below. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is HIGH, a LOW output (0) results. So again think of this as a switch to control lights; if both the switches are off you will have no light, but unlike the AND GATE this gate can function with only one input so the light can turn on with only one switch. So the light can be turned on with any number of switches down and will only be off when the switches are up. BOTH OF THEM. So back to gates, you can get an output from a OR GATE with only one input you don't need both inputs, the only time you will have no output is when there is absolutely no input.

Finally a 'NOT' gate

In digital logic, an inverter or NOT GATE is a very simple logic gate. The truth table is shown below. This gate is practically a opposite gate there is no high or low input or out put its either or. So back to the lovely light switches. When the light switch is pressed down to theoretically put the light on, the light turns off. When you put the switch back up again to theoretically turn the light off then the light turns on. So in computer terms, if there is no input there will be solid output, if there is a solid input there will be no output.


Now Logic Tables...

Logic Gates are how the input and output and binary all add up to create an output... So each of these gates have a different way of using binary to input and output. As described in each gate analysis logic gates are gates which channel input and output in different ways and are shown on the logic tables. There are many other logic gates and tables but these are the most common and defiantly the most simple. Just remember that logic gates are binary digits, and on a table 1 means there is a HIGH input and 0 means there is LOW/NO input which depending on the gate itself result in either 1 OR 0. 1 meaning yes there is an output, and 0 meaning no there is no output.