GCSE Computer Science/Logic gates

Logic gates are the basic building blocks of any digital system. It is a circuit having one or more than one input and only one output. There are four main types of gate; NOT, AND, OR and XOR. There are also three gates which are the inverse of one of the main gates; NAND, NOR and XNOR. These inverse gates are the equivalent of the main gate type followed by a NOT.

Truth Tables
''- Draw truth tables and recognise a logic gate from its truth table - Produce truth tables for given logic circuits'' - 2016 CIE Syllabus p12

A truth table shows all of the possible inputs and outputs for a logic circuit. The example below is for a NOT gate. It shows that if the input is a 1 then the output is a 0 and vice-versa.

Gate Types
''- Understand and define the functions of NOT, AND, OR, NAND, NOR and XOR (EOR) gates, including the binary output produced from all the possible binary inputs - Recognise and use ... standard symbols used to represent logic gates'' - 2016 CIE Syllabus p12

NOT
A NOT gate will invert, or flip, any input. If the input is a 1 then it will output a 0 and vice-versa.

AND
An AND gate will return a 1 if, and only if, both of the inputs were a 1. If either, or both, inputs are a 0 then the output will also be 0.

OR
An OR gate will return a 1 if either, or both, of the inputs are a 1. It will only return a 0 if both inputs were a 0.

XOR
The XOR gate will return a 1 if either, but not both, of the inputs are a 1. If both, or neither, input is a 1 then it will return 0.

NAND
The NAND gate is the opposite of an AND gate.

NOR
The NOR gate is the same as an OR followed by a NOT, so is the inverse of an OR gate.

XNOR
The XNOR gate is the same as an XOR followed by a NOT, so is the inverse of an XOR gate.

Logic Circuits
''- use logic gates to create electronic circuits - produce a logic circuit to solve a given problem or to implement a given written logic statement'' - 2016 CIE Syllabus p12

A logic circuit is an electronic circuit which includes one or more logic gates. Below is an example of a logic circuit together with its logic table: