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

Specification link - 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.

IN	OUT
0	1
1	0

Gate Types

Specification link - 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

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

A	Y
0	1
1	0

AND

An AND Gate

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.

A	B	Y
0	0	0
0	1	0
1	0	0
1	1	1

OR

An OR Gate

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.

A	B	Y
0	0	0
0	1	1
1	0	1
1	1	1

XOR

An XOR Gate

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.

A	B	Y
0	0	0
0	1	1
1	0	1
1	1	0

NAND

A NAND Gate

The NAND gate is the opposite of an AND gate.

A	B	Y
0	0	1
0	1	1
1	0	1
1	1	0

NOR

A NOR Gate

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

A	B	Y
0	0	1
0	1	0
1	0	0
1	1	0

XNOR

An XNOR Gate

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

A	B	Y
0	0	1
0	1	0
1	0	0
1	1	1

Logic Circuits

Specification link - 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:

Logic circuit

A	B	A AND B	A OR B	Q
0	0	0	0	0
0	1	0	1	0
1	0	0	1	0
1	1	1	1	1