GCSE Computer Science/Logic gates

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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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

An XOR Gate

The XOR gate will a 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[edit]

A NAND Gate

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

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

NOR[edit]

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[edit]

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[edit]

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