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By the end of this subtopic, you should be able to:
use the symbols for NOT, AND, OR, NAND, NOR and XOR
understand and define what each of these logic gates does
remember that, in this syllabus, all gates except NOT have two inputs only
construct the truth table for each gate
construct a logic circuit from:
construct a truth table from:
construct a logic expression from:
In computer science, logic is a way of making decisions using simple conditions. These conditions have only two possible values:
Logic is based on Boolean values. This means every input and output is either 1 or 0. There is nothing in between.
For example, a system might use these inputs:
A logic circuit takes these inputs and produces an output, also as 1 or 0.
A logic gate is a building block used in digital circuits. It takes one or more binary inputs and gives one binary output.
You can think of a logic gate as a very small decision-maker. It follows a rule. That rule tells it what output to give for each possible input.
For this syllabus, you need to know six gates:
A very important syllabus point is this:
You must know both the function of each gate and the symbol used to show it in a circuit.
Because plain text cannot draw the exact exam symbol neatly, the notes below describe the symbol clearly. In the exam, you should recognise and draw the standard gate symbols.
The NOT gate has one input and one output.
Its job is to reverse the input.
So, NOT means the opposite.
If input A goes into a NOT gate, the expression is written as:
NOT A
The symbol is usually drawn as a triangle pointing to the right, with a small circle on the output side. That small circle shows that the signal is being inverted, which means flipped.
| A | NOT A |
|---|---|
| 0 | 1 |
| 1 | 0 |
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