Introduction
Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms which are used for the true and false states are shown in the table below. It is best to be familiar with them all.
| Logic states | |
| True | False |
| 1 | 0 |
| High | Low |
| +Vs | 0V |
| On | Off |
NOR. Capital letters are normally used to make it clear that the term refers to a logic gate.
Note that logic gates are not always required because simple logic functions can be performed with switches or diodes:
- Switches in series (AND function)
- Switches in parallel (OR function)
- Combining IC outputs with diodes (OR function)
Logic gate symbols
There are two series of symbols for logic gates:- The traditional symbols have distinctive shapes making them easy to recognise so they are widely used in industry and education.
- The IEC (International Electrotechnical Commission) symbols are rectangles with a symbol inside to show the gate function. They are rarely used despite their official status, but you may need to know them for an examination.
Inputs and outputs
Gates have two or more inputs, except a NOT gate which has only one input. All gates have only one output. Usually the letters A, B, C and so on are used to label inputs, and Q is used to label the output. On this page the inputs are shown on the left and the output on the right.
The inverting circle (o)
Some gate symbols have a circle on their output which means that their function includes inverting of the output. It is equivalent to feeding the output through a NOT gate. For example the NAND (Not AND) gate symbol shown on the right is the same as an AND gate symbol but with the addition of an inverting circle on the output.
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