Digital electronics relies on the actions of just seven types of logic gates, called AND, OR, NAND (Not AND), NOR (Not OR), XOR (Exclusive OR) XNOR (Exclusive NOR) and NOT.
Fig illustrates a selection of the basic logic gates that are available from a number of manufacturers in standard families of integrated circuits. Each logic family is designed so that gates and other logic ICs within that family (and other related families) can be easily combined, and built into larger logic circuits to carry out complex functions with the minimum of additional components.
In binary logic there are only two states allowed, 1 and 0 or ‘on and off’. the word NOT in the world of binary logic therefore means ‘the opposite of’. If something is not 1 it must be 0, if it is not on, it must be off. So NAND (not AND) simply means that a NAND gate performs the opposite function to an AND gate.
A logic gate is a small transistor circuit, basically a type of amplifier, which is implemented in different forms within an integrated circuit. Each type of gate has one or more (most often two) inputs and one output.
The principle of operation is that the circuit operates on just two voltage levels, called logic 0 and logic 1. These values are represented by two different voltage levels. In 5 volt logic, 1 is ideally represented by 5V and 0 by 0V, and in 3.3V logic 1 is ideally represented by 3.3V and logic 0 by 0V. When either of these voltage levels is applied to the inputs, the output of the gate responds by assuming a 1 or a 0 level, depending on the particular logic of the gate. The logic rules for each type of gate can be described in different ways; by a written description of the action, by a truth table, or by a Boolean algebra statement.