The letters are then linked by a Boolean symbol indicating the logical action of the gate. The letters have no meaning in themselves, other than just to label the various points in the circuit. to indicate inputs, and letters from the second half of the alphabet, very commonly X or Y (and sometimes Q or P) to label an output. These use letters from the beginning of the alphabet, such as A, B, C etc. The actions of any of these gates can also be described using Boolean statements.

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. 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. 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. These values are represented by two different voltage levels. The principle of operation is that the circuit operates on just two voltage levels, called logic 0 and logic 1. Each type of gate has one or more (most often two) inputs and one output. So NAND (not AND) simply means that a NAND gate performs the opposite function to an AND gate.Ī logic gate is a small transistor circuit, basically a type of amplifier, which is implemented in different forms within an integrated circuit. If something is not 1 it must be 0, if it is not on, it must be off. the word NOT in the world of binary logic therefore means ‘the opposite of’. In binary logic there are only two states allowed, 1 and 0 or ‘on and off’. 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.

2.1.1 illustrates a selection of the basic logic gates that are available from a number of manufacturers in standard families of integrated circuits. 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.įig.

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