Logic Gates:

The term logic in electronics refers to the representation and logical manipulation of numbers usually in a code employing two symbols. i.e., bits.

An electronic logic circuit is one whose inputs and outputs can take only one of two states. Where the output of such a circuit depends only on the present state of the input to the circuit, it is called a COMBINATIONAL LOGIC CIRCUIT.

Logic circuits may have many inputs and many outputs and be made up of a large number of elements called LOGIC GATES.

Most modern electronic logic networks are constructed from two state components in the form of integrated circuits fabricated in a single piece of pure silicon and often referred to as a CHIP. They are available as transistor-transistor logic (TTL) and complementary symmetry metal oxide semiconductor (CMOS or COSMOS) which supersede earlier resistor-transistor logic (RTL) and diode-transistor logic (DTL).

Logic circuits are most widely used in computers and calculators, but their use also extends to a wide range of control and test equipment.

Figure shows the logic convention.

As the 'positive logic' representation is favoured by the majority of designers and manufacturers, it is intended to adopt this representation throughout this section. Positive logic refers to the use of a 1 to represent the true or more positive level (e.g. +5v) and 0 to represent the fault, or less positive level (e.g. 0v).