# What is Primary and Secondary Logic Gates

Do you know about logic gates? There are two types of logic gates. Let’s discuss what is primary and secondary logic gates with their detail classification.

## Primary logic gates

The common use of logic elements is to act as switches, although they have no moving parts. They open to pass on a pulse of electricity or close to shut it off.

This is why they are known as gates. The primary gates are OR, AND and NOT.

OR Gate

An OR gate has an output 1 if any of inputs are 1. The symbol and truth table for a two-input. Here output, Y=A+B, where + denotes OR operation.

Two-input OR gate, (a) Truth Table, (b) Symbol

Figure Illustrates, the close relationship between the 2-input OR gate and an electrical switching circuit. Such a circuit normally contains some source of every (a battery), an

Parallel Circuit

Output device (a lamp), and one or more switches all connected by wires. A switch is a two-state device that is either closed (on) or opens (off). In Figure 3.2 switch A and B, are connected in parallel.

The lamp will light if switch A is closed, or if switch B is closed, or if both switches are closed. This is the property described by the truth table for the Or gate, where 1 denotes that the switch or lamp is on and 0 indicates that it is off.

AND Gate

An AND gate has an output 1 if all of its inputs are 1. The symbol and truth table for a two-input AND gates are shown in figure. Here output, Y=A.B. where denotes AND operation.

Two-input And gate, (a) Truth Table, (b) Symbol

Figure is a circuit showing two switches. A and B, connected in series. The lamp will light only when both A and B are close. This is exactly the property described by the truth table for the AND gate, here 1 denotes that the circuit element is on and 0 denotes that it if off.

Series Circuit

Not Gate

A NOT gate has one input and one output. It has the effect of reversing the input signal and is also called an inverter. The symbol and truth table for a NOT gate are shown in figure. Here output Y=A, where-indicates NOT operation.

NOT gate, (a) Truth Table, (b) Symbol

## Secondary Logic Gates

Some secondary gates are NAND, NOR, EXOR etc. NAND and NOR gates are called universal gats because any one them can be used to realize any logic expression.

NAND Gates

A NAND gate has the same effect as an AND gate followed by a NOT gate, Hence the output will be opposite of the AND gate. The symbol and truth table for a two-input NAND gate are shown in Figure.

NAND gate, (a) Truth Table, (b) Symbol

NOR Gate

A NOR gate has the same effect as OR gate followed by a NOT gate. Hence the output will be the opposite of OR gate. The symbol and truth table for a two-input NOR gate are shown in Figure.

NOR gate, (a) Truth Table, (b) Symbol

EXOR Gat

It is widely used in digital circuits. Symbol and truth table for a two-input EXOR gate are shown in Figure 3.8. Here output Y=A + Â denotes EXOR operation.

EXOR gate, (a) Truth Table, (b) Symbol

Basic gates (AND, OR, NOT) and universal gates (NAND, NOR) can be used in combination to make up digital compute circuits.

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