LOGIC GATES

A logic gate is an elementary building block of a digital circuit. Most Logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary condition low (0) or high (1). The logic gate of a terminal can, and generally change often, as the circuit processes data. It processes one or more input signal in a logical fashion. Depending on the input value or voltage, the logic gate will either output a value of ‘1’ for ON or a value of ‘0’ for OFF.

There are three basic logic gates: AND, OR and NOT.

BINARY CODE

Logic gates are digital circuits and they utilize a binary numbering system known as binary code. Binary code is the same language used by computer which uses only 1 or 0 as numbers.

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 output.

 

LOGIC SYMBOL OF THE “AND” GATE

AND Gate

HOW DOES THE AND GATE WORK?

‘AND’ gates are like two or more switches in series. All the switches have to be closed (ON or value of 1) in order to make the lamp (output) turn on. If all the inputs are not ‘ON’, the output is ‘OFF’.

TRUTH TABLE FOR “AND” GATE

INPUT INPUT OUTPUT
A B C
1
1
1 1 1

 

All the value of the AND gate must be a ‘1’ in order or the output value to be ‘1’. Any other input combination will result in zero.

OR GATE

An ‘OR’ gate is like two or more switches in parallel. Only one switch need to be closed (ON or value of 1) in order to make the lamp (output C) turn ON with a value of 1.

 

LOGIC SYMBOL FOR “OR” GATE

Or gate

TRUTH TABLE OF “OR” GATE

INPUT INPUT OUTPUT
A B C
1 1
1 1
1 1 1

A value of ‘1’ applied to either or both inputs of the OR gate will result in an output value of ‘1’. A value of ‘0’ applied to both inputs will result in an output of ‘0’.

 

NOT GATE

NOT gate have only one input and output. It reverses the input signal value. If the input is 1, the output will be 0 and if the input is 0 then the output will be 1.

LOGIC SYMBOL FOR “NOT” GATE

Not Gate

TRUTH TABLE FOR “NOT” GATE

INPUT OUTPUT
A C
1
1

“NOT” gate can be referred as inverter, whatever the input signal is the output is always the opposite.

The teacher summarizes the lesson and allows student to ask questions to clear doubts.

Assessment :

1.    Define Logic gate

2.    Give the types of logic gate we have

3.    Draw the symbol for each gate

Assignment:

Make a research online about Alternate Logic gate

 

See also

Differences between primary and secondary memory

Types of Secondary Storage

Memory Unit

Central Processing Unit

COMPUTER HARDWARE

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