There are several basic logic gates used in performing operations in digital systems. The common ones are
- OR Gate
- AND Gate
- NOT Gate
- XOR Gate
Additionally, these gates can also be found in a combination of one or two. Therefore, we get other gates, such as NAND Gate, NOR Gate, EXOR Gate and EXNOR Gate.
OR Gate
In an OR gate, the output of an OR gate attains state 1 if one or more inputs attain state 1.
The Boolean expression of the OR gate is Y = A + B, read as Y equals A ‘OR’ B.
The truth table of a two-input OR basic gate is given as
AND Gate
In the AND gate, the output of an AND gate attains state 1 if and only if all the inputs are in state 1.
The Boolean expression of AND gate is Y = A.B
The truth table of a two-input AND basic gate is given as
NOT Gate
In a NOT gate, the output of a NOT gate attains state 1 if and only if the input does not attain state 1.
The Boolean expression is
It is read as Y equals NOT A.
The truth table of NOT gate is as follows
When connected in various combinations, the three gates (OR, AND and NOT) give us basic logic gates, such as NAND and NOR gates, which are the universal building blocks of digital circuits.
NAND Gate
This basic logic gate is the combination of AND and NOT gates.
The Boolean expression of the NAND gate is
The truth table of a NAND gate is given as
NOR Gate
This gate is the combination of OR and NOT gates.
The Boolean expression of the NOR gate is
The truth table of a NOR gate is as follows
Exclusive-OR gate (XOR Gate)
In an XOR gate, the output of a two-input XOR gate attains state 1 if one adds only input and attains state 1.
The Boolean expression of the XOR gate is
or
The truth table of an XOR gate is
Exclusive-NOR Gate (XNOR Gate)
In the XNOR gate, the output is in state 1 when both inputs are the same, that is, both 0 or both 1.
The Boolean expression of the XNOR gate 
The truth table of an XNOR gate is given below