Logic OR Gate

Introduction :

Logic gates are the digital electronics devices (or) circuits, which accept one (or) more number of inputs, but give only one number of output.

Why do we need logic gates in the first place? Logic gates are the building blocks for modern digital computation. With the help of logic gates, any kind of logic function can be performed. There are so many different types of logic gates, These are classified depending on their logical operations, ( EX-OR, EX-NOR )among them some are classified as Basic logic gates ( AND, OR, NOT ) and some are classified as Universal logic gates ( NAND, NOR ). Here the OR logic gate is classified as a Basic logic gate. The electronic circuit that performs this OR logic function is called the OR logic gate.

Logic gates are the basic building blocks, for all the modern-day digital electronics devices.

OR logic gate :

The logic behind the OR logic gate is, “If any one of the inputs to a logic gate is logic high, then its output is logic high”. This type of logic gate is known as the OR logic gate.

What is meant by logic high?

Here logic high means the input voltage of one of the inputs is slightly higher than the input voltage of the other inputs. The most common logic high input voltages are (+5 V, +1 V, +3.3 V etc.,). In digital terms logic high is represented as “1” in positive logic and as “0” in negative logic. In digital terminology “1” is not a mathematical constant one (1), it’s just a representation of logic high state in positive logic, similarly “0” is not a mathematical constant zero (0), it’s just a representation of logic high state in negative logic.

In Boolean algebra terms,

logic high = 1 = high voltage ( positive logic )

logic high = 0 = high voltage ( negative logic )

What is meant by logic low?

Here logic low means the input voltage of one of the inputs is slightly lower than the input voltage of the other inputs. The most common logic low input voltages are ( 0 V, -5 V, -3.3 V, -1 V etc.,). In digital terms logic low is represented as “0” in positive logic and as “1” in negative logic. In digital terminology “0” is not a mathematical constant zero (0), it’s just a representation of logic low state in positive logic, similarly “1” is not a mathematical constant one (1), it’s just a representation of logic low state in negative logic.

In Boolean algebra terms,

logic low = 0 = low voltage ( positive logic )

logic low = 1 = low voltage ( negative logic )

Note :

1. OR logic is the same as the binary addition.
2. Any open-ended input is treated as logic “0” in the Positive logic OR gate.
3. Any open-ended input is treated as logic “1” in the Negative logic OR gate.

Basic logic gates :

AND gate, OR gate, NOT gate are called basic logic gates. Why are they called Basic logic gates?

Any logic function (or) logic gate can be created by using the mixture of AND, OR, and NOT logic gates. This is why these gates are called Basic logic gates.

Ex :

OR operator :

OR operator is represented as “+”, Read it as “OR”. It acts like a binary addition.

EX :

Boolean Expression = A + B , Read it as A “OR” B

Classification of logic OR gates :

OR gates are classified depending on their number of inputs. The most commonly used OR gates are

• Two input logic OR gate.
• Three input logic OR gate.

Two input logic OR gate :

If an OR logic gate accepts two inputs and provides the desired OR logic as output then that type of logic gate is called a two-input logic OR gate.

Its logic function is,

Positive logic :

“When any one of the inputs is logic high (1), only then the output is logic high (1)”. Positive logic gates are widely used.

Negative logic :

“When any one of the inputs is logic high (0), only then the output is logic high (0)”.

Logic Symbol :

Switch representation :

If,

n = number of switches (inputs)

m = maximum possible number of output combination

Ex :

number of inputs (n) = 2

Maximum possible number of output combination (m) = 2² = 4

Truth table :

Positive logic :

If any one input is,

0 = OR gate acts as a buffer with the output being the other input.

1 = OR gate is disabled with output is one (1).

With this, we can say that,

Positive logic OR gate is equal to the Negative logic AND gate

Unused input :

TTL logic :

In TTL logic, if any input is open (or) floating, it will act as “1”

ECL logic :

In ECL logic, if any input is open (or) floating, it will act as “0”

In positive logic OR gate, the open (or) floating input follows ECL logic, which means open (or) floating input in Positive logic OR gate is considered as “0”.

Negative logic :

If any one input is,

0 = OR gate is disabled with zero (0) as output.

1 = OR gate acts as a buffer with the output being the other input.

With this, we can say that,

Negative logic OR gate is equal to the Positive logic AND gate

Unused input :

TTL logic :

In TTL logic, if any input is open (or) floating, it will act as “1”

ECL logic :

In ECL logic, if any input is open (or) floating, it will act as “0”

In negative logic OR gate, the open (or) floating input follows TTL logic, which means open (or) floating input in Negative logic OR gate is considered as “1”.

Three input logic OR gate :

If an OR logic gate accepts three inputs and provides the desired OR logic as output then that type of logic gate is called a three-input logic OR gate.

Its logic function is,

Positive logic :

“When any one of the inputs is logic high (1), only then the output is logic high (1)”. Positive logic gates are widely used.

Negative logic :

“When any one of the inputs is logic high (0), only then the output is logic high (0)”.

Logic Symbol :

Switch representation :

If,

n = number of switches (inputs)

m = maximum possible number output of combination

Ex:

number of inputs (n) = 3

Maximum possible number of output combination (m) = 2³ = 8

Truth table :

Positive logic :

If any one input is,

0 = OR gate gives OR of the other two inputs as output.

1 = OR gate is disabled with output one (1)

With this, we can say that,

Positive logic OR gate is equal to the Negative logic AND gate

Unused input :

TTL logic :

In TTL logic, if any input is open (or) floating, it will act as “1”

ECL logic :

In ECL logic, if any input is open (or) floating, it will act as “0”

In positive logic OR gate, the open (or) floating input follows ECL logic, which means open (or) floating input in Positive logic OR gate is considered as “0”.

Negative logic :

If any one input is,

0 = OR gate is disabled with output zero (0).

1 = OR gate gives OR of the other two inputs as output.

With this, we can say that,

Negative logic OR gate is equal to the Positive logic AND gate

Unused input :

TTL logic :

In TTL logic, if any input is open (or) floating, it will act as “1”

ECL logic :

In ECL logic, if any input is open (or) floating, it will act as “0”

In negative logic OR gate, the open (or) floating input follows TTL logic, which means open (or) floating input in Negative logic OR gate is considered as “1”.

Boolean law :

AND gate follows all these Boolean laws.

• Commutative law :

This law states that the binary product of the two inputs is equivalent to the binary product performed by changing the order of the same inputs.

i.e.

where, A & B are logic inputs.

• Associative law :

This law states that if we have three logic inputs, then the binary product between the binary product of the second and third logic input, to the first logic input, is equal to the binary product between the binary product of the first and second logic input, to the third logic input.

i.e.

where, A,B & C are logic inputs.

• Distributive law :

This law states that, if we have three logic inputs, then the binary addition of the first logic input with the binary product of the second logic input and third logic input is equal to the binary product between the binary addition of the first logic input with the second logic input and the binary addition of the first logic input with the third logic input.

i.e.

where, A,B & C are logic inputs.

Conclusion :

Now we all know, what is logic OR gate and how it operates. Logic gates are the basic building blocks for any kind of digital electronic device/system. OR logic gate is known as a Basic logic gate because, with the combination of AND, OR, NOT logic gate, it is possible to make any kind of digital logical functions.