Logic Gates in Computer System Architecture for UGC NET Computer Science notes

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In this post,let us understand logic gates in computer system architecture in a simple way.

Introduction:

Before learning logic gates,we must first understand  binary information.

What is binary?

Binary is number system that uses only two digits.

0 and 1.

Digital computers operate using  two states.

0 --->OFF

1------>ON

This is called the binary system.

Binary  is represented in digital computers by physical quantities called electric signals.

Electric signals such as voltage exist throughout the computer in either one of the two recognizable states.

0----->Low voltage

1------>High voltage

These two states represent a binary variable that can be equal to 0 or 1.

Binary logic means digit 0 or 1.

Binary logic deals with  variables that can take 0 or 1 and operations defined in them.

It is used to describe in Algebraic or tabular form.

What are Logic gates?

A Logic gate is an electronic circuit that manipulates binary information.

Logic gates are block of hardware ,that produce the signals of binary 1 or 0 when input logic requirements are satisfied.

A variety of logic gates are commonly used in digital computers.

Each gate has a distinct graphic symbols and its operations  can be described by means of an algebraic expression.

The input output relationship of the binary variables for each gate can be represented in  tabular form by a truth table.

Explanation:

Now see the below flow

Binary values(0,1)

           |

 Electric Signals(Voltage levels)

            |

Electronic Circuits

             |

Logic Gates

           |

Digital Computer Operations

The above diagram in a simple line:

Logic gates are electronic circuits that process binary values represented as  electric signals to perform digital computer operations.

Now the below table shows the  eight digital logic gates.

AND Gate.

  x=A.B

Truth Table:

A.         B.      x

0.          0.      0

0.          1.      0

1.          0.      0

1           1.      1

Explanation

•  The above diagram is an AND gate logic circuit.
• In truth table,it has two input variables and one output variable.
• If any of the inputs is 0 ,the output is 0.Otherwise ,the output is 1.
• Simply:AND gate=Multiplication

OR Gate

.    x=A+B

Truth table:

A.       B.       x

0.        0.        0

0.        1.        1

1.        0         1

1.         1.       1

Explanation:

• The above diagram is an OR Gate logic circuit.
• In truth table,it has two variables and one output variable.
• if any input is 1,the output is 1.Otherwise,the output is 0.
• Simply :OR Gate=Addition

NOT Gate:

 

   ___

x=A

Truth table:

           

A.        x

1.        0

0.        1

Explanation:

• The above diagram is NOT gate Logic circuit.
• It has only one input variable and only one output variable.
• if the input variable is 1,the output is 0.
• Simply say ,NOT Gate=Output is the opposite of Input.

Buffer Gate

 

y=A

Truth table

 

A.       y

0.        0

1.        1

Explanation:

• The above diagram is buffer gate logic circuit.
• In truth table,it has only one input variable and  one output variable.
• If the input is 1,the output is 1.If the input is 0,the output is 0
• Simply :Buffer Gate=Output is same as Input.

NAND gate

   

     

      ____

 x=A.B

Truth Table:

A.          B.        x

0.           0.         1

0.           1.         1

1            0.         1

1.           1.         0

Explanation:

• The above diagram is NAND gate logic circuit.
• In truth table,it has two variables and one output variable.
• If two inputs are 1,the output is 0.Otherwise the output is 1.
• Simply :NAND Gate=AND Gate followed by NOT gate.

NOR Gate

 

    _____

x=A+B

Truth Table

A.        B.          x

0.         0.          1

0.         1.          0

1.         0.          0

1.         1.          0

Explanation:

• The above diagram is NOR gate logic circuit.
• In truth table, it has two variables and one output variable.
• If two inputs are 0,the output is 1.Otherwise the output is 0.
• Simply:NOR Gate=OR Gate followed by NOT gate.

Exclusive OR (XOR)

x= A⊕B

Truth Table

A.         B.          x

0           0           0

0.          1.          1

1.          0.          1

1.          1.          0

Explanation:

• The above diagram is an Exclusive OR gate logic circuit.
• .In truth table,it has two input variables and one output variable.
• If the inputs are the  same,the output is 0.Otherwise the output is 1.
• Simply say, XOR=Exclusive OR gate.

Exclusive-NOR

      ______

x= A ⊕ B 

Truth Table

A.       B.           x

0.        0.           1

0.        1.           0

1.        0.           0

1.        1.           1

Explanation:

• The above diagram is an Exclusive NOR gate logic circuit.
• In truth table,it has two input variables and one output variable.
• If two inputs are same value,the output is 1.Otherwise the output is 0.
• Simply:XNOR gate =Exclusive NOR gate.   

Conclusion:

I hope,this post helps you to  understand logic gates in a simple way.