Logic Gates





The gate is an electronic circuit to receive more than one input and deliver single output. Gates are often called logic circuits because they can be analyzed with Boolean algebra. The computer system is a set of gates.

The ALU (Arithmetic Logic Unit) of computer system is responsible for mathematical and logical processing of data and compiler interacts with ALU at the time of logical operations.The gates are applied in main memory, ALU, resisters etc. Logic gates can also be constructed from relays, diode, fluidics and optical elements.

There are two methods of symbol presentation for logic gates. In traditional methods, special types of symbols are used for each types of gate, but in IEC (International Electrotechnical Commission) type of symbols contains rectangle with gate function inside.

 

Examples of Gate

Examples of Gate


(a) AND Gate: Two or more input signals are applied to obtain single output signals. If all the input signals are high, output signal is obtained as high. The AND operation is called Intersection. The first electronic AND gate is invented by Walther Bothe, inventor of the coincidence circuit.

AND Gate

AND Gate


True table:

A

B

A.B

0

0

0

0 1 0
1 0

0

1 1

1

 

Its logical expression: a .b=min(a, b)

Example: 0.1= min (0, 1) = 0

The logical or Boolean expression or function for AND gate: f(A,B)= A.B

 

(b) OR Gate: More than one input signals are converted into single outputs. If any one signal is high, the output becomes high. Bipolar transistors and MOSEFTs can also be used to build OR gate. It is some time called UNION operation.

OR Gate

OR Gate


Truth Table:

A

B

Y=A+B

0

0

0

0

1

1

1

0

1

1

1

1

Its logical expression: a + b = max (a, b)

Example: 0 + 1 = max (0, 1) =1

The logical or Boolean function is written as: f (A, B) = A+B

 

(c) NOT Gate: The NOT gate has two or more input signals but only one output signals. The output signals are always reverse of input signals. If all the input signals are low, single high output signal is obtained. It is called inverter because it inverts the input and its operation is called Inversion.

NOT Gate

NOT Gate


True table:

A

A

0

1

1

0



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