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  ../images/main/bullet_green_ball.gif Logic Circuits
   

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Boolean algebra is ideal for expressing the behavior of logic circuits.

   

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A circuit can be expressed as a logic design and implemented as a collection of individual connected logic gates.

   

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  ../images/main/bulllet_4dots_orange.gif Fixed Logic Systems

A fixed logic system has two possible choices for representing true and false.

   

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  ../images/main/bullet_star_pink.gif Positive Logic

In a positive logic system, a high voltage is used to represent logical true (1), and a low voltage for a logical false (0).

   

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  ../images/main/bullet_star_pink.gif Negative Logic

In a negative logic system, a low voltage is used to represent logical true (1), and a high voltage for a logical false (0).

   

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In positive logic circuits it is normal to use +5V for true and 0V for false.

   

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  ../images/main/bulllet_4dots_orange.gif Switching Circuits

The abstract logic described previously can be implemented as an actual circuit. Switches are left open for logic 0 and closed for logic 1.

   

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  ../images/main/bullet_star_pink.gif Two variable AND circuit X.Y
   

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  ../images/main/bullet_star_pink.gif Two variable OR circuit X + Y
   

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  ../images/main/bullet_star_pink.gif Four variable circuit U.V.(X + Y)
   

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  ../images/main/bulllet_4dots_orange.gif Truth Table

A truth table is a means for describing how a logic circuit's output depends on the logic levels present at the circuit's inputs.

   

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In the following twos-inputs logic circuit, the table lists all possible combinations of logic levels present at inputs X and Y along with the corresponding output level F.

   

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X

Y

F = X*Y

0

0

0

0

1

0

1

0

0

1

1

1

   

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When either input X AND Y is 1, the output F is 1. Therefore the "?" in the box is an AND gate.

   

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Copyright 1998-2014

Deepak Kumar Tala - All rights reserved

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