
Logic Circuits
Boolean algebra is ideal for expressing the behavior of logic circuits.
A circuit can be expressed as a logic design and implemented as a collection of individual connected logic gates.
Fixed Logic Systems
A fixed logic system has two possible choices for representing true and false.
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).
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).
In positive logic circuits it is normal to use +5V for true and 0V for false.
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.
Two variable AND circuit X.Y
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Four variable circuit U.V.(X + Y)
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.
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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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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