Logic Circuit II (Alternative Logic Gate)

Logic Circuit II (Alternative Logic Gates)

Definition of Alternative Logic Gate
Alternative logic gates are those that are the combination of two logic gates.

Types of Alternative Logic Gates
There are generally two types
1. NAND gate
2. NOR gate

NAND Gate
NAND gate is the combination of both an AND gate and a NOT gate. It operates the same as an AND gate but the output will be the opposite.

Logic Symbol for the NAND Gate

Truth Table for NAND Gate

INPUT
A
INPUT
B
OUTPUT
C
o
0
1
0
1
1
1
0
1
1
1
0

NAND Gate Equation
The NAND gate equation can also be expressed by a Boolean algebra equation.
For 2 input NAND gate the equation is:



The expression is the same as the AND gate with an over bar above the entire portion of the equation representing the input. This equation reads X equals A and B NOT.

NOR Gate
The NOR gate is the combination of both OR gate and a NOT gate. It operates the same as an OR gate, but the output will be the opposite.

Logic symbol for the NOR gate


Truth Table for the NOR Gate

INPUT
A
INPUT
B
OUTPUT
C
0
0
1
0
1
0
1
0
0
1
1
0

NOR Gate Equation
The NOR gate operation can also be expressed by a Boolean algebra equation. For a 2-input NOR gate the equation is:


The expression reads X equals to A or B NOT.

Uses of Logic Gates
Logic gates can be used to:
i. build digital circuit
ii. design complex ICs
iii. construct a simple alarm
iv. control traffic light

XOR (Exclusive-OR) gate
The XOR gate acts in the same way as the logical either/or. The output is true if either, but not both, of the input, is true. Another way of looking at this circuit is to observe that the output is 1 if the inputs are different, but 0 if the input are the same.

Logic Symbol for the NOR Gate



Truth Table for the XOR Gate

INPUT
A
INPUT
B
OUTPUT
C
0
0
0
0
1
1
1
0
1
1
1
0

XOR Equation



Or



XOR as a Simple Comparator
The XOR gate can be used to construct a Simple Comparator. A comparator is a combinational logic circuit that compares the magnitudes of two binary quantities to determine which one has the greater magnitude. XOR can the used as a basic comparator.

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