NOR As Universal Gate

NOR gates are so-called "universal gates" that can be combined to form any other kind of logic gate.
A NOR gate is logically an inverted OR gate. By itself has the following truth table:
Truth Table
Input A  Input B  Output Q
                          0            0              1
                          0            1              0
                          1            0              0 
                          1            1              0

NOR AS NOT
This is made by joining the inputs of a NOR gate. As a NOR gate is equivalent to an OR gate leading to NOT gate, this automatically sees to the "OR" part of the NOR gate, eliminating it from consideration and leaving only the NOT part.
in fig no.2


NOR AS OR
The OR gate is simply a NOR gate followed by a NOT gate.
fig 3


NOR AS AND
An AND gate gives a 1 output when both inputs are 1; a NOR gate gives a 1 output only when both inputs are 0. Therefore, an AND gate is made by inverting the inputs to a NOR gate.
fig 4

NOR AS NAND
A NAND gate is made using an AND gate in series with a NOT gate.
fig 5

NOR AS XOR
An XOR gate is made by connecting the output of 3 NOR gates (connected as an AND gate) and the output of a NOR gate to the respective inputs of a NOR gate. This expresses the logical fomula (A AND B) NOR (A NOR B). This construction entails a propagation delay three times that of a single NOR gate.
fig 6

NOR AS XNOR
An XNOR gate can be constructed from four NOR gates implementing the expression "(A NOR N) NOR (B NOR N) where N = A NOR B".This construction has a propagation delay three times that of a single NOR gate, and uses more gates.
fig 7


for more details please see the video 

in video how the current flow form gate...
in video how the current flow form low to high trastion and hig to low transation


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