Logic NOR Gate
The Logic NOR Gate or InclusiveNOR gate is a
combination of the digital logic OR gate with that of an inverter or NOT
gate connected together in series.The NOR (Not – OR) gate has an output that is normally at logic level “1” and only goes “LOW” to logic level “0” when ANY of its inputs are at logic level “1”. The Logic NOR Gate is the reverse or “Complementary” form of the OR gate we have seen previously.
Logic NOR Gate Equivalent
Then we can define the operation of a 2input digital logic NOR gate as being:
“If both A and B are NOT true, then Q is true”
Transistor NOR Gate
A simple 2input logic NOR gate can be constructed using RTL Resistortransistor switches connected together as shown below with the inputs connected directly to the transistor bases. Both transistors must be cutoff “OFF” for an output at Q.The Digital Logic “NOR” Gate
2input NOR Gate
Symbol

Truth
Table


2input
NOR Gate

B

A

Q

0

0

1


0

1

0


1

0

0


1

1

0


Boolean
Expression Q = A+B

Read as
A OR B gives NOT Q

3input NOR Gate
Symbol

Truth Table


3input
NOR Gate

C

B

A

Q

0

0

0

1


0

0

1

0


0

1

0

0


0

1

1

0


1

0

0

0


1

0

1

0


1

1

0

0


1

1

1

0


Boolean
Expression Q = A+B+C

Read as
A OR B OR C gives NOT Q

As with the OR function, the NOR function can also have any number of individual inputs and commercial available NOR Gate IC’s are available in standard 2, 3, or 4 input types. If additional inputs are required, then the standard NOR gates can be cascaded together to provide more inputs for example.
A 4input NOR Function
If the number of inputs required is an odd number of inputs any “unused” inputs can be held LOW by connecting them directly to ground using suitable “Pulldown” resistors.
The Logic NOR Gate function is sometimes known as the Pierce Function and is denoted by a downwards arrow operator as shown, A↓B.
The “Universal” NOR Gate
Like the NAND gate seen in the last section, the NOR gate can also be classed as a “Universal” type gate. NOR gates can be used to produce any other type of logic gate function just like the NAND gate and by connecting them together in various combinations the three basic gate types of AND, OR and NOT function can be formed using only NOR‘s, for example.Various Logic Gates using only NOR Gates
Commonly available digital logic NOR gate IC’s include:
TTL Logic NOR Gates
 74LS02 Quad 2input
 74LS27 Triple 3input
 74LS260 Dual 4input
CMOS Logic NOR Gates
 CD4001 Quad 2input
 CD4025 Triple 3input
 CD4002 Dual 4input