Dec 122020
 

NOR Gate

The NOR gate is a logic gate that outputs a logic “0” in all combinations of its inputs except when they are all logic “0”, in this case the output is a logic “1”.

See the truth tables in diagrams below.

There are two accepted symbols for this gate. One of the American standard (ANSI) and another of the European standard (IEC).

NOR gate symbols. American standard (ANSI) - European standard (IEC)

A NOR gate can be implemented by concatenating an OR gate with a NOT gate, as shown in the following image.

NOR gate equivalent circuit

From the previous image it can be seen that the output of the OR gate is: X = A + B. This output is passed through the inverter or NOT gate and is obtained: X = A + B.

NOR Gate Truth Table

As in the case of the OR gate, this gate can be found in versions with 2, 3 or more inputs.

  • In TTL technology: 7402 (2 inputs), 7427 (3 inputs), etc.
  • In CMOS technology: 4001 (2 inputs), 4025 (3 inputs), etc.

Truth tables for these two types of gates are as follows:

2 and 3 Input NOR Gate Truth Tables

As you can see, the output X is only “1”, when all the inputs are “0”.

How to create a NOT gate with a NOR gate?

Like the NAND logic gate, an interesting case of this gate is when inputs A and B or A, B and C (in the case of a 3-input NOR gate) are joined, to get a single input. In this case the output (X) has exactly the opposite value of the input. View the first and last rows of the truth tables above.

NOT gate made with NOR gate - Truth table

In other words: With a NOR gate you can get the behavior of a NOT gate.

Below is the internal layout of the 7427 TTL IC which has triple 3-input NOR gates. These gates have the 3-input truth table shown in a previous image.

7427 - Triple 3-input TTL NOR gate

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