The NOR Gate

Introduction

Having discussed the NAND gate in depth, we now intend to dive into another important Universal gate: the NOR Gate. The NOR gate is also similar to the NAND gate formation technique. In this NOR gate, it is the combination of an OR Gate and a NOT Gate. We have discussed the two gates ‘OR and NOT‘, in our earlier posts. Please note the key points regarding these two gates now, so that the NOR gate logic function is easier to understand.

What is a NOR gate?

NOR gate is nothing but the combination of NOT and OR gates, both in the verbatim sense as well as in the technical sense. This can be well explained with the use of a figure, as shown below. See how the OR gate and NOT gate are connected to achieve the NOR gate function. The output of OR gate is fed to the NOT gate and the resultant output is NOR gate logic function.

The term NOR is a contraction of the expression NOT-OR.
A NOR gate, therefore, is an OR gate followed by an inverter. You know that an inverter is also called NOT gate. The output of a NOR gate is binary 1 if all its inputs are binary 0. Mathematically, a NOR gate function is represented as
Q = A+B (Q equals A and B not).

The NOR Gate

The symbol for the NOR gate is given below. When A and B are the inputs, the output is the inverse of A and B, say it is Q, which is A+B not.

The shape of the NOR gate is such that the symbol of OR gate has a notch at the outside tip of the symbol.

IC 7402 Quad 2-input NOR gate

As an illustration of the NOR gate, we can consider the simple TTL IC 7402. IC 7402 contains four Identical two-input NOR Gates In a single Package. As such, the IC is called a 2-input NOR gate. The pin configuration of the IC is shown in the figure below.

All the four gates are completely independent except for a common power supply input pin. As usual, pin 14 is connected to the positive power supply, whereas pin 7 is grounded. Not all gates are used in every application. At those times, simply ignore the gates not used because each gates are independant as mentioned earlier.

Truth table

It is high time that we should study the truth table for this logic function. The truth table gives clarity of all the functions of the NOR gate. The truth table for the NOR gate is given below, which clearly shows the input and output logic levels.

NOR Gate Truth Table

Inputs		Output
A	B	Q
0	0	1
0	1	0
1	0	0
1	1	0

OR+NOT=NOR Truth Table

The truth table for the NOR gate is derived from the truth table of the OR gate by inverting the output column. This is shown below. This table gives a clear idea of the formation of the final NOR truth table.

OR+NOT=NOR Truth Table

Input		OR Output	NOR Output
A	B	Q	Q'
0	0	0	1
0	1	1	0
1	0	1	0
1	1	1	0

Conclusion

We have extensively discussed the gate function, its symbol, with an illustrative IC companion to master the logic gates in our digital technique tutorial. This is the second logic gate of the Universal gate family. We have concluded studying the two universal gates under this segment. I hope the beginners especially would have acquired the basic knowledge of the Universal gates by deep studying these two logic gates in our tutorial.

We shall practically do one experiment in our future post to prove the logic functions by DIY practicals, so that the beginners get a strong basic foundation on this subject matter. Please give your valuable comments on the foregoing discussions held so that any improvement can be attempted if necessary in our future editions.