As pointed out in previous paragraph that NOR Gate is the combination of OR and NOT Gate with input of NOT Gate connected at the output of the OR Gate. Thus corresponding to each combination of the inputs of the NOR Gate, it will have an output that is the complement of the output of the OR Gate. The schematic symbol along with the basic circuit of the NOR Gate is as shown in the following image:
As shown in the image above NOR Gate process the inputs A and B in such a way that first OR Gate manipulates the input variables A and B to generate A+B and then NOT gate acts on A+B to generate complement of the A+B. The Bubble symbol at the OR gate represents the negation of the output of the OR Gate.
NOR Gate Truth Table:
The relation between the state of the output variable and that of the input variables is represented in the form of the table. This table is called the Truth Table. The Truth Table of the NOR Gate along with its schematics symbol is shown below:
The output of the NOR Gate is HIGH if and only if both of its inputs are LOW otherwise its output is LOW, note that the output of the NOR Gate is exact complement of the output of the OR Gate in which output will be LOW if only if both inputs are LOW otherwise the output is HIGH for all other combinations of the input variables.
Universality of NOR Gate:
NOR Gate has a very useful property which makes it unique and important among all other Gates. The Boolean expression of any complexity can be implemented using NOR Gate only that is NOR Gate alone can be employed to realize all possible Boolean expressions without the need of any other Gate. This property of NOR Gate is called Functional Completeness, due to this property the entire microprocessor can be designed using NOR Gate only! NOR Gate shares this property with NAND Gate and both of the Gates are called Universal Gates.
NOR Gate Circuit:
Now let us understand the circuit that implements the NOR Gate. NOR Gate can be implemented in a variety of ways depending upon the electronic components used to design the circuit. For example diodes, transistors, resistors and combination of these components can implement the NOR Gate. The most popular techniques for designing the NOR gates are TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal Oxide Semiconductor Transistor) logics. An example of the basic circuit implementing the NOR Gate functionality is shown in the figure below:
The inputs A and B of the NOR Gate are connected at the base of the transistors T1 and T2 respectively and the output is taken from the collector. The transistor here acts as the switch so when the signal is applied at the base of the transistor the transistor starts conducting and shorts the output to the ground similarly when no signal is applied at the input the output is connected to the Vcc as shown. When signal is applied neither at input A nor B none of the transistors T1 and T2 turn on and the output terminal remains connected to the Vcc and thus output of the NOR Gate remains HIGH. When signal is applied either at input A or B corresponding transistor T1 or T2 turns on and shorts the output terminal to the ground thus output of the NOR Gate gets LOW. Similar is the case when signal is applied at both terminals. This how the transistors and resistors can be used to implement the NOR Gate functionality.
NOR Gate IC 4001:
NOR gates are available in the IC packages. As mentioned earlier that TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal Oxide Semiconductor) technologies are used to design NOR gate. One of the most popular IC for NOR Gate is 4001 which is a QUAD two inputs NOR Gate IC which means that this IC contains four independent two input NOR Gates. The pinout and connection diagram of the 4001 IC is shown below:
I hope this article will be helpful for. In the next article I will come up with other important topics. Till then stay connected, keep reading and enjoy learning.