NOR GATE |
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LogicThe NOR gate is an electronic circuit that performs joint denial. The output of the NOR Boolean operator is true only when all the inputs are false. Otherwise, the output is false. The NOR gate is known as the joint denial operator because to say neither A nor B is the same as to say not A and not B (DeMorgan’s theorem). For example, to say neither enjoyment nor sorrow is our destined end or way is the same as to say not enjoyment and not sorrow is our destined end or way (Longfellow). Consider the statement if Dave passes neither calculus nor physics he will lose his scholarship. In analyzing this statement we observe that Dave can suffer four possible circumstances, as shown in Table 1. Because all the other logic gates can be constructed using the NOR gate, in digital systems we say the NOR gate is universal. Mathematicians use the term functionally complete and linguists say expressively adequate. It is typical in engineering to use 1 instead of TRUE and 0 instead of FALSE. Therefore we rewrite the data from Table 2 in Table 3 accordingly.
Table 3: Truth Table of complete evaluation For greater detail on the logic significance of the word NOR, read the Boolean Algebra article. Physical ImplementationIn order to apply the principles of Boolean algebra to create real machines that can think and make decisions, we have had to find ways to physically implement the logic operators AND, OR, NOT, etc. To that end, modern day engineering uses transistor networks called logic gates. Hence, a logic gate is actually a group of transistors so arranged as to behave as a Boolean operator. From a circuit complexity perspective, the most basic logic gate is the NOT gate (aka the Inverter). The NOT gate is made of two transistors, as shown in Figure 1. The next most basic logic gate is the NOR gate, which is effectively two Inverters as shown in Figure 2. Hence, we only need four transistors to build a NOR gate.
Figure 1: Interactive transistor circuit of the NOT logic operator
Figure 2: Interactive transistor circuit of the NOR logic operator
TransistorsThe use of transistors to build logic gates is quite modern. Before transistors we used other devices, such as vacuum tubes (aka thermionic valves). And very soon we may use DNA, or some other abundant material. There are many types of transistors. Our circuits in figures 1 and 2, for example, use complementary metal–oxide semiconductor (CMOS) technology. Our choice of CMOS is arbitrarily based on the fact that CMOS is by far the dominant technology in use today. The dominance is due to how well CMOS performs in all the important categories: fabrication cost, packing density, loading capacity (i.e. fan-out), operational speed (i.e. propagation delay), noise margin, and power dissipation (i.e. green technology). There is of course more to transistors than can be presented here; especially since transistors are used for more than just digital systems. And so we refer you to any good micro–electronics textbook. Alternate DesignBelow we show two additional typical constructions of the NOR gate. Each of the constructions presents specific conveniences to designers. If you are very new to digital systems design, you may not understand the importance of the figures below. Still, we include them in this article for the people who may need them. 
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