IC 4001 NOR gate (IC digital)

From Wikipedia, the free encyclopedia

This article is about NOR in the sense of an electronic logic gate (e.g. CMOS 4001). For NOR in the purely logical sense, see Logical NOR, for other uses of similar terms, see Nor.

INPUT A B		OUTPUT A NOR B

The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0). If one or both input is HIGH (1), a LOW output (0) results. NOR is the result of the negation of the OR operator, thus forming a complete operation the combination of which can be combined to generate any other logical function. By contrast, the OR operator is monotonic as it can only change LOW to HIGH but not vice versa.

In most, but not all, circuit implementations, the negation comes for free (e.g. CMOS) — in some logic families (e.g. TTL) it is required and OR has to be implemented as a NOR followed by an inverter. A significant exception is some forms of domino logic.

It is because of their diversity that NOR and NAND gates are used more than any other gates in the processor design. As both of these gates can be used to replicate any other gate they are produced in much higher numbers than other logic gates. This makes them cheaper to buy, which is another factor causing them to be the most popular of gates.

This entry was posted on Tuesday, July 10th, 2007 at 9:19 am and is filed under Digital. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.