NOR gate

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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
0	0	1
0	1	0
1	0	0
1	1	0

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.

1 Symbols
2 Hardware Description and Pinout
3 Implementations
- 3.1 Alternatives
4 See also

[edit] Symbols

There are two symbols for NOR gates: the American ('military') symbol and the IEC ('European' or 'rectangular') symbol. For more information see Logic Gate Symbols.

American NOR Symbol

IEC NOR Symbol

[edit] Hardware Description and Pinout

NOR Gates are basic logic gates, and as such they are recognised in TTL and CMOS ICs. The standard, 4000 series, CMOS IC is the 4001, which includes four independent, two-input, NOR gates. The pinout diagram is as follows:

Pinout Diagram of a 4001 Quad NOR DIL-format IC

 1  Input A1
 2  Input B1
 3  Output Q1
 4  Output Q2
 5  Input B2
 6  Input A2
 7  V_SS
 8  Input A3
 9  Input B3
 10 Output Q3
 11 Output Q4
 12 Input B4
 13 Input A4
 14 V_DD

This device is available from most semiconductor manufacturers such as Philips. It is usually available in both through-hole DIL and SOIC format. Datasheets are readily available in most Datasheet Databases. Two NOR gates can be used to make a latch.

As well as the standard 2-Input NOR Gate, 3- and 4-Input NOR Gates are also available:

4000: Dual 3-Input NOR Gate and Inverter
4002: Dual 4-Input NOR Gate

[edit] Implementations

[edit] Alternatives

NOR Gate Constructed Using Only NAND Gates

If no specific NOR gates are available, one can be made from NAND gates in the configuration shown below. Interestingly, any logic gate can be made either from a combination of NAND gates or from a combination of NOR gates.