NOR gate
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| 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.
The nor gate like a NAND gate is an universal gate in the sense that any boolean function can be implemented by nor gates.
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.
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[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.
[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:
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1 Input A1 2 Input B1 3 Output Q1 4 Output Q2 5 Input B2 6 Input A2 7 VSS 8 Input A3 9 Input B3 10 Output Q3 11 Output Q4 12 Input B4 13 Input A4 14 VDD |
[edit] Availability
These devices are available from most semiconductor manufacturers such as Fairchild Semiconductor, Philips or Texas Instruments. These are usually available in both through-hole DIL and SOIC format. Datasheets are readily available in most datasheet databases.
The standard 2-, 3-, 4- and 8-input NOR gates are available:
- CMOS
- 4001: Quad 2-input NOR gate
- 4025: Triple 3-input NOR gate
- 4002: Dual 4-input NOR gate
- 4078: Mono 8-input NOR gate
- TTL
- 7402: Quad 2-input NOR gate
- 7427: Triple 3-input NOR gate
- 7425: Dual 4-input NOR gate
- Mono 8-input NOR gate is not available in TTL
[edit] Implementations
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[edit] Alternatives
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.
