The NOR gate is a digital

logic gate A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...

that implements logical NOR - it behaves according to the

truth table A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional argumen ...

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 In logic, negation, also called the logical complement, is an operation that takes a proposition P to another proposition "not P", written \neg P, \mathord P or \overline. It is interpreted intuitively as being true when P is false, and false ...

of the OR operator. It can also in some senses be seen as the inverse of an

AND gate The AND gate is a basic digital logic gate that implements logical conjunction (∧) from mathematical logic AND gate behaves according to the truth table. A HIGH output (1) results only if all the inputs to the AND gate are HIGH (1). If not all ...

. NOR is a

functionally complete In logic, a functionally complete set of logical connectives or Boolean operators is one which can be used to express all possible truth tables by combining members of the set into a Boolean expression.. ("Complete set of logical connectives").. (" ...

operation—NOR gates can be combined to generate any other logical function. It shares this property with the

NAND gate In digital electronics, a NAND gate (NOT-AND) is a logic gate which produces an output which is false only if all its inputs are true; thus its output is complement to that of an AND gate. A LOW (0) output results only if all the inputs to the ...

. 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—including

CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...

and

TTL TTL may refer to: Photography * Through-the-lens metering, a camera feature * Zenit TTL, an SLR film camera named for its TTL metering capability Technology * Time to live, a computer data lifespan-limiting mechanism * Transistor–transistor lo ...

. In such logic families, OR is the more complicated operation; it may use a NOR followed by a NOT. A significant exception is some forms of the

domino logic Domino logic is a CMOS-based evolution of the dynamic logic techniques based on either PMOS or NMOS transistors. It allows a rail-to-rail logic swing. It was developed to speed up circuits, solving the premature cascade problem, typically by in ...

family. The original

Apollo Guidance Computer The Apollo Guidance Computer (AGC) was a digital computer produced for the Apollo program that was installed on board each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidan ...

used 4,100 integrated circuits (IC), each one containing only two 3-input NOR gates.

Symbols

There are three symbols for NOR gates: the American (ANSI or 'military') symbol and the IEC ('European' or 'rectangular') symbol, as well as the deprecated DIN symbol. For more information see Logic Gate Symbols. The ANSI symbol for the NOR gate is a standard OR gate with an inversion bubble connected. The bubble indicates that the function of the or gate has been inverted. NorAdder

Hardware description and pinout

NOR Gates are basic logic gates, and as such they are recognised in

and

ICs. The standard, 4000 series, CMOS IC is the 4001, which includes four independent, two-input, NOR gates. The pinout diagram is as follows:

Availability

These devices are available from most semiconductor manufacturers such as

Fairchild Semiconductor Fairchild Semiconductor International, Inc. was an American semiconductor company based in San Jose, California. Founded in 1957 as a division of Fairchild Camera and Instrument, it became a pioneer in the manufacturing of transistors and of int ...

Philips Koninklijke Philips N.V. (), commonly shortened to Philips, is a Dutch multinational conglomerate corporation that was founded in Eindhoven in 1891. Since 1997, it has been mostly headquartered in Amsterdam, though the Benelux headquarters i ...

Texas Instruments Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globall ...

. These are usually available in both through-hole DIP and SOIC format. Datasheets are readily available in most datasheet databases. In the popular CMOS and TTL

logic families In computer engineering, a logic family is one of two related concepts: * A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compati ...

, NOR gates with up to 8 inputs are available: *

** 4001: Quad 2-input NOR gate ** 4025: Triple 3-input NOR gate ** 4002: Dual 4-input NOR gate ** 4078: Single 8-input NOR gate *

** 7402: Quad 2-input NOR gate ** 7427: Triple 3-input NOR gate ** 7425: Dual 4-input NOR gate (with strobe, obsolete) ** 74260: Dual 5-Input NOR Gate ** 744078: Single 8-input NOR Gate In the older RTL and ECL families, NOR gates were efficient and most commonly used.

Implementations

The diagrams above show the construction of a 2-input NOR gate using

NMOS logic N-type metal-oxide-semiconductor logic uses n-type (-) MOSFETs (metal-oxide-semiconductor field-effect transistors) to implement logic gates and other digital circuits. These nMOS transistors operate by creating an inversion layer in a p-type t ...

circuitry. If either of the inputs is high, the corresponding N-channel

MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

is turned on and the output is pulled low; otherwise the output is pulled high through the

pull-up resistor In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as switches and transistors, which physically in ...

The diagram below shows a 2-input NOR gate using

technology. The

diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...

s and

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...

s on the inputs are to protect the CMOS components from damage due to

electrostatic discharge Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an short circuit, electrical short or dielectric breakdown. A buildup of static electricity can be caused ...

(ESD) and play no part in the logical function of the circuit. Cmosunbuff

Functional completeness

The NOR gate has the property of

functional completeness In logic, a functionally complete set of logical connectives or Boolean operators is one which can be used to express all possible truth tables by combining members of the set into a Boolean expression.. ("Complete set of logical connectives").. (" ...

, which it shares with the NAND gate. That is, any other logic function (AND, OR, etc.) can be implemented using only NOR gates.Mano, M. Morris and Charles R. Kime. ''Logic and Computer Design Fundamentals, Third Edition.'' Prentice Hall, 2004. p. 73. An entire processor can be created using NOR gates alone. As NAND gates are also functionally complete, if no specific NOR gates are available, one can be made from NAND gates using NAND logic.

References