Logic gate

In, a logic gate is an idealized or physical device implementing a ; that is, it performs a on one or more  inputs and produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero and unlimited, or it may refer to a non-ideal physical device (see  for comparison).

Logic gates are primarily implemented using s or s acting as, but can also be constructed using s, electromagnetic s , , , , , or even elements. With amplification, logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of, and therefore, all of the algorithms and that can be described with Boolean logic.

Logic circuits include such devices as s,, s (ALUs), and , all the way up through complete s, which may contain more than 100 million gates. In modern practice, most gates are made from s (metal–oxide–semiconductor ).

Compound logic gates (AOI) and OR-AND-Invert (OAI) are often employed in circuit design because their construction using MOSFETs is simpler and more efficient than the sum of the individual gates.

In, s are used.

Electronic gates
To build a logic system, s,  (vacuum tubes), or s can be used. The simplest family of logic gates using is called  (RTL). Unlike simple diode logic gates (which do not have a gain element), RTL gates can be cascaded indefinitely to produce more complex logic functions. RTL gates were used in early s. For higher speed and better density, the resistors used in RTL were replaced by diodes resulting in (DTL). (TTL) then supplanted DTL. As integrated circuits became more complex, bipolar transistors were replaced with smaller s (s); see and. To reduce power consumption still further, most contemporary chip implementations of digital systems now use logic. CMOS uses complementary (both n-channel and p-channel) MOSFET devices to achieve a high speed with low power dissipation.

For small-scale logic, designers now use prefabricated logic gates from families of devices such as the  by, the   by , and their more recent descendants. Increasingly, these fixed-function logic gates are being replaced by s, which allow designers to pack many mixed logic gates into a single integrated circuit. The field-programmable nature of s such as has reduced the 'hard' property of hardware; it is now possible to change the logic design of a hardware system by reprogramming some of its components, thus allowing the features or function of a hardware implementation of a logic system to be changed. Other types of logic gates include, but are not limited to:

Electronic logic gates differ significantly from their relay-and-switch equivalents. They are much faster, consume much less power, and are much smaller (all by a factor of a million or more in most cases). Also, there is a fundamental structural difference. The switch circuit creates a continuous metallic path for current to flow (in either direction) between its input and its output. The semiconductor logic gate, on the other hand, acts as a high-, which sinks a tiny current at its input and produces a low-impedance voltage at its output. It is not possible for current to flow between the output and the input of a semiconductor logic gate.

Another important advantage of standardized integrated circuit logic families, such as the 7400 and 4000 families, is that they can be cascaded. This means that the output of one gate can be wired to the inputs of one or several other gates, and so on. Systems with varying degrees of complexity can be built without great concern of the designer for the internal workings of the gates, provided the limitations of each integrated circuit are considered.

The output of one gate can only drive a finite number of inputs to other gates, a number called the ' limit'. Also, there is always a delay, called the '', from a change in input of a gate to the corresponding change in its output. When gates are cascaded, the total propagation delay is approximately the sum of the individual delays, an effect which can become a problem in high-speed circuits. Additional delay can be caused when many inputs are connected to an output, due to the distributed of all the inputs and wiring and the finite amount of current that each output can provide.

History and development
The was refined by  (published in 1705), influenced by the ancient 's binary system. Leibniz established that, by using the binary system, the principles of arithmetic and logic could be combined.

In an 1886 letter, described how logical operations could be carried out by electrical switching circuits. Eventually, s replaced relays for logic operations. 's modification, in 1907, of the can be used as a logic gate. introduced a version of the 16-row as proposition 5.101 of  (1921). , inventor of the, got part of the 1954 in physics, for the first modern electronic AND gate in 1924. designed and built electromechanical logic gates for his computer (from 1935–38).

From 1934 to 1936, engineer  introduced  in a series of papers showing that , which he discovered independently, can describe the operation of switching circuits. His work was later cited by, who elaborated on the use of Boolean algebra in the analysis and design of switching circuits in 1937. Using this property of electrical switches to implement logic is the fundamental concept that underlies all electronic digital s. Switching circuit theory became the foundation of design, as it became widely known in the electrical engineering community during and after, with theoretical rigor superseding the ad hoc methods that had prevailed previously.

(MOS) logic originates from the (metal-oxide-semiconductor field-effect transistor), invented by  and  at  in 1959. They first demonstrated both and  in 1960. Both types were later combined and adapted into (CMOS) logic by  and  at  in 1963.

Active research is taking place in s.

Symbols
There are two sets of symbols for elementary logic gates in common use, both defined in / Std 91-1984 and its supplement ANSI/IEEE Std 91a-1991. The "distinctive shape" set, based on traditional schematics, is used for simple drawings and derives from MIL-STD-806 of the 1950s and 1960s. It is sometimes unofficially described as "military", reflecting its origin. The "rectangular shape" set, based on ANSI Y32.14 and other early industry standards as later refined by IEEE and IEC, has rectangular outlines for all types of gate and allows representation of a much wider range of devices than is possible with the traditional symbols. The IEC standard, 60617-12, has been adopted by other standards, such as  60617-12:1999 in Europe,  EN 60617-12:1999 in the United Kingdom, and  EN 60617-12:1998 in Germany.

The mutual goal of IEEE Std 91-1984 and IEC 60617-12 was to provide a uniform method of describing the complex logic functions of digital circuits with schematic symbols. These functions were more complex than simple AND and OR gates. They could be medium scale circuits such as a 4-bit counter to a large scale circuit such as a microprocessor.

IEC 617-12 and its successor IEC 60617-12 do not explicitly show the "distinctive shape" symbols, but do not prohibit them. These are, however, shown in ANSI/IEEE 91 (and 91a) with this note: "The distinctive-shape symbol is, according to IEC Publication 617, Part 12, not preferred, but is not considered to be in contradiction to that standard." IEC 60617-12 correspondingly contains the note (Section 2.1) "Although non-preferred, the use of other symbols recognized by official national standards, that is distinctive shapes in place of symbols [list of basic gates], shall not be considered to be in contradiction with this standard. Usage of these other symbols in combination to form complex symbols (for example, use as embedded symbols) is discouraged." This compromise was reached between the respective IEEE and IEC working groups to permit the IEEE and IEC standards to be in mutual compliance with one another.

A third style of symbols, DIN 40700 (1976), was in use in Europe and is still widely used in European academia, see the logic table in.

In the 1980s, schematics were the predominant method to design both and custom ICs known as s. Today custom ICs and the  are typically designed with  (HDL) such as  or.

Truth tables
Output comparison of 1-input logic gates.

Output comparison of 2-input logic gates.

Universal logic gates
(during 1880–81) showed that (or alternatively ) can be used to reproduce the functions of all the other logic gates, but his work on it was unpublished until 1933. The first published proof was by in 1913, so the NAND logical operation is sometimes called ; the  is sometimes called Peirce's arrow. Consequently, these gates are sometimes called universal logic gates.

De Morgan equivalent symbols
By use of, an AND function is identical to an OR function with negated inputs and outputs. Likewise, an OR function is identical to an AND function with negated inputs and outputs. A NAND gate is equivalent to an OR gate with negated inputs, and a NOR gate is equivalent to an AND gate with negated inputs.

This leads to an alternative set of symbols for basic gates that use the opposite core symbol (AND or OR) but with the inputs and outputs negated. Use of these alternative symbols can make logic circuit diagrams much clearer and help to show accidental connection of an active high output to an active low input or vice versa. Any connection that has logic negations at both ends can be replaced by a negationless connection and a suitable change of gate or vice versa. Any connection that has a negation at one end and no negation at the other can be made easier to interpret by instead using the De Morgan equivalent symbol at either of the two ends. When negation or polarity indicators on both ends of a connection match, there is no logic negation in that path (effectively, bubbles "cancel"), making it easier to follow logic states from one symbol to the next. This is commonly seen in real logic diagrams – thus the reader must not get into the habit of associating the shapes exclusively as OR or AND shapes, but also take into account the bubbles at both inputs and outputs in order to determine the "true" logic function indicated.

A De Morgan symbol can show more clearly a gate's primary logical purpose and the polarity of its nodes that are considered in the "signaled" (active, on) state. Consider the simplified case where a two-input NAND gate is used to drive a motor when either of its inputs are brought low by a switch. The "signaled" state (motor on) occurs when either one OR the other switch is on. Unlike a regular NAND symbol, which suggests AND logic, the De Morgan version, a two negative-input OR gate, correctly shows that OR is of interest. The regular NAND symbol has a bubble at the output and none at the inputs (the opposite of the states that will turn the motor on), but the De Morgan symbol shows both inputs and output in the polarity that will drive the motor.

De Morgan's theorem is most commonly used to implement logic gates as combinations of only NAND gates, or as combinations of only NOR gates, for economic reasons.

Data storage
Logic gates can also be used to store data. A storage element can be constructed by connecting several gates in a "" circuit. More complicated designs that use s and that change only on a rising or falling edge of the clock are called edge-triggered "". Formally, a flip-flop is called a bistable circuit, because it has two stable states which it can maintain indefinitely. The combination of multiple flip-flops in parallel, to store a multiple-bit value, is known as a register. When using any of these gate setups the overall system has memory; it is then called a system since its output can be influenced by its previous state(s), i.e. by the sequence of input states. In contrast, the output from is purely a combination of its present inputs, unaffected by the previous input and output states.

These logic circuits are known as computer. They vary in performance, based on factors of, complexity, and reliability of storage, and many different types of designs are used based on the application.

Three-state logic gates
A three-state logic gate is a type of logic gate that can have three different outputs: high (H), low (L) and high-impedance (Z). The high-impedance state plays no role in the logic, which is strictly binary. These devices are used on of the  to allow multiple chips to send data. A group of three-states driving a line with a suitable control circuit is basically equivalent to a, which may be physically distributed over separate devices or plug-in cards.

In electronics, a high output would mean the output is sourcing current from the positive power terminal (positive voltage). A low output would mean the output is sinking current to the negative power terminal (zero voltage). High impedance would mean that the output is effectively disconnected from the circuit.

Implementations
Since the 1990s, most logic gates are made in (complementary metal oxide semiconductor) technology that uses both NMOS and PMOS transistors. Often millions of logic gates are in a single.

There are several with different characteristics (power consumption, speed, cost, size) such as:  (resistor–diode logic),  (resistor-transistor logic),  (diode–transistor logic),  (transistor–transistor logic) and CMOS. There are also sub-variants, e.g. standard CMOS logic vs. advanced types using still CMOS technology, but with some optimizations for avoiding loss of speed due to slower PMOS transistors.

Non-electronic implementations are varied, though few of them are used in practical applications. Many early electromechanical digital computers, such as the, were built from gates, using electro-mechanical s. Logic gates can be made using  devices, such as the  or mechanical logic gates, including on a molecular scale. Logic gates have been made out of (see ) and used to create a computer called MAYA (see ). Logic gates can be made from effects (though  usually diverges from boolean design; see ). gates use effects.

In principle any method that leads to a gate that is (for example, either a NOR or a NAND gate) can be used to make any kind of digital logic circuit. Note that the use of 3-state logic for bus systems is not needed, and can be replaced by digital multiplexers, which can be built using only simple logic gates (such as NAND gates, NOR gates, or AND and OR gates).