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Input/Output Standards

Input/Output standards

This is a list of electrical input/output standards that have more or less well-defined electrical properties like operating voltage, driving current, level switching behavior, circuit implementation etc., or are (industry) standards. Different classes: single-ended, differential, voltage-referenced.
- Bus-hold
- Center Tap Terminated (CTT)
- Complementary Metal Oxide Semiconductor (CMOS)
- Current mode logic (CML)
- Digital Visual Interface (DVI)
- Emitter coupled logic (ECL)
- FireWire (IEEE1394)
- Gunning Transceiver Logic Plus (GTL+)
- Gunning Transceiver Logic Terminated (GTL)
- High-Speed Transceiver Logic (HSTL)
- Low Voltage CMOS (LVCMOS)
- Low voltage differential signaling (LVDS)
- Low Voltage Positive Referenced Emitter Coupled Logic (LVPECL)
- Low Voltage Transistor-Transistor Logic (LVTTL)
- Open collector
- Open drain
- Peripheral Component Interconnect (PCI)
- Positive Referenced Emitter Coupled Logic (PECL)
- Push-pull output
- Stub Series Terminated Logic (SSTL)
- Sustained tri-state
- Transistor-transistor logic (TTL)
- Tri-state
- Universal Serial Bus (USB)

External links


- http://www.andreas-schwope.de/ASIC_s/Schnittstellen/Buffer_Types/body_buffer_types.html
- http://www.actel.com/documents/AXIOSelectionAN.pdf

Electrical

Electricity is a general term applied to phenomena involving a fundamental property of matter called an electric charge. This article will introduce and explain some of the basic principles of electricity.

Related concepts

being radiated as light as the air of Earth's atmosphere is shifted from gas to plasma and back. ]] In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms.
- Electric charge: a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
- Electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity.
- Electric potential the potential energy per unit charge associated with a static (time-invariant) electric field.
- Electric current: a movement or flow of electrically charged particles.
- Electrical energy: energy made available by the flow of electric charge through a conductor or from the forces between charged particles.
- Electric power: The rate at which electric energy is converted into another form, such as light, heat, or mechanical energy (or converted from another form into electric energy).

History

Ancient

According to Thales of Miletus, writing circa 600 BCE, a form of electricity was known to the Ancient Greeks who found that rubbing fur on various substances, such as amber, would cause a particular attraction between the two. The Greeks noted that the amber buttons could attract light objects such as hair and that if they rubbed the amber for long enough they could even get a spark to jump. The origin of the word "electricity" is from the Greek word ēlektron, a word the ancient Greeks used for both "amber" and "electrum," and derives from an old root, ēlek- = "shine." The same word was used for both amber and electrum, probably because of the pale yellow color of some varieties of electrum (see electrum). An object found in Iraq in 1938, dated to about 250 BCE and called the Baghdad Battery, resembles a galvanic cell and is believed by some to have been used for electroplating. Additionally, some egyptologists associate the ancient goddess Hathor with artificial light (see Hathor temple). But, remaining unproven are the conjectures that these and other similar ancient artifacts had electrical function and that their associated ancient technology contributed to the development of modern electrical knowledge.

Modern

In 1600 the English scientist William Gilbert returned to the subject in De Magnete, and coined the modern Latin word electricus from ηλεκτρον (elektron), the Greek word for "amber", which soon gave rise to the English words electric and electricity. He was followed in 1660 by Otto von Guericke, who is regarded as having invented an early electrostatic generator. Other European pioneers were Robert Boyle, who in 1675 stated that electric attraction and repulsion can act across a vacuum; Stephen Gray, who in 1729 classified materials as conductors and insulators; and C. F. Du Fay, who first identified the two types of electricity that would later be called positive and negative. The Leyden jar, a type of capacitor for electrical energy in large quantities, was invented at Leiden University by Pieter van Musschenbroek in 1745. William Watson, experimenting with the Leyden jar, discovered in 1747 that a discharge of static electricity was equivalent to an electric current. In June, 1752, Benjamin Franklin promoted his investigations of electricity and theories through the famous, though extremely dangerous, experiment of flying a kite during a thunderstorm. Following these experiments he invented a lightning rod and established the link between lightning and electricity. If Franklin did fly a kite in a storm, he did not do it the way it is often described (as it would have been dramatic but fatal). It was either Franklin (more frequently) or Ebenezer Kinnersley of Philadelphia (less frequently) who created the convention of positive and negative electricity. Franklin's observations aided later scientists such as Michael Faraday, Luigi Galvani, Alessandro Volta, André-Marie Ampère, and Georg Simon Ohm whose work provided the basis for modern electrical technology. The work of Faraday, Volta, Ampere, and Ohm is honored by society, in that fundamental units of electrical measurement are named after them. Volta worked with chemicals and discovered that chemical reactions could be used to create positively charged anodes and negatively charged cathodes. When a conductor was attached between these, the difference in the electrical potential (also known as voltage) drives a current between them through the conductor. The potential difference between two points is measured in units of volts in recognition of Volta's work. The invention of the electric telegraph showed that commercial and practical use could be made of electrical phenomena. By the end of the 19th century electrical engineering became a distinct profession, separate from the physicist or inventor. The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, inventor of the polyphase induction motor; Samuel Morse, inventor of the telegraph; Antonio Meucci, an inventor of the telephone; Thomas Edison inventor of the phonograph and a practical incandescent light bulb; George Westinghouse, inventor of the electric locomotive; Charles Steinmetz, theoretician of alternating current; Alexander Graham Bell, another inventor of the telephone and founder of a sucessful telephone business. The rapid advance of electrical technology in the latter 19th and early 20th centuries lead to commercial rivalry such as the so-called War of the Currents), between Edison's direct-current system or Westinghouse's alternating-current method. Often concurrent research in widely scattered locations lead to multiple claims to the invention of a device or system.

Electric charge

Electric charge is a property of certain subatomic particles (e.g., electrons and protons) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity" is used interchangeably with the phrases "charge of electricity" and "quantity of charge." There are two types of charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law.

Electric field

The concept of electric field was introduced by Michael Faraday. The electrical field force acts between two charges, in the same way that the gravitational field force acts between two masses. However, electric field is a little bit different. Gravitational force depends on mass, whereas electric force depends on the electric charge on both objects. A positive charge exerts away from the object and a negative charge pulls towards the object equally in all directions; thus it is symetric. The most common experience with electric charge in everyday life is that of static cling - when two particular types of materials are rubbed together, they tend to stick together, at least for a while.

Electric potential

The electric potential difference between two points is defined as the work done per unit charge (against electrical forces) in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradients indicates the direction of heat flows. Similarly, there is an electric potential at every point in space, and its gradient in the the electric field indicates where charges move.

Electric current

The electric charge which occurs naturally within conductors can be forced to flow, while the charges within insulators are locked in place and cannot be moved. Devices that use charge flow principles in materials are called electronic devices. A flow of electric charge is called an electric current. A direct current (DC) is a unidirectional flow; alternating current (AC) is a flow whose time average is zero, but whose energy capability (RMS level) is not zero. With AC the electric current repeatedly changes direction. Electric current is measured in Amperes Ohm's Law is an important relationship describing the behaviour of electric currents: See also: electrical conduction For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.

Electrical energy

Electrical energy, is the flow of electrons or ions. When electrons are flowing through a wire or through hundreds of feet of air in the case of lightning it is because they are being forced to do so by an electrical field. A force is exerted on the electrons and they move. Work is done on the charged particles. A force is pushing them through a distance. More properly, they are moving from outer orbitals of one atom to another, being pushed by the electromotive force. While the electrons are in motion they contain kinetic energy. Consquently, atomic level electricity is a form of kinetic energy.

Electric power

Electric power is the capacity of the circuit for performing work in a particular amount of time. When a charge moves in a conductor, work is done by that charge. Devices can be made which convert this work into heat (Electric arc furnaces), light (light bulbs and Fluorescent lamps), or motion, i.e. kinetic energy (electric motors). The unit for all forms of power is the watt (symbol: W). In practice, however, this is generally reserved for the real power component. Apparent power is conventionally expressed in volt-amperes (VA) since it is the simple multiple of rms voltage and current. The unit for reactive power is given the special name "VAR", which stands for volt-amperes-reactive.

SI electricity units

See also


- Electromagnetism
- Electrical phenomenon
- Electrostatics

Devices


- Battery
- Conductor
- Insulator

Engineering


- Green electricity
- Electrical wiring

Safety


- Electric shock
- High-voltage hazards

Electrical phenomena in nature


- Matter: — since atoms and molecules are held together by electric forces.
- Lightning: electrical discharges in the atmosphere.
- The Earth's magnetic field — created by electric currents circulating in the planet's core.
- Sometimes due to solar flares, a phenomenon known as a power surge can be created.
- Piezoelectricity: the ability of certain crystals to generate a voltage in response to applied mechanical stress.
- Triboelectricity: electric charge taken on by contact or friction between two different materials.
- Bioelectromagnetism: electrical phenomena within living organisms.
- Bioelectricity — Many animals are sensitive to electric fields, some (e.g., sharks) more than others (e.g., people). Most also generate their own electric fields.
- Gymnotiformes, such as the electric eel, deliberately generate strong fields to detect or stun their prey.
- Neurons in the nervous system transmit information by electrical impulses known as action potentials.

External links


- [http://amasci.com/miscon/whatis.html What is electricity?]
- [http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=electricity Merriam-Webster: Electricity]
- [http://www.bibliomania.com/2/9/72/119/21387/1.html Tyndall: Faraday as Discovery: Identity of Electricities]
- [http://www.eia.doe.gov/fuelelectric.html US Energy Department Statistics]
- [http://www.mouthshut.com/readreview/38842-1.html How to save on your electricity bills]
- [http://users.pandora.be/worldstandards/electricity.htm Electricity around the world]
- [http://www.tufts.edu/as/wright_center/fellows/bob_morse_04/ A Comprehensive Collection of Franklin’s Electrical Works: The Electrical Writings of Benjamin Franklin], Created and Collected by Robert A. Morse (2004)
- [http://www.telesensoryview.com/steverosecom/Articles/UnderstandingBasicElectri.html Understanding Electricity and some Electronics in 10 minutes](Steve Rose, Maui)
- [http://amasci.com/miscon/eleca.html Electricity Misconceptions]
- ko:전기 ja:電気 simple:Electricity

Complementary Metal Oxide Semiconductor

:"CMOS" can also refer to nonvolatile memory on the motherboard of a personal computer; see Nonvolatile BIOS memory. Nonvolatile BIOS memory CMOS (pronounced "see-moss"), which stands for complementary metal-oxide-semiconductor, is a major class of integrated circuits. CMOS chips include microprocessor, microcontroller, static RAM, and other digital logic circuits. The central characteristic of the technology is that it only uses significant power when its transistors are switching between on and off states. Consequently, CMOS devices use little power and do not produce as much heat as other forms of logic. CMOS also allows a high density of logic functions on a chip. The word "complementary" refers to the fact that the design uses pairs of transistors for logic functions, only one of which is switched on at any time. The phrase "metal-oxide-semiconductor" is a reference to the nature of the fabrication process originally used to build CMOS chips. That process created field effect transistors having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Instead of metal, today the gate electrodes are almost always made from a different material, polysilicon, but the name CMOS nevertheless continues to be used for the modern descendants of the original process. (See also MOSFET.) A chip with a large number of CMOS transistors packed tightly together is sometimes known as CHMOS (for "Complementary High-density metal-oxide-semiconductor"). Standard discrete CMOS logic functions were originally available only in the 4000 series of logic integrated circuits. Many families in the 74 series are now fabricated in CMOS, NMOS, BiCMOS or another variant.

Development history

CMOS circuits were invented in 1963 by Frank Wanlass at Fairchild Semiconductor. The first CMOS integrated circuits were made by RCA in 1968 by a group led by Albert Medwin. Originally a low-power but slow alternative to TTL, CMOS found early adopters in the watch industry and in other fields where battery life was more important than speed. Some twenty-five years later, CMOS has become the predominant technology in digital integrated circuits. This is essentially because area occupation, operating speed, energy efficiency and manufacturing costs have benefited and continue to benefit from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In addition, the simplicity and comparatively low power dissipation of CMOS circuits have allowed for integration densities not possible on the basis of bipolar junction transistors. Early CMOS circuits were very susceptible to damage from electrostatic discharge (ESD). Subsequent generations were thus equipped with sophisticated protection circuitry that helps absorb electric charges with no damage to the fragile gate oxides and PN-junctions. Still, antistatic handling precautions for semiconductor devices continue to be followed to prevent excessive energies from building up. Manufacturers recommend using antistatic precautions when adding a memory module to a computer, for instance. On the other hand, early generations such as the 4000 series that used aluminum as a gate material were extremely tolerant of supply voltage variations and operated anywhere from 3 to 18 volts DC. For many years, CMOS logic was designed to operate from the then industry-standard of 5 V imposed by TTL. By 1990, lower power dissipation was usually more important than easy interfacing to TTL, and CMOS voltage supplies began to drop along with the geometric dimensions of the transistors. Lower voltage supplies not only saved power, but allowed thinner, higher performance gate insulators to be used. Some modern CMOS circuits operate from voltages below one volt. In the early fabrication processes, the gate electrode was made of aluminum. Later CMOS processes switched to polycrystalline silicon ("polysilicon"), which can better tolerate the high temperatures used to anneal the silicon after ion implantation. As of 2004 there is some research into using metal gates once again, but all commonly used processes have polysilicon gates.

Technical details

CMOS (complementary metal oxide semiconductor) refers to both a particular style of digital circuitry design, and the family of processes used to implement that circuitry on integrated circuits (chips). CMOS logic on a CMOS process dissipates less energy and is more dense than other implementations of the same functionality. As this advantage has grown and become more important, CMOS processes and variants have come to dominate, so that as of 2004 the vast majority of integrated circuit manufacturing by dollar volume is on CMOS processes.

Structure

CMOS logic uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits found in computers, telecommunications and signal processing equipment. Although CMOS logic can be implemented with discrete devices (for instance, in an introductory circuits class), typical commercial CMOS products are integrated circuits composed of millions (or hundreds of millions) of transistors of both types on a rectangular piece of silicon of between 0.1 and 4 square centimeters. These bits of silicon are commonly called chips, although within the industry they are also referred to as die, perhaps because they are the result of dicing (that is, cutting up) the circular silicon wafer which is the basic unit of semiconductor device fabrication. In CMOS logic gates a collection of n-type MOSFETs is arranged in a pull-down network between the output and the lower-voltage power supply rail (often named Vss). Instead of the load resistor of NMOS logic gates, CMOS logic gates have a collection of p-type MOSFETs in a pull-up network between the output and the higher-voltage rail (often named Vdd). The p-type transistor network is complementary to the n-type transistor network, so that when the n-type is off, the p-type is on, and vice-versa. CMOS logic dissipates less power than NMOS logic because CMOS dissipates power only when switching (dynamic power). On a typical ASIC in a modern 90 nanometer process, switching the output might take 120 picoseconds, and happen once every ten nanoseconds. NMOS logic dissipates power whenever the output is low (static power), because there is a current path from Vdd to Vss through the load resistor and the n-type network. P-type MOSFETs are complementary to n-type because they turn on when their gate voltage goes sufficiently below their source voltage, and because they can pull the drain all the way to Vdd. Thus, if both a p-type and n-type transistor have their gates connected to the same input, the p-type MOSFET will be on when the n-type MOSFET is off, and vice-versa.

Example: NAND gate

right As an example, shown on the right is a circuit diagram of a NAND gate in CMOS logic. If both of the A and B inputs are high, then: both the n-type transistors (bottom half of the diagram) will conduct, neither of the p-type transistors (top half) will conduct, and a conductive path will be established between the output and Vss, bringing the output low. If either of the A or B inputs is low, one of the n-type transistors will not conduct, one of the p-type transistors will, and a conductive path will be established between the output and Vdd, bringing the output high. Another advantage of CMOS over NMOS is that both low-to-high and high-to-low output transitions are fast since the pull-up transistors have low resistance when switched on, unlike the load resistors in NMOS logic. In addition, the output signal swings the full voltage between the low and high rails. This strong, more nearly symmetric response also makes CMOS more resistant to noise. See Logical effort for a method of calculating delay in a CMOS circuit.

Power

CMOS circuits dissipate power by charging and discharging the various load capacitances (mostly gate and wire capacitance, but also drain and some source capacitances) whenever they are switched. The charge moved is the capacitance multiplied by the voltage change. Multiply by the switching frequency to get the current used, and multiply by voltage again to get the characteristic switching power dissipated by a CMOS device: P = C V^2 f . A different form of power consumption became noticeable in the 1990s as wires on chip became narrower and the long wires became more resistive. CMOS gates at the end of those resistive wires see slow input transistions. During the middle of these transitions, both the NMOS and PMOS networks are partially conductive, and current flows directly from Vdd to Vss. The power thus used is called crowbar power. Careful design which avoids weakly driven long skinny wires has ameliorated this effect, and crowbar power is nearly always substantially smaller than switching power. Both NMOS and PMOS transistors have a threshold gate-to-source voltage, below which the current through the device drops exponentially. Historically, CMOS designs operated at supply voltages much larger than their threshold voltages (Vdd might have been 5 V, and Vth for both NMOS and PMOS might have been 700 mV). As supply voltages have come down to conserve power, voltage thresholds have had to come down as well. The exponential current curve has not changed, however, and as a result a modern NMOS transistor with a Vth of 200 mV has a significant subthreshold leakage current. Designs (e.g. desktop processors) which try to optimize their fabrication processes for minimum power dissipation during operation have been lowering Vth so that leakage power begins to approximate switching power. As a result, these devices dissipate considerable power even when not switching.

See also


- Magic is open-source software often used as a layout tool for CMOS circuits.

External links


- [http://tech-www.informatik.uni-hamburg.de/applets/cmos/cmosdemo.html CMOS gate description and interactive illustrations]
- [http://members.aol.com/lasicad/ LASI] is a "general purpose" IC layout CAD tool. It is a free download and can be used as a layout tool for CMOS circuits. Category:Electronic design Category:Computer hardware Category:Digital electronics Category:Logic families ja:Complementary Metal Oxide Semiconductor

Current mode logic

Current mode logic (CML) is one logic class of the CMOS family. Its main structure is differential CMOS circuits which can operate at very high frequencies.

Emitter coupled logic

In electronics, emitter coupled logic (or ECL) is a design which uses transistors to steer current through gates which compute logical functions (as does every logic family). By comparison, TTL and related families use transistors as digital switches (ECL is also digital), where transistors are either cut off or saturated, depending on the state of the circuit. This distinction explains ECL's chief advantage: that because the transistors are always in the active region, they can change state very rapidly, so ECL circuits can operate at very high speed; and also its major disadvantage: the transistors are continually drawing current, which means the circuits require high power, and thus generate large amounts of waste heat. ECL gates use differential amplifier configurations at the input stage. A bias configuration supplies a constant voltage at the midrange of the low and high logic levels to the differential amplifier, so that the appropriate logical function of the input voltages will control the amplifier and the base of the output transistor (this output transistor is used in common emitter configuration?). The propagation time for this arrangement can be less than a nanosecond, making it the fastest logic family through several years. Other noteworthy characteristics of the ECL family include the fact that the large current requirement is approximately constant, and does not depend significantly on the state of the circuit. This means that ECL circuits generate relatively little power noise, unlike many other logic types which typically draw far more current when switching than quiescent, for which power noise can become problematic. In an ALU - where a lot of switching occurs - ECL can draw lower mean current than CMOS. ECL circuits usually operate with negative power supplies, and use logic levels incompatible with other families, which means that interoperation between ECL and other designs is difficult. The fact that the high and low logic levels are relatively close mean that ECL suffers from small noise margins, which can be troublesome if one wants to use these levels outside a chip. The drawbacks associated with ECL have meant that it has been used mainly when high performance is a vital requirement, and other families (particularly advanced CMOS variants) have been gradually taking over ECL use in some applications. However, some experts predict increasing use of ECL in the future, particularly in conjunction with more widespread adoption of advanced semiconductors such as GaAs, which has always been the semiconductor of the future, but cannot be produced as cheap/clean as Si and is toxic.

See also


- resistor-transistor logic (RTL)
- diode-transistor logic (DTL)
- transistor-transistor logic (TTL)
- Complementary Metal Oxide Semiconductor (CMOS)
- Positive Emitter Coupled Logic (PECL)
- Low Voltage Positive Emitter Coupled Logic (LVPECL)
- Embeddable Common Lisp Category:Logic families

Gunning Transceiver Logic Terminated

Gunning Transceiver Logic or GTL is a type of logic signalling used to drive electronic backplane buses. It has a voltage swing between 0.4 volts and 1.2 volts, much lower than that used in TTL and CMOS logic, and symmetrical parallel resistive termination. The maximum signalling frequency is specified to be 100 MHz, although some applications use higher frequencies. GTL is defined by JEDEC standard JESD 8-3 (1993). The bus used in the Pentium Pro, Pentium II and Pentium III processors made by Intel uses GTL+ (or GTLP, developed by Fairchild Semiconductor), an upgraded version of GTL which has defined slew rates and higher voltage levels.

See also


- Front side bus

Reference


- Fairchild Semiconductor, Application Note AN-1070, GTLP vs. GTL: A Performance Comparison from a System Perspective, 1997 Category:Computer buses Category:Logic families

High-Speed Transceiver Logic

High-Speed Transceiver Logic or HSTL is a technology-independent standard for signalling between integrated circuits. The nominal signalling range is 0 V to 1.5 V, though variations are allowed, and signals may be single-ended or differential. It is designed for operation beyond 180 MHz. The following classes are defined by standard EIA/JESD8-6 from EIA/JEDEC:
- Class I (unterminated, or symmetrically parallel terminated)
- Class II (series terminated)
- Class III (asymmetrically parallel terminated)
- Class IV (asymmetrically double parallel terminated) Note: Symmetric parallel termination means that the termination resistor at the load is connected to half the output buffer's supply voltage. Double parallel termination means that parallel termination resistors are fitted at both ends of the transmission line. See also SSTL (Stub Series Terminated Logic).

Low voltage differential signaling

Low voltage differential signaling, or LVDS, is an electrical signalling system that can run at very high speeds over cheap, twisted-pair copper cables. It was introduced in 1994, and has since become very popular in computers, where it forms part of very high-speed networks and computer buses.

Differential vs. single-ended signalling

LVDS is a differential signalling system, which means that it transmits two different voltages which are compared at the receiver. See Differential signalling for details. LVDS uses the difference in voltage between two wires to signal information. The transmitter injects a small current, nominally 3.5 milliamperes, into one wire or the other, depending on the logic level to be sent. The current passes through a resistor of about 100 to 120 ohms (matched to the characteristic impedance of the cable) at the receiving end, then returns in the opposite direction along the other wire. From Ohm's law, the voltage difference across the resistor is therefore about 350 millivolts. The receiver senses the polarity of this voltage to determine the logic level. (This is a type of current loop signalling). The small amplitude of the signal and the tight electric- and magnetic-field coupling between the two wires reduces the amount of radiated electromagnetic noise. The low common-mode voltage (the average of the voltages on the two wires) of about 1.25 V allows LVDS to be used with a wide range of integrated circuits with power supply voltages down to 2.5 V or lower. The low differential voltage, about 350 mV as stated above, causes LVDS to consume very little power compared to other systems. For example, the static power dissipation in the LVDS load resistor is 1.2 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal. This power efficiency is maintained at high frequencies because of the low voltage swing. LVDS is not the only differential signalling system in use. See Differential signalling for a list of other differential signalling systems. LVDS is currently the only scheme that combines low power dissipation with high speed.

Applications

LVDS only became popular in the latter half of the 1990s. Prior to that point it could signal faster than the computers it was running in, and the need to run twice as many wires for the same amount of data outweighed the speed benefits. Yet multimedia and supercomputer users, both of whom needed to move large amounts of data over links several meters long (from a disk drive to a workstation, for instance) maintained a widespread interest in LVDS. Two examples of LVDS use in computer buses come from HyperTransport and FireWire, both of which trace their ancestry back to the post-Futurebus work which also led to SCI. LVDS is supported in SCSI standards (Ultra-2 SCSI and later) to allow higher data rates and longer cable lengths. Serial ATA and SpaceWire utilizes LVDS to allow high speed data transfer. LVDS can also transport video data from graphics adapters to computer monitors, particularly flat panels, using the Flat Panel Display (FPD) Link, LVDS Display Interface (LDI) and OpenLDI standards. These standards allow a maximum pixel clock of 112 MHz, which suffices for a display resolution of 1400 x 1050 (SXGA+) at 60 Hz refresh. A dual link can boost the maximum display resolution to 2048 x 1536 (QXGA) at 60 Hz. FPD-Link works with cable lengths up to about 5 m, and LDI extends this to about 10 m.

SerDes

LVDS is often used for serial data transmission, which involves sending data bit-by-bit down a single wire (as opposed to parallel transmission, in which several bits, usually in multiples of eight, are sent down many wires at once). Its high speed, and its use of in-channel synchronisation, makes it possible to send serial data faster than could be done with a parallel bus, and using fewer wires. The device for converting between serial and parallel data is called a serializer/deserializer, abbreviated to SerDes.

Bus LVDS

When serial data transmission (see SerDes, above) is not fast enough, data can be transmitted in parallel form using an LVDS pair for each bit. This system is called bus LVDS, or BLVDS. Standard LVDS transmitters are designed for point-to-point links, but multipoint bus systems can be made using modified LVDS transmitters with high-current outputs that can drive multiple termination resistors.

SCI-LVD

The present form of LVDS was preceded by an earlier attempt, SCI-LVD, which was a subset of the Scalable Coherent Interconnect (SCI) specified in the IEEE 1596.3 standard. It was designed for interconnecting multiprocessing systems.

Standards

The ANSI/TIA/EIA-644-A (published in 2001) standard defines LVDS. This recommends a maximum data rate of 655 Mbit/s over twisted-pair copper wire, but predicts a possible speed of over 1.9 Gbit/s for an ideal transmission medium.

External links


- [http://www.national.com/appinfo/lvds/0,1798,100,00.html Introduction to LVDS]

References


- An Overview of LVDS Technology, National Semiconductor, AN-971, July 1998.
- LVDS Owner's Manual, National Semiconductor, 3rd Edition, 2004

See also


- high-voltage differential signalling (HVDS) Category:computer buses Category:Logic families

Open drain

Open drain is one of the many different electrical input/output standards in use today in digital designs. Open-drain refers to the drain terminal of a MOSFET transistor. An Open-drain output is either actively sinking voltage low (typically considered logic 0) in the active state or is high impedance (no current flowing, typically logic 1) in the non-active state. More than one open-drain output can be attached to a single wire. If all outputs attached to the wire are in their non-active state, a pull-up will hold the wire at a high voltage state. If at least one of the device outputs is in the active state, then the signal wire voltage will be low.

Positive Referenced Emitter Coupled Logic

Positive Emitter Coupled Logic, or PECL, is a further development of the emitter coupled logic (ECL) technology and requires a positive 5V supply instead of a negative -5V supply. The Low Voltage Positive Emitter Coupled Logic (LVPECL) is an improved version of PECL to meet today's low voltage requirements. PECL is a differential signaling system and mainly used in high speed and clock distribution circuits.

See also


- Emitter coupled logic (ECL)
- Low Voltage Positive Emitter Coupled Logic (LVPECL)
- Low voltage differential signaling (LVDS) Category:Logic families

Stub Series Terminated Logic

Stub Series Terminated Logic (SSTL) devices are a family of electronic devices for driving transmission lines. They are specifically designed for driving the DDR (double-data-rate) SDRAM modules used in computer memory. Two voltage levels for SSTL are defined:
- SSTL-2, 2.5 V, defined in EIA/JESD8-9B 2002
- SSTL-3, 3.3 V, defined in EIA/JESD8-8 1996 Terminations can be Class I (one series resistor at the source and one parallel resistor at the load) or Class II (one series resistor at the source and two parallel resistors, one at each end). See also HSTL (High-Speed Transceiver Logic).

Universal Serial Bus

:For other meanings of the abbreviation USB see USB (disambiguation). USB (disambiguation) USB (disambiguation) USB (disambiguation) Universal Serial Bus (USB) provides a serial bus standard for connecting devices, usually to a computer, but it also is in use on other devices such as set-top boxes, game consoles such as Sony's PlayStation 2, Microsoft's Xbox 360, Nintendo's Revolution and PDAs.

Overview

A USB system has an asymmetric design, consisting of a host controller and multiple devices connected in a tree-like fashion using special hub devices. There is a limit of 5 levels of branching hubs per controller. Up to 127 devices may be connected to a single host controller, but the count must include the hub devices as well. A modern computer likely has several host controllers so the total useful number of connected devices is beyond what could reasonably be connected to a single controller. There is no need for a terminator on any USB bus, as there is for SPI-SCSI and some others. The design of USB aimed to remove the need for adding separate expansion cards into the computer's ISA or PCI bus, and improve plug-and-play capabilities by allowing devices to be hot swapped or added to the system without rebooting the computer. When the new device first plugs in, the host enumerates it and loads the device driver necessary to run it. device driver USB can connect peripherals such as mice, keyboards, gamepads and joysticks, scanners, digital cameras, printers, hard disks, and networking components. For multimedia devices such as scanners and digital cameras, USB has become the standard connection method. For printers, USB has also grown in popularity and started displacing parallel ports because USB makes it simple to add more than one printer to a computer. As of 2004 there were about 1 billion USB devices in the world. As of 2005, the only large classes of peripherals that cannot use USB (because they need a higher data rate than USB can provide) are displays and monitors, data acquisition devices that use firewire ports, and high-quality digital video components.

Standardization

The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. Notable members have included Apple Computer, Hewlett-Packard, NEC, Microsoft, Intel, and Agere. The USB specification is at version 2.0 as of January 2005. Hewlett-Packard, Intel, Lucent, Microsoft, NEC and Philips jointly led the initiative to develop a higher data transfer rate than the 1.1 specification to meet the bandwidth demands of developing technologies. The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001. Previous notable releases of the specification were 0.9, 1.0, and 1.1. Each iteration of the standard is completely backward compatible with previous versions. Smaller USB plugs and receptors called Mini-A and Mini-B are also available, as specified by the On-The-Go Supplement to the USB 2.0 Specification. The specification is of revision 1.0a currently.

Technical details

2001 USB connects several devices to a host controller through a chain of hubs. In USB terminology devices are referred to as functions, because in theory what we know as a device may actually host several functions, such as a router that is a Secure Digital Card reader at the same time. The hubs are special purpose devices that are not officially considered functions. There always exists one hub known as the root hub, which is attached directly to the host controller. These devices/functions (and hubs) have associated pipes (logical channels) which are connections from the host controller to a logical entity on the device named an endpoint. The pipes are synonymous to byte streams such as in the pipelines of Unix, however in USB lingo the term endpoint is (sloppily) used as a synonym for the entire pipe, even in the standard documentation. These endpoints (and their respective pipes) are numbered 0-15 in each direction, so a device/function can have up to 32 active pipes, 16 inward and 16 outward. (The OUT direction shall be interpreted out of the host controller and the IN direction is into the host controller.) Endpoint 0 is however reserved for the bus management in both directions and thus takes up two of the 32 endpoints. In these pipes, data is transferred in packets of varying length. Each pipe has a maximum packet length, typically 2^n bytes, so a USB packet will often contain something on the order of 8, 16, 32, 64, 128, 256, 512 or 1024 bytes. Each endpoint can transfer data in one direction only, either into or out of the device/function, so each pipe is uni-directional. All USB devices have at least two such pipes/endpoints: namely endpoint 0 which is used to control the device on the bus. There is always an inward and an outward pipe numbered 0 on each device. The pipes are also divided into four different categories by way of their transfer type:
- control transfers - typically used for short, simple commands to the device, and a status response, used e.g. by the bus control pipe number 0
- isochronous transfers - at some guaranteed speed (often but not necessarily as fast as possible) but with possible data loss, e.g. realtime audio or video
- interrupt transfers - devices that need guaranteed quick responses (bounded latency), e.g. pointing devices and keyboards
- bulk transfers - large sporadic transfers using all remaining available bandwidth (but with no guarantees on bandwidth or latency), e.g. file transfers When a device (function) or hub is attached to the host controller through any hub on the bus, it is given a unique 7 bit address on the bus by the host controller. The host controller then polls the bus for traffic, usually in a round-robin fashion, so no device can transfer any data on the bus without explicit request from the host controller. To access an endpoint, a hierarchical configuration must be obtained. The device connected to the bus has one (and only one) device descriptor which in turn has one or more configuration descriptors. These configurations often correspond to states, e.g. active vs. low power mode. Each configuration descriptor in turn has one or more interface descriptors, which describe certain aspects of the device, so that it may be used for different purposes: for example, a camera may have both audio and video interfaces. These interface descriptors in turn have one default interface setting and possibly more alternate interface settings which in turn have endpoint descriptors, as outlined above. An endpoint may however be reused among several interfaces and alternate interface settings. The hardware that contains the host controller and the root hub has an interface toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer. In practice, these are hardware registers (ports) in the computer. At version 1.0 and 1.1 there were two competing HCD implementations. Compaq's Open Host Controller Interface (OHCI) was adopted as the standard by the USB-IF. However, Intel subsequently created a specification they called the Universal Host Controller Interface (UHCI) and insisted other implementers pay to license and implement UHCI. VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI. The main difference between OHCI and UHCI is the fact that UHCI is more software-driven than OHCI is, making UHCI slightly more processor-intensive but cheaper to implement (excluding the license fees). The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations which increased cost. During the design phase of USB 2.0 the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Extended Host Controller Interface (EHCI). Only EHCI can support high-speed transfers. Each EHCI controller contains four virtual HCD implementations to support Full Speed and Low Speed devices. The virtual HCD on Intel and Via EHCI controllers are UHCI. All other vendors use virtual OHCI controllers. On Microsoft Windows platforms, one can tell whether a USB port is version 2.0 by opening the Device Manager and checking for the word "Enhanced" in its description; only USB 2.0 drivers will contain the word "Enhanced." On Linux systems, the lspci command will list all PCI devices, and a controllers will be named OHCI, UHCI or EHCI respectively, which is also the case in the Mac OS X system profiler.

Device classes

Devices that attach to the bus can be full-custom devices requiring a full-custom device driver to be used, or may belong to a device class. These classes define an expected behaviour in terms of device and interface descriptors so that the same device driver may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device. The most used device classes are:
- USB human interface device class, keyboards, mice, etc.
- USB mass storage device class used for keydrives, portable hard drives, Multi Media Card readers, digital cameras, digital audio players etc. This device class presents the device as a block device (almost always used to store a file system).
- USB communications device class ("CDC") used for modems (and winmodems), network cards (and cross-over cables), ISDN connections, Fax
- USB printer device class, printer-like devices
- USB audio device class, sound card-like devices
- USB video device class, webcam-like devices, motion image capture devices Device classes are decided upon by the Device Working Group of the USB Implementers Forum.

USB signaling

Standard USB signaling

webcam USB signals are transmitted on a twisted pair of data cables, labelled D+ and D−. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. D+ and D− operate together; they are not separate simplex connections.
Transfer speed
USB supports three data rates.
- A Low Speed rate of 1.5 Mbit/s (183 KiB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice and joysticks.
- A Full Speed rate of 12 Mbit/s (1.4 MiB/s). Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices. All USB Hubs support Full Speed.
- A Hi-Speed rate of 480 Mbit/s (57 MiB/s). (Commonly called USB 2.0) Not all USB 2.0 devices are Hi-Speed. A USB device should specify the speed it will use by correct labeling on the box it came in or sometimes on the device itself. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or High-Speed after passing a compliancy test and paying a licensing fee. Hi-Speed devices should fall back to the slower data rate of Full Speed when plugged into a Full Speed hub. Hi-Speed hubs have a special function called the Transaction Translator that segregates Full Speed and Low Speed bus traffic from Hi-Speed traffic. The Transaction Translator in a Hi-Speed hub (or possibly each port depending on the electrical design) will function as a completely separate Full Speed bus to Full Speed and Low Speed devices attached to it. This segregation is for bandwidth only; bus rules about power and hub depth still apply.

Mini USB signaling

USB-IF Most of the pins of a mini USB connector are the same as a standard USB connector, except pin 4. Pin 4 is called ID and is connected to pin 5 for a mini-A and is either unconnected or connected to pin 5 through a resistor for a mini-B.

USB connectors

The connectors which the USB committee specified were designed to support a number of USB's underlying goals, and to reflect lessons learned from the varied menagerie of connectors then in service. In particular:
- The connectors are designed to be robust. Many previous connector designs were fragile, with pins or other delicate components prone to bending or breaking, even with the application of only very modest force. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is further protected by an enclosing metal sheath. As a result USB connectors can safely be handled, inserted, and removed, even by a small child. The encasing sheath and the tough moulded plug body mean that a connector can be dropped, stepped upon, even crushed or struck, all without damage; a considerable degree of force is needed to significantly damage a USB connector.
- It is difficult to incorrectly attach a USB connector. Connectors cannot be plugged-in upside down, and it is clear from the appearance and kinesthetic sensation of making a connection when the plug and socket are correctly mated.
- The connectors are particularly cheap to manufacture.
- The connectors enforce the directed topology of a USB network. USB does not support cyclical networks, so the connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. RJ-45 cabling) gender-changers are never used, making it difficult to create a cyclic USB network.
- A moderate insertion/removal force is specified. USB cables and small USB devices are held in place by the gripping force from the receptacle (without the need for the screws, clips, or thumbturns other connectors require). The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances or by those with motor disabilities.
- The connector construction always ensures that the external sheath on the plug contacts with its counterpart in the receptacle before the four connectors within are connected. This sheath is typically connected to the system ground, allowing otherwise damaging static charges to be safely discharged by this route (rather than via delicate electronic components). This means of enclosure also means that there is a (moderate) degree of protection from electromagnetic interference afforded to the USB signal while it travels through the mated connector pair (this is the only location when the otherwise twisted data pair must travel a distance in parallel).
- The USB standard specifies relatively low tolerances for compliant USB connectors, intending to minimize incompatibilities in connectors produced by different vendors (a goal that has been very successfully achieved). Unlike most other connector standards, the USB spec also defines limits to the size of a connecting device in the area around its plug. This was done to avoid circumstances where a device complied with the connector specification but its large size blocked adjacent ports. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does. The USB 1.0, 1.1 and 2.0 specifications define two types of connectors for the attachment of devices to the bus: A, and B. However, the mechanical layer has changed in some examples. For example, the IBM UltraPort is a proprietary USB connector located on the top of IBM's laptop LCDs. It uses a different mechanical connector while preserving the USB signaling and protocol. Other manufacturers of small items also developed their own small form factor connector, and a wide variety of these have appeared. For specification purposes, these devices were treated as having a captive cable. An extension to USB called USB On-The-Go allows a single port to act as either a host or a device - chosen by which end of the cable plugs into the socket on the unit. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This facility targets units such as PDAs where the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance. USB On-The-Go has therefore defined two small form factor connectors, the mini-A and mini-B, and a hermaphroditic socket (mini-AB), which should stop the proliferation of proprietary designs. Wireless USB is a promising future standard being developed to extend the USB standard while maintaining backwards compatibility with USB 1.1 and USB 2.0 on the protocol level. The maximum length of a USB cable is 5 meters; greater lengths require hubs [http://www.usb.org/developers/usbfaq/#cab1].

Power supply

The USB connector provides a single nominally 5 volt wire from which connected USB devices may power themselves. In practice, delivered voltage can drop well below 5 V, to only slightly above 4 V. The compliance spec requires no more than 5.25 V anywhere and no less than 4.375 V at the worst case; a low-power function after a bus-powered hub. In typical situations the voltage is close to 5 V. A given segment of the bus is specified to deliver up to 500 mA. This is often enough to power several devices, although this budget must be shared among all devices downstream of an unpowered hub. A bus-powered device may use as much of that power as allowed by the port it is plugged into. Bus-powered hubs can continue to distribute the bus provided power to connected devices but the USB specification only allows for a single level of bus-powered devices from a bus-powered hub. This disallows connection of a bus-powered hub to another bus-powered hub. Many hubs include external power supplies which will power devices connected through them without taking power from the bus. Devices that need more than 500 mA must provide their own power. When USB devices (including hubs) are first connected they are interrogated by the host controller, which enquires of each their maximum power requirements. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available (and will generally shut down devices or hubs in order to keep power consumption within the available resource). A number of devices use this power supply without participating in a proper USB network. The typical example is a USB-powered reading light, but fans, battery chargers (particularly for mobile telephones) and even miniature vacuum cleaners are available. In most cases, these items contain no electronic circuitry, and thus are not proper USB devices at all. This can cause problems with some computers—the USB specification requires that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host's permission, into high-power mode. Some devices intended for connection to laptops draw more power than is permitted by the specification for a single USB port; to avoid requiring an exernal power supply, these devices come with dual cables, and the user is instructed that the device must be plugged-into two USB ports. On a laptop with only two ports, this means only one such device can be used at a time, unless a powered hub is added. A number of peripherals for IBM laptops (now made by Lenovo) are designed to use dual USB connections in this manner. USB-powered devices attempting to draw large currents without requesting the power will not work with certain USB controllers, and will either disrupt other devices on the bus or fail to work themselves (or both). Those problems with the abuse of the USB power supply have inspired a number of April Fool hoaxes, like the introduction of a USB-powered George Foreman iGrill [http://www.thinkgeek.com/stuff/looflirpa/igrill.shtml] and a desktop USB Fondue Set [http://www.thinkgeek.com/stuff/41/fundue.shtml].

USB compared to other standards

Storage

Fondue USB implements connections to storage devices using a set of standards called the USB mass-storage device class. This was initially intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices. USB is not intended to be a primary bus for a computer's internal storage: buses such as ATA (IDE) and SCSI fulfill that role. However, USB has one important advantage in making it possible to install and remove devices without opening the computer case, making it useful for external drives. Today, a number of manufacturers offer portable USB hard drives that offer performance comparable to conventional ATA (IDE) drives. These external drives, called enclosures, are often composed of translating devices that connect to USB on one side and to conventional IDE, ATA, ATAPI, or SCSI drives on the other. A drive is installed into the enclosure and the enclosure is then plugged into the computer, thus creating the function of a regular USB mass-storage device. FireWire technology is also commonly used with portable hard drives, some of which include both USB and FireWire ports. FireWire tends to perform better in speed benchmark tests. However, USB ports are more common on consumer-level computers, which enhances the portability of a USB drive.

Human-interface devices (HIDs)

USB has not completely replaced AT keyboard connections and PS/2 keyboard and mouse connections, but virtually all PC motherboards manufactured today have one or more USB ports. As of 2004, most new motherboards have multiple USB 2.0 high-speed ports, though some are internal, and require a "header" connection to be accessible from the front or rear of the computer case. Similarly, support for joysticks, keypads, tablets and other human-interface devices is progressively migrating from MIDI, "game", and PS/2 connectors to USB. It is now quite common for a mouse or keyboard to be a USB device, which is shipped with a small USB-to-PS/2 adaptor connected to the end of its cable, so it can be used with either USB or PS/2 ports. Apple computers have used USB mice and keyboards exclusively since January 1999.

USB 2.0 vs FireWire

USB 2.0 transmits data at up to 480 megabits per second (Mbps) while FireWire 400 (IEEE 1394a) handles data at up to 400 Mbps [http://www.choice.com.au/viewArticle.aspx?id=104527&catId=100274&tid=100008&p=1]. However USB 2.0 is not commonly considered to be faster than FireWire 400. USB uses more CPU resources than FireWire and its data transfer rate is degraded as more load is applied to the CPU (by running concurrent tasks). Finally, the more recent IEEE 1394b specification of FireWire supports data rates up to 3.2 gigabits per second.

Version history

USB


- USB 1.0 FDR: Released in November 1995, the same year that Apple adopted the IEEE 1394 standard known as FireWire.
- USB 1.0: Released in January 1996.
- USB 1.1: Released in September 1998.
- USB 2.0: Released in April 2000. The major feature of this standard was the addition of high-speed mode. This is the current revision.
- USB 2.0: Revised in December 2002. Added three speed distinction to this standard, allowing all devices to be USB 2.0 compliant even if they were previously considered only 1.1 or 1.0 compliant. The makes the backwards compatibility explicit, but more difficult to determine a device's throughput without seeing the symbol. As an example, a computer's port could be incapable of USB 2.0's hi-speed fast transfer rates, but still claim USB 2.0 compliance (since it supports some of USB 2.0).

USB On-The-Go Supplement


- USB On-The-Go Supplement 1.0: Released in December 2001.
- USB On-The-Go Supplement 1.0a: Released in June 2003. This is the current revision.

Extensions to USB

The PictBridge standard allows for interconnecting consumer imaging devices. It typically uses USB as the underlying communication layer. Microsoft's Xbox game console uses standard USB 1.1 signalling, but features a proprietary connector rather than the standard USB connector. Similarly IBM UltraPort uses standard USB signalling, but uses a proprietary connection format. The USB Implementers Forum is working on a wireless networking standard based on the USB protocol. Wireless USB is intended as a cable-replacement technology, and will use Ultra wideband wireless technology for data rates of up to 480 Mbit/s. Wireless USB is well suited to wireless connection of PC centric devices, just as Bluetooth is now widely used for mobile phone centric personal networks (at much lower data rates). See http://www.usb.org/developers/wusb/ for more details.

See also


- ACCESS.bus
- FireWire (also known as IEEE 1394, or I.link)
- USB Flash Drive
- USB streaming
- U3
- Serial cable (obsoleted by USB and Wi-Fi)

External links


- [http://www.usb.org/ Home of USB Implementers Forum, Inc.], including [http://www.usb.org/developers/docs/ the USB 2.0 specification]
- [http://www.lvr.com/usb.htm USB Central] for developers of USB devices and hosts
- [http://www.bootdisk.com/usb.htm USB for DOS]
- [http://www.linux-usb.org/ Linux USB Project], containing much technical information and documentation
- [http://www.windowsnetworking.com/articles_tutorials/usbmain.html USB Networking Introduction]
- [http://usbmount.alioth.debian.org/ Linux usbmount].
- [http://www.beyondlogic.org/usbnutshell/usb-in-a-nutshell.pdf USB in a NutShell] - a primer for developers
- [http://developer.intel.com/technology/usb/uhci11d.htm Universal Host Controller Interface (UHCI)] Category:Computer buses Category:USB ko:USB ja:Universal Serial Bus nb:USB simple:Universal Serial Bus th:ยูเอสบี

Bunbury

Bunbury is a placename in more than one country:
- Bunbury, Western Australia
- Bunbury, Cheshire, United Kingdom Bunbury is also a family name: Enrique Bunbury is lead singer in the spanish rock band Heroes del Silencio. Edward Herbert Bunbury was a historian of Greek and Roman geography. Henry William Bunbury is a British caricaturist. Bunbury is also a fictional character in Oscar Wilde's play The Importance of Being Earnest. One might say "meta-fictional", since even in the play, Bunbury did not exist but the supposed chronic illness of this supposed friend was used as an excuse for Algernon Moncrieff to leave his current whereabouts and go and "have a good time" elsewhere without the risk of detection.(see Bunburying)

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