Universal Serial bus

of these articles describes universal the Serial bus; for other meanings of USB see USB (term clarifying).
The USB Logo

that universal Serial bus (USB) is a bus system for the connection of a computer with accessory equipment. A USB connection occupieslittle place and can supply simple devices such as mice, telephones or keyboards with river. With USB equipped devices can be interconnected during operation (Hot Plugging), attached devices and their characteristics can be recognized automatically. Modern computers haveusually two to six USB interfaces. Too little if USB connections are available, one can operate over inexpensive of stroke several - up to 127 - USB devices at an interface.

USB is suitable for many devices such as printer, scanner, Webcams, mouse, keyboard, in addition,newer like USB stick or Dongles. Since the introduction of the USB 2.0-Spezifikation relatively high data rates are possible, thus the USB is suitable for the connection of further device types such as non removable disks, TV-interfaces and photo cameras. USB replaces increasingly older serial and parallelConnections, in addition, PCI - bus systems, PCMCIA - Slots or Firewire.

Table of contents

History and development

USB2.0 PCI Card

the idea of a universal bus for peripheral devices decrease/go back among other things to a development of Apple : Apple Desktop bus (ADB) the Apple Macintosh became - model row already in in the computersyears imported and a data rate of 10.000 bits /s had the 1980er. Already the ADB pursued the goal, the cable tangle up of reducing by and under the desk and connected in addition for example simply the mouse with the keyboard and thiswith the computer; separate data lines were saved in such a way.

Intel developed the today's universal serial bus (USB 1,0 ) in the year 1996. The bus was designed for the connection of devices to the PC. It should the follow-up for onewhole row from PC interfaces used at that time begin and these standardize. Therefore the USB specification was limited not only on keyboard and mouse, but included also different peripheral devices such as printers and scanners also. The first standard, USB 1,0, saw a bit rateof maximally 12 MBit/s (1.5 MByte/s) forwards. Mass storages - as for instance non removable disks - were seen at the beginning not as a substantial application of USB, were not in principle however supported.

As the first chip-corrode supported 1997 the Intel products 430HX and 430VX the USB. Of itapart the acceptance of the technology was at the beginning something hesitating, most devices from this time was appropriate still for older interfaces. Substantial factor for the market acceptance of USB was 1998 the introduction iMac from Apple. With this computerreplaced Apple the internal ADB by the USB - thus formed within a short time an installed basis, which was dependent on the USB.

Starting from at the end of of 1998 the revised specification USB 1,1, those followed primarily errors andAnd added the interrupt Out transfer repaired ambiguity in the 1,0 specification. The speed remained the same however. USB 1.x represented therefore no competition to Apple FireWire - to standard (IEEE 1394), that from the outset a data rate of 100MBit/s offered and rapidly on 400, 800 and finally 1,600 MBit/s (IEEE 1394b, starting from April 2002) was accelerated.

In the year 2000 the specification for USB 2,0, those above all the data rate followed on up to 480MBit/s (60 MByte/s) extended and so the meaningful connection of non removable disks or video devices made possible. Products for it appeared however only starting from 2002 at the market.


USB is a serial bus, the individual bits of the packet becomes thustransfer successively. Data communication been made differentially by two twisted lines, one transmits the data signal invariably, the other one the inverted signal. The signal receiver forms the differential voltage of both signals; the voltage increase between 1 - and 0-Pegeln is twice as large thereby. This increasesthe transmission reliability, suppresses Gleichtaktstörungen and improves besides the electromagnetic compatibility.

Two further lines serve if necessarily for the current supply of the attached devices. By use of four veins in cable can this more thinly and more inexpensive than with parallel interfacesare implemented. A high data rate can be attained at relatively small expenditure, since several signals with identical electrical and temporal behavior will not transfer must.

The bus specification plans a central host CONTROLLER, which takes over the co-ordination of the attached devices. To itcan up to 127 different devices be attached. To a USB haven in each case individual USB equipment can be attached. If to a host several devices are to be attached, therefore a distributor (stroke) for the coupling of these devices must ensure. From the employment of stroke result tree structures, which end all in the host CONTROLLER.

The USB replaces the older PC interfaces RS232 (serial), Gameport, the Centronics interface as well as the PS/2-Schnittstelle for keyboard and mouse. Compared with these USB offersclearly higher data transmission rates. USB transmits its data however in packages, for some time-critical applications is therefore less suitable it - for instance with only packages occupied with few bytes, which lower the data transmission rate, or if collecting from bytes toFill a package the transmission would retard.

Despite its name - universal Serial bus - the USB is not a physical data bus. With such several devices are attached parallel to a line. The designation „bus “refers to thoselogical cross-linking, the actual electrical execution takes place only with point-to-point connections.

USB plugs and - cable

USB plug

USB Standardstecker type A and B. Not true to scale, with pin numbers, plan view
USB Ministecker type A and B. Not true to scale, with pin numbers,Plan view
Verschiedene USB-Stecker. Von links nach rechts: Typ A, Typ B, Typ Mini-B 5-polig (Standard), Typ Mini-B 4-polig (Mitsumi), Typ Mini-B 4-polig (Aiptek) - ohne Anspruch auf Vollständigkeit, Draufsicht
different USB plugs. From left to right: Type A, type B, type mini b 5-pin (standard), type mini b 4-polig (Mitsumi), type mini b 4-polig (Aiptek) - without requirement on completeness, plan view

the plugs of a USB cable is polarity - and permutation arranges. InDirection of the host CONTROLLER (Upstream) are used flat plug (type A). To the attached equipment (Downstream) the cables are fixed installed either or attached over approximately square plugs (type B). For devices alsosmaller available space (e.g. digital cameras) exist also more compact plug variants, the mini USB plugs. Those are there depending upon equipment manufacturer different designs with differently many connection pins (up to 8) - see picture, it gives however still further designs. Embodied in the USB standard is onlythe five-pin, removes roof-similar plugs (illustrated in the photo in the center).

USB cables

in a USB cable are needed four veins. Two veins transmit thereby the data. The other two veins supply the attached equipment with a tensionof 5 V. The USB specification devices with an achievement of up to 2,5 W may refer appropriate devices can thus via the bus be along-supplied up to 500 mA from the USB.

The cables must depending upon speed differently become shielded. Cables, which correspond only to the specification low speed, may be no B-plug have, but must fixed at the equipment installed or use a manufacturer-specific plug. The reason for it is that the small screen of the cable to problemsDevices with higher speeds to lead can.

The length of a cable from the stroke to the equipment is limited to five meters. Low speed cables are limited by the specification to 3 m, this are technically however unfounded and presumably become in a future versionescape the specification. The specification excludes extensions. One can overcome longer distances, by USB stroke are interposed. So-called active USB extension cords correspond to a bus power OD stroke (see below ) with only one Downstream haven and a firmly attached cable at the Upstream haven in their functions.Since the electrical effects of these cables in the USB bus correspond to those of a bus Powered of USB stroke with 5 m cables, additionally the restrictions should be considered with the nesting of USB stroke with their use.

An alternative to bridging lengths over 30 m are the USB LINE in such a way specified Extender. A USB LINE Extender consists of two components: A cousin module, which are attached to the computer, and a remote module for the connection of the USB equipment. For distance bypass between these two components Ethernetkabel are used. The maximum lengththe Ethernetkabels is limited on 45 m.

Colour codes and Pinouts
pin name color description
1 VCC red +5V
2 D white DATA -
3 D+ green to DATA +
4 GND black Ground

the USB standard puts apart from the allocation of the interfacealso the names of the individual plug pins and the vein color firmly. The number of a plug pin can be read off in the schematic diagrams stated above.

speeds and data rates

USB it permits to equipment with 1,5 Mbit/s (Low speed) to transfer 12 Mbit/s (fill speed) or with 480 Mbit/s (High speed, starting from USB 2,0 available) data. These rates are based on the system clock of the respective USB speed and represent the physical bit rate. The actually usableData rate is appropriate - for z. B. by Datenoverhead - under it; with current systems in the order of magnitude 320 Mbit/s.

If the interface of equipment is indicated as „USB 2,0 “, that does not mean necessarily that this equipment offers also the High speed data rate of 480 MBit/s.Point of view of the offerers participates that a USB-2.0-kompatibles equipment can use in principle each of the three speeds and the 2.0-Kompatibilität primarily meant that the newest version of the specification is kept. 480 MBit/s may be thus only expected, ifEquipment with the Logo „USB 2,0 rear speed “is excellent.

Communication with USB is steered by the host CONTROLLER, which usually sits in the PC. Only this can read or to equipment send data of equipment. [[Ein]]Equipment may send data to the host CONTROLLER only if it is queried by this. With time-critical data streams - for instance with mouse movements, which ruckeln are not - the host CONTROLLER must itself therefore, in order to avoid stagnations, frequently enough with the equipment inquires,whether it wants to send data.

Direct communication between USB devices is actually not possible in accordance with the USB standard; this was only made possible by the extension USB on-the-go. The FireWire was created - standard, that for similar targeted applications as USB and with thisin competition, offers contrary to USB the possibility of a Peer ton Peer stands - for communication between devices, without the control is necessary by a host. Thus for instance the structure of a network is possible over FireWire.

USB stroke

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4 Port USB-Hub mit eigener Stromversorgung (self powered)
4 haven USB stroke with own current supply (self powered)

a USB stroke is a special kind of USB equipment, which passes the USB signal on on additional of haven for the connection of several further devices. Are commercial USB stroke with up to sevenDownstream haven.

Stroke can refer their river from the bus (bus Powered), or over an own current supply order (Self Powered). Most Self Powered of stroke is supplied via a plug power pack with river. Some monitors inserted also a USB stroke,over the current supply of the monitor to be along-fed. Self Powered of stroke have the advantage that each equipment up to 500 mA, attached at them, can refer river. With bus Powered of stroke the stroke and all devices attached to it may together maximally500 mA refer. Hybrid Self and bus Powered of stroke are possible - the stroke is Self Powered if a power pack is attached to it and otherwise bus Powered. Bus Powered of stroke are partly designated actively also than „passively “and Self Powered of stroke than „“, thisis technically however wrong.

With the nesting of stroke the borders are specified by those maximally 127 possible USB devices and by signal running time - each stroke increases the running time, the nesting level is to maximally 5 (stroke) levels below the host CONTROLLER and/or.the root stroke limits. The maximum distance between two devices connected with USB is because of the limit of 5 m per USB cable with 30 m - 6 cables with 5 m each length and between them 5 of stroke.

USB2.0 and stroke

Low, filling and High speed devices can be mixed at a USB-2.0-Host nearly at will, without speed disadvantages develop. Stroke after the USB-1.x-Standard can be used at USB-2.0-Hosts. Devices, which are attached directly or indirectly to such a stroke, can however only the speed fill speed reach, thus 12 MBit/s.

A USB-2.0-Host and a USB-2.0-Hub always communicate with High speed, even if at the stroke Low or filling speed devices are attached. It is task of the stroke, the data of these devices inHigh speed minutes to pack, the stroke inserted in addition one or more so-called „Transaction of translator “. Determined the number of Transaction of translator, how many slow devices can be attached to a USB-2.0-Hub, without out-braking this. Become to a host more slowDevices attached as Transaction of translator available are, then the data rate of all devices on speeds, attached to this host, breaks in clearly under those of a USB-1.1-Hosts.

the different host CONTROLLERs

the USB CONTROLLER chips in the PCs adhere to onefrom three standards established. These differ in each case in their efficiency and the implementation from certain functionalities - either settle them the chip automatically, „in hardware “, or the device driver must provide in terms of software for it. For USB equipment is the usedCONTROLLER (nearly) completely transparency, however is every now and then important to PC it for the user, determines to be able, which kind uses chip of the computers, in order to be able to select the correct driver.

  • Host CONTROLLER interface ( UHCI) became universal in November 1995 of Intel specifies. The current version of the document carries the revision number 1,1, therefore also all present USB devices call themselves USB-1.1-kompatibel. UHCI chips offer support for USB devices with 1,5 or 12 MBit/s range. They become exclusively of the manufacturers Intel and VIA Technologies blocks.
  • Open host CONTROLLER interface (OHCI) is a specification, which by Compaq , Microsoft and national Semiconductor were developed together. Version 1.0 of the standard was published in December 1995, the current version carries the version number 1.0aand comes from September 1999. A OHCI CONTROLLER has in principle the same abilities as its UHCI counterparts, settled however more tasks in hardware and is marginally faster thereby than a UHCI CONTROLLER. This difference lies mostly within ranges, the straight stillare measurable, therefore one can neglect this in the use; Equipment developers must consider this however. With the USB controllers on motherboards, those of Intel or VIA do not come concern it with large probability OHCI.
  • Enhanced host CONTROLLER interface (EHCI)is the designation of the chip, which is used with USB 2,0. The EHCI completes thereby only the transmissions in the rear speed mode (480 MBit/s). If one puts USB-1.1-Geräte to a haven with EHCI chip, the EHCI CONTROLLER hands the data traffic at one behindit lying UHCI or OHCI CONTROLLERs further (these CONTROLLERs are typically on the same chip). If no EHCI driver is available, the High speed devices is through-handed likewise to the USB 1.1-Controller and works then with slower speed, if this at all possibleis.

the USB CONTROLLER addresses configurations, interfaces

and Alternate Settings internal the attached devices with seven bits is enough for identification, whereby the 127 maximally connectable devices result. The address 0 is occupied automatically by devices,RESET signal receive. If to or several haven new devices are detected, then the host CONTROLLER switches one on this haven, sends to the equipment attached there a RESET and tries the equipment to then identify. Afterwards it divides the equipment oneclear address also. Since in each case a haven with equipment not configured yet is activated, it comes to no address collisions. The host CONTROLLER asks usually first for a DEVICE descriptor, which contains among other things the manufacturer and product ID. With further descriptors the equipment communicates , which alternative configurations it possesses, into which it from its device driver be switched can. With a Webcam these alternatives could consist about of whether the camera is switched on or whether only the microphone runs. For thatCONTROLLER is relevant with the fact that the different configurations bring also different requirements of electric current with itself. Without special release by the operating system equipment may not use any more than 100 mA river. Within a configuration the equipment can define different interfaces,those over one or more terminator points order in each case. Different need at reserved range is signaled over Alternate so mentioned Settings. An example of it is a Webcam, which can send pictures in two different dissolutions. The Alternate Setting 0 is activated,if equipment data transferred would not like and thus pausiert.

to require, the USB standard does not define device classes over for each equipment own drivers different device classes, which can be steered by generic drivers. In this way are USB keyboards, mice, USB mass storages and other devices with their fundamental functions immediately ready to be launched, without only a driver would have to be installed by CD. Manufacturer-specific extensions (require then the own driver) are possible. The information, to which device classes itself equipmentcounts, can in the DEVICE descriptor (if the equipment only one class belonged) or in an interface descriptor (with devices, which belong to several classes) to be accommodated.


move modes, the this offers move modes of the USB to the attached devices for everyoneindividual terminator point to specify know.

terminator points

of USB devices have Unteradressen of the equipment a number of durchnummerierten „terminator points “, to a certain extent. The terminator points are present on the hardware side in the devices and become of the USB THEM (Serial interface engine) serves. Over these terminator points from each other independent data streams can run. Devices with several separate functions (e.g.Webcams, which transfer video and audio) have several terminator points. The transmissions of and to the terminator points take place usually unidirectionally, for bi-directional transmissions are therefore a IN - and a OUT - terminator point necessarily (IN and OUT refer in each case to the view of the host CONTROLLER). An exception of it are terminator points, which use the so-called control transfer mode.

In each USB equipment a terminator point with address must 0 available its, over which the recognition and configuration of the equipment run, beyond that he can take over also still further functions. Terminator point 0 always uses the control transfer mode.

USB equipment may have maximally 31 terminator points: the control terminator point (thatactually two terminator points summarizes) and ever 15 in and 15 Out terminator points. Low speed devices are limited to terminator point 0 plus maximally two further terminator points in the interrupt transfer mode with maximally 8 bytes per transfer.

isochronous transfer

the isochronous transferis suitable for data, which need a guaranteed range. This kind of transfer is available for filling speed and High speed devices. If the Alternate Setting defines a terminator point with isochronous transfer, then the host CONTROLLER driver reserves the necessary range. This range is not available,thus the activation of the Alternate Settings fails, and no isochronous communication can be developed with this equipment.

The necessary range results from the product of the inquiry interval and the FIFO - size. Filling speed devices know each ms up to 1023Bytes per isochronous terminator point transferred (1023 kByte/s), High speed devices can implement Micro Frame (125 µs) with up to 1024 bytes up to 3 transmissions for each (24 MByte/s). If several isochronous terminator points are available in equipment, the data rate increases accordingly. ThoseTransmission is secured with a check number (CRC16), however with a transfer error by the hardware is not repeated. The receiver can recognize whether the data became to transfer correctly. Isochronous transmissions e.g. become. of the USB audio Class uses with externalUSB Soundkarten use finds.


interrupt transfer interrupt transfer for the transmission of small data sets, which are available at not exactly assignable times. In the final POINT Descriptor communicates the equipment, in which maximum time intervals it would like to be asked for new data.The smallest possible inquiry interval amounts to with Low speed 10 ms, with filling speed 1 ms and with High speed up to 3 inquiries in 125 µs. With Low speed know per inquiry up to 8 byte, with filling speed up to 64 byte and with High speedup to 1024 byte to be transferred. From it maximum data rates of 800 Byte/s result in the case of Low speed, 64 kByte/s in the case of filling speed and up to 24 MByte/s in the case of High speed. The data are secured with a check number (CRC16) and becomeTransfer errors up to 3 time by the hardware repeats. Devices of the HID class (human interface DEVICE, for example keyboards, mice and Joysticks, the data transfer over the interrupt transfer.

Bulk transfer

Bulk transfer are for largeData sets meant, which are however not time-critical. These transfers are priorisiert and by the CONTROLLER are accomplished low, if all isochronous and interrupt transfer and are remaining still range are final. Bulk transfers are secured by a check number (CRC16) and becomeby the hardware up to three times repeats. Low speed devices cannot use this kind of transfer. Filling speed devices use FIFO sizes of 8, 16, 32 or 64 bytes. High speed devices always use 512 byte large FIFO.

control transfer

control transfer are a specialKind of Datentransfers, which require a terminator point, which can accomplish both in and Out operations. Control transfers are generally confirmed in both directions, so that transmitters and receivers can be always safe that the data also arrived. Therefore becomesthe terminator point 0 in the control transfer mode uses. Control transfers are for example after detecting the USB equipment and important for the change of first communication elementarily.

support in the operating systems

  • Mac OS supports USB 1,1 starting from Mac OS 8.1. Inthe time the extent of the devices, which are supported with class drivers, was extended clearly, since Mac OS 8,5 become most usual device classes supported. Mac OS X in all versions supports USB 1,1 and starting from 10.2.8 also USB 2.0.
  • Windows 95 has a rudimentary support of USB 1,0, which is considered however as error-prone.
  • Windows 98 supports USB 1,0, starting from the version Windows 98 SE also USB 1.1.
  • Windows NT does not have any USB support, from third manufacturers is however system expansions for it available.Equipment manufacturers test their products rarely with such extensions, therefore these system expansions apply only to special cases as suited.
  • Windows 2000 (SP3) and XP (with Patch) support USB 1,1 and USB 2.0. The USB Hostcontroller is however sometimes incorrectly recognized, most manufacturersguess/advise to install the drivers of the chip sentence manufacturer.
  • Linux supports 2.2, 2,4 and 2,6 USB CONTROLLERs in the Kernel versions. Into the Kernelversionen 2,4 and 2,6 is drivers for UHCI, OHCI and EHCI CONTROLLERs integrated, as well as support for usual USB terminals.
  • Amiga OS3.x supportsas standard equipment no USB. Only with software and hardware of third offerers a binding of USB-1.1-Geräten is possible. Starting from Amiga OS4, depending upon hardware, USB 1,1 and 2,0 is supported.
  • NetBSD, FreeBSD and OpenBSD support UHCI, OHCI and EHCIas well as usual terminals. NetBSD was 1998 the first free operating system with USB support.
  • Newer operating system versions with Unix - basis use ever more frequently libusb than basis library.
  • Where the operating system does not have the necessary support for USB devices, the Bios can after activating „USBLegacy support “(English. create support for hereditary loads “) in its attitudes remedy for „USB, since thereby USB input devices appear to the operating system opposite as PS/2-Geräte.

USB on-the-go

by USB on-the-go (OTG) can communicate according to equipped devices directly with one another. Thus can without a computer, which takes over the host function, to be done. With devices with USB OTG one of the two terminals takes over a reduced host functionality, the devices can communicate therefore with one another. Possible operational areas are for example the connection of digital camera and printer or thatExchange of music files between two MP3-Playern.

USB OTG products are marked by the USB Logo by additional green arrow on the lower surface and white „on-The-Go “- signature. The USB OTG specification became to 18. December 2001 discharged, in some cases still comes it to Inkompatibilitäten betweenDevices of different manufacturers.


momentary occupy two initiatives the term „WirelessUSB “. The older of the two initiated by the company Cypress, meanwhile is jumped up Atmel than second chip manufacturer on the course. The Cypress WirelessUSB is actually nonewireless USB to build but a technology around wireless terminals which are connected with the computer then by a receiver/a transmitter (Transceiver), attached at the USB. In addition a transmission technique in the license-free 2,4-GHz-Band is used, the data rate amounts to up to 62,5 kbit/s andis limited thereby for input devices completely sufficiently, for other applications however often too scarcely.

The second WirelessUSB project is substantially more fastidious, the pertinent consortium by Intel is stated. A goal is it to create a technology with that the full 480MBit/s of the High speed move mode to be wirelessly transferred can. A short range under 10 m is intended; the transmission is to be based on a Ultra Wideband technology. First demonstrations of this technology are not to be expected however before the second half of 2005.

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See also

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