:''For other meanings of the abbreviation USB see USB (disambiguation).'' Universal Serial Bus (USB) provides a serial port computer 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 and PDAs. == Overview == A USB system has an asymmetric design, consisting of a Server 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 SCSI and some others. The design of USB aimed to remove the need for adding separate expansion cards into the computer's Industry Standard Architecture or Peripheral Component Interconnect 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 enumeration it and loads the device driver necessary to run it. USB can connect computer peripheral such as computer mouse, computer keyboards, gamepads and joysticks, image scanners, digital cameras, computer printers, hard disks, and Computer network components. For multimedia devices such as image 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 computer display, and high-quality digital video components. ===Standardization=== The design of USB is standardized by the USB Implementers Forum (USBIF), 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. This version was standardized by the USBIF 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 == 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 pipeline (software) 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 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. power management 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 Memory-mapped I/O (port (computing)) 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 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 Peripheral Component Interconnect devices, and a controllers will be named OHCI, UHCI or EHCI respectively. ===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, computer keyboard, computer mouse, 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 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, computer 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==== {| |+ Standard USB connector pinout ! Pin !! Function (host) !! Function (device) |- | 1 || VBUS (4.75–5.25 V) || VBUS (4.4–5.25 V) |- | 2 || D− || D− |- | 3 || D+ || D+ |- | 4 || Ground (electricity) || Ground (electricity) |} 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. Contrary to popular belief, D+ and D− operate together; they are not separate simplex connections. USB supports three data rates. A Low Speed rate of 1.5 Mbit (192 KBytes) per second that is mostly used for Human Input Devices (HID) such as keyboards, mice and joysticks. USB has a Full Speed rate of 12 Mbit (1.5 MBytes) per second. 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. USB 2.0 added a Hi-Speed rate of 480 Mbit (60 MBytes) per second. 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 USBIF certifies devices and provides special marketing logos for each speed. 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==== {| |+ Mini USB connector pinout |- ! Pin !! Function |- | 1 || VBUS (4.4–5.25 V) |- | 2 || D− |- | 3 || D+ |- | 4 || ID |- | 5 || Ground |} 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 impossible 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 notebook Liquid crystal displays. 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 Personal Digital Assistants 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. ===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. A given segment of the bus is specified to deliver up to 500 Ampere. 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 or higher than 5 volts 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 Ampere maximum) and state how much current they need, before switching, with the host's permission, into high-power mode. 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 Grill ''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 === [[Image:Cruzertitaniumusbkey.jpg|thumb|A keydrive, a typical USB mass storage device]] USB implements connections to storage device 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 Advanced Technology Attachment (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 USB enclosure, 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. You install a drive into the enclosure and then plug the enclosure into the computer; thus functioning as a regular USB mass-storage device. Firewire technology is also commonly used with portable hard drives and some include both types of ports. Firewire tends to perform better in speed Benchmark (computing) 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 accessibility from the front or rear of the computer case. Similarly support for joysticks, keypads, tablets and other human-interface devices is progressively migrating over from MIDI, "game", and PS/2 connectors. 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. If one has the choice of connecting a keyboard to either a USB or a PS/2 port, it is important to determine whether the BIOS supports the USB keyboard when the operating system cannot enter its normal state, because on some older computers an operating system driver is needed to use a USB keyboard. Thus on some computers a USB keyboard cannot be used for troubleshooting when the OS will not start. Apple computers have used USB mice and keyboards exclusively since January 1999. Compaq included USB as early as April 1996. ==Version history== ===USB=== *USB 1.0 FDR: Came out in November 1995, the same year that Apple Computer adopted the IEEE 1394 standard known as Firewire. *USB 1.0: Came out in January 1996. *USB 1.1: Came out in September 1998. *USB 2.0: Came out in April 2000. The major feature of this standard was the addition of high-speed mode. This is the current revision. ===USB On-The-Go Supplement=== *USB On-The-Go Supplement 1.0: Came out in December 2001. *USB On-The-Go Supplement 1.0a: Came out 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 provides more power to devices, and 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 (IEEE1394) * USB Flash Drive * USB streaming * U3 == External links == * [http://www.usb.org/ USB home, including documentation] * [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.barefeats.com/usb2.html USB 2.0 vs Firewire] - benchmark using external drives on Macs * [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)] Computer buses USB th:ยูเอสบี
== Use of SCSI Command Set by USB == Q: I think it would be useful to mention that USB uses the SCSI command set, despite adopting a different physical architecture. This is actually highlighted in the SCSI article. As the current article stands SCSI and USB seem like completely unrelated technologies. User:Nickg 08:50, 20 Aug 2004 (UTC) A: See USB Mass Storage Rationale of SCSI over USB. == Please specify USB version == "USB has a Full Speed rate of 12 Mbit per second." Is this version 1.0 or 1.1? :no one knows. - User:Omegatron 14:36, Jan 15, 2005 (UTC) *? whaddya mean nobody knows? VERSION NUMBERS MEAN NOTHING TO THE SPEED. DO NOT PAY ATTENTION TO VERSION NUMBERS. Full speed is 12Mb. Always, has been, since .9 at least. User:SchmuckyTheCat 19:25, 15 Jan 2005 (UTC) :* update to myself, Full Speed has ''always'' been in the spec, with bandwidth testing of multiple FS devices being included as reference at 0.9 User:SchmuckyTheCat 17:11, 8 Mar 2005 (UTC) == Can someone please explain usb cable types == I know there are at least two and maybe more types of cables for USB - Could someone differentiate them? Is one type of plug/cable for USB 1 and another for USB 2? Or does the cable/plug type not matter? I know they are compatible, does one cable/plug limit the bandwidth? I have tried to find this on the internet but my searches only turn up vendors trying to sell stuff - it is overwhelming. Could someone please research this and maybe even put pictures of the USB plug types up on the page? Oh, and I know that a USB 1 hub would have to be replaced to handle the bandwidth of USB 2, but I do not know if the cables make a difference. ---- ''The link to the USB.ORG site includes access to the specs for cables which include pictures (drawings). The A end hooks to the host and the B end hooks to the device. As noted above, in the extension called On-the-go, there is also a hermaphroditic socket which will accept either the A or the B end of the mini-version of the standard cables. There is no plug/socket change for High speed (480Mpbs) but the cable spec was tightened in the 2.0 version to allow for the higher transfer rates. A 1.1 Spec. hub will work on a 2.0 system but will limit the maximum speed of any down-stream devices to 12Mbps regardless of whether the downstream devices are High-speed capable or not. - richard'' == USB A-A cables == Can someone write in detail on the topic of USB A-A cables? From what I know, there are 2 types of these: *Plain A-A cablle *A-A with electronics I don't know what the first one is used for, it costs ~$5 (nominal cost - cable and 2 terminals). The latter can be used to connect 2 computers and includes some kind of device, so that files can be copied (~$50). I have some info that files cannot be copied from computer to computer on the plain ones because USB is an asynchronous protocol, but I don't understrand the technical details. Any info on this? Any software workarounds for file transfers using the cheap cables possible? User:Helix84 13:25, 3 Nov 2004 (UTC) There shouldn't be any topic of USB A-A cables. The ones that have electronics in the middle are just another device. The electronics just expose a bulk interface on each side, one to each host controller. These usually require some special software that sends files or whatever over the bulk interface to the same software running on the other machine. These aren't deserving of their own topic, they're just another device. Those that don't have electronics in the middle are invalid according to the spec. Since the first machine you plug one into is sending 5v and the next machine you plug it into EXPECTS to send 5v, you're just likely to blow the motherboard of one or both machines. Just bad, bad, bad. Of course you occasionally run into some no-name stupid device that uses an A-A as a device cable, avoid them like the plague. If they couldn't even get the cable spec right it's unlikely the device will work well AT ALL. -- user:SchmuckyTheCat 8 Dec 2004 : At some point I was going to add a section on those little USB PC-to-PC networky things (but I confess I've never used one, never seen one, and I'm really not sure what they're officially called). That section would have a one-liner explaining why an A-A cable wouldn't work. I suspect that in addition to the "you cooked my motherboard" phenomenon, there's the unavoidable fact that the low-layer protocol handlers (built into the relevant host controllers) aren't built this way (unlike those in USB-to-go) and anyway the resistors that balance the datapair will be doubled (which isn't good). - User:John Fader 03:31, 9 Dec 2004 (UTC) : we bought a bunch of one brand and reverse engineered our own driver for a few purposes: throughput, loopback, and bit verification testing. yeah, it might be worth a few sentences of "dispelling common misperception" in the discussion of cable types. - User:SchmuckyTheCat 22:42, 8 Dec 2004 == USB 2 HS vs. FireWire == I would like to comment on USB2.0 being in direct competition with IEEE 1394. Here are a list of features that makes then resolutely different: ===Application domain=== *USB addresses needs for a wide range of devices (mouse, keyboard, modem, hard disk drives, scanner, printer...) that do not exist in IEEE 1394. *IEEE 1394 addresses needs of audio-video devices such as videorecorder, digital camera that have no real equivalent on USB. ''USB's isochronous mode does apply to streaming devices including videorecorders and audio devices such as speakers, microphones, etc. The High-speed (480Mbps) use of isochronous is directly comparable to use of IEEE 1394/Firewire for these devices - richard'' ===Communication paradigm=== *USB provides host to peripheral communication. A host computer is required in the system. *IEEE 1394 provides non-centralized networking. You may build a network with simply a VCR and a TV tuner. Sylvwild ''Yes, the system requres a host - but the USB On-the-go extension allows for the possibility of a unit that is usually used as a device to become a host for the purposes of a point to point conversation as you describe. The initial setup is determined by which end of the cable is plugged in to which unit but is switchable under software control so that the initial device may assume host and vice versa. The USB OTG sockets are hermaphroditic - can accept either the A or B end of the cable. - richard'' == "Throughput" == "USB 2.0 boasts 480Mbps throughput" I don't think so. USB 2.0 does signaling on the wire at 480 megacycles per second. The physical layer transports up to 480 million bits in one second. But that doesn't mean it has 480 Mbps ''throughput''. In fact, it pretty much guarantees the throughput is less than that. Looking at [http://www.usb.org/developers/docs/ the spec], the fastest way to transfer data seems to be with a high-speed bulk transaction with a data payload of 512. This gets you (see p.55) 53248000 bytes/second of bandwidth, or just under 426 Mbps throughput. And that's assuming you can saturate the line with 100% high-speed bulk transactions; I think you have to have other transactions going on to request all that data, which is why even the theoretical throughput is lower than 426 Mbps. Am I missing something? I haven't seen the Firewire spec, but it probably has something similar going on. I doubt it has 400 Mbps of throughput. But it does seem to perform better than USB 2.0: this should be a good indicator that USB doesn't really "boast" more "throughput". == USB 1.0, 1.1, 2.0 == "Confusingly, the USB Forum has renamed USB 1.1 to USB 2.0 Full Speed; and USB 2.0 to USB 2.0 High Speed." Are those definitely absolutely the same? Please provide a reference. I find conflicting info online. - User:Omegatron 22:39, Nov 6, 2004 (UTC) subsequent edits deleted that info, it was wrong. == Maximum cable length == I think about adding a mention of maximum cable length, for people (as I was) concerned about the distance between two equipments. There's an answer on the USB.org [http://www.usb.org/faq/ans5/ USB Info: Frequently Asked Questions]. Any comment ? - --User:Olivier Debre 09:02, 18 Jan 2005 (UTC) :Certainly this should be included. Explain why cable length matters and explain the changes in recommendations by the USB.org. (Originally it was a length, but they changed it to a signal integrity spec instead.) - User:Omegatron 21:49, Jan 26, 2005 (UTC) == interrupts and synchronization == Which way are frames initiated? Does the host/hub send out a frame to each device every ms, or do the devices send out frames every ms? Since the clocks of the devices would vary slightly I would imagine the host does. How does the USB host interrupt the CPU? Where can I find more details about these, since they may be too technical for this article? - User:Omegatron 21:47, Jan 26, 2005 (UTC) *the USB host sits on an internal bus in the PC. HCD --> bus --> CPU I don't know of any implementation of USB that isn't PCI, but maybe Mac's or other non-x86 architectures use some other internal bus, I've never had to care. On Windows the USB stack transfers URB messages between driver components, URBs are equivalent to IRPs in the rest of the Windows kernel messagaging interface. User:SchmuckyTheCat 03:58, 31 Jan 2005 (UTC) :: :-) Well, at least I know of some acronyms to look up now... - User:Omegatron 04:06, Jan 31, 2005 (UTC) :: Host controller device (USB host), Interrupt return pointer, USB Request Block - User:Omegatron 04:11, Jan 31, 2005 (UTC) *also, i gave you an incomplete answer. the bus has 10% of the traffic reserved for control purposes - so devices DO keep in synch. it also uses control to make sure everybody gets a chance to talk, but who talks is dependent on the type of traffic they want to send (see the transfer types under technical details). transfer types have priority: control, interrupt, isoch, bulk. interrupt devices can interrupt but they don't send large amounts of data, so this shouldn't interfere. isoch devices reserve bandwidth in an OS dependent fashion (on windows it's first-come first-served and no hoarding) so they can talk as much as they want in their reserved pipe. bulk devices talk when the HCD tells them too and they get whatever bandwidth is left over. they have to scale back when something else is going on. simple enough. but then: see also how the virtual 1.1 HCDs in EHCI create virtual busses on the same wire, and how the TT virtualizes 1.1 traffic in a 2.0 hub. i think i'll do a re-write of portions of the article for this, the article is currently too technical for simple concepts to come out of it. :I don't really understand all of that. One thing I was asking was, since the host and devices each have their own clocks, which clock is used to synchronize transfers between them? I assume the host clock, and the devices only send data when they are spoken to? - User:Omegatron 18:42, Jun 6, 2005 (UTC) == Types of USB connectors == Would anybody who knows this topic well care to add pictures and information about the various types of USB connectors, such as Type A, Type B, and mini USB? — User:Brim 08:28, Feb 5, 2005 (UTC) == Which information should be hidden or removed? == I simply wanted to add information that some people might want to know: USB 1.1 introduced Full Speed, or 12.5 Mbit/s transfer speed. In my first edit, the wording was poor but not factually incorrect when I said 1.1 had Full Speed. Then an unnamed editor reverted it, stating that version 2.0 also has support for Full Speed, not just 1.1. Not a valid argument at all for a revert, but a true statement nonetheless, I thought, so I simply added more information to this for clarity and said 1.1 introduced Full Speed. This was ''yet again'' reverted by the same editor. It seems like this person has an ''agenda of removing information'' from the main article. Strange, since one would think an Encyclopedia would be a place where you ''add'', as opposed to ''remove'', information (especially the ones people would likely to be interested in). As a consumer, I would like to know as a basic fact what speeds are associated with the version number of a USB port. User: SchmuckyTheCat thinks otherwise, and believes that this information must be hidden from plain view. At his/her request, I have created a section where s/he could respond, hopefully without the caps-lock on. --User:69.214.224.74 20:19, 8 Mar 2005 (UTC) : First off, please don't make a technical discussion into a personal conflict by making comments about other users and what you imagine their motives to be. Let's just stick to the article, okay. Now, it's clear that there's disagreement both about what speeds were introduced when, and how we should present this. I think we should have a "feature table" showing the different versions of USB, their features, and significant changes. Now, as to which version did what: sources ''need'' to be cited, particularly so now that there is controversy. A source in this case is ideally the version, page, and line number (and ideally a quote) from the official USB spec. When this (surely an unimpeachable) source is available, bickering without citing exact references is a waste of everyone's time. -- 20:50, 8 Mar 2005 (UTC) ::This info was wrong in the past and someone fixed it. See above. Please don't change it unless you know for certain you are right. Cite your sources to prove it. - User:Omegatron 22:38, Mar 8, 2005 (UTC) :*Full speed has always been part of the spec, probably since the version number was just an idea in a few engineers head. It was never introduced, it has always been. I'm actually interested to see why you think this is true. Is there another website or product literature saying this? I understand the user confusion, but the version number of a port doesn't correspond to speed. Like all technical interfaces, the new interface is "faster, better, cheaper" than the last but backwards compatible. You never, for instance, talk about RAM as being DDR v2.0. It's DDR266 or PC2100, and of course the faster stuff slows down in a slower slot. Unfortunately, some USB device marketers started advertising the version number as a speed indicator, which is enitrely misleading. A USB 2.0 device might only do Full Speed (12mb). What sells more, a 12mb device that says "12mb" or a 12mb device that says "USB 2.0"? A device might even do 480Mb, but the manufacturer fails to get it certified (maybe it fails other parts of the spec). They can't put the Hi-Speed logo on the box, but they can say 2.0. "Hi-Speed" is trademarked. "2.0" is not. A smart consumer should look for certification logos and declared speeds, not version numbers. User:SchmuckyTheCat 15:16, 9 Mar 2005 (UTC) :* Also, see previous discussions on this on this talk page under "Please specify version number" and "USB 1.0, 1.1, 2.0" User:SchmuckyTheCat 18:07, 9 Mar 2005 (UTC) What is the difference between USB 1.0 and 1.1? If they supported the same device speeds, someone had better go tell list of device bandwidths, which may or may not be confusing Low-Speed/Full-Speed with 1.0/1.1. (It lists 1.1 as being faster than 1.0; can someone who knows the difference go look?) What benefits did moving to 1.1 offer for devices that made up for breaking compatibility with 1.0 host controllers? User:Grendelkhan|User_talk:Grendelkhan 19:05, 2005 Mar 9 (UTC) * krikey, what a mess that list is. I will go correct. If someone writes a USB history section the difference between 1.0 and 1.1 would be: pretty much nothing. User:SchmuckyTheCat 19:44, 9 Mar 2005 (UTC) == impedance == cable has a characteristic impedance of 90 ohms. each driver should be 45 ohms ± 10%. for high-speed, each receiver shuold also be 45 ohms, standard nonreflective impedance matching. not sure if the 45 ohms are ''required'' for low/full speed devices though. - User:Omegatron 20:22, Jun 6, 2005 (UTC) :Actually it's more complicated than that: "When the full-speed driver is not part of a high-speed capable transceiver, the impedance of each of the drivers (ZDRV) must be between 28 Ω and 44 Ω, i.e., within the gray area in Figure 7-4. When the full-speed driver is part of a high-speed capable transceiver, the impedance of each of the drivers (ZHSDRV) must be between 40.5 Ω and 49.5 Ω, i.e., within the gray area in Figure 7-5." - User:Omegatron 20:36, Jun 6, 2005 (UTC)
#REDIRECT Universal Serial Bus
See other meanings of words starting from letter:
Words begining with Universal_Serial_Bus:
Universal_Serial_Bus
Universal_Serial_Bus
Universal_serial_bus
Universal Serial Bus
:''For other meanings of the abbreviation USB see USB (disambiguation).'' Universal Serial Bus (USB) provides a serial port computer 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 and PDAs. == Overview == A USB system has an asymmetric design, consisting of a Server 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 SCSI and some others. The design of USB aimed to remove the need for adding separate expansion cards into the computer's Industry Standard Architecture or Peripheral Component Interconnect 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 enumeration it and loads the device driver necessary to run it. USB can connect computer peripheral such as computer mouse, computer keyboards, gamepads and joysticks, image scanners, digital cameras, computer printers, hard disks, and Computer network components. For multimedia devices such as image 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 computer display, and high-quality digital video components. ===Standardization=== The design of USB is standardized by the USB Implementers Forum (USBIF), 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. This version was standardized by the USBIF 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 == 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 pipeline (software) 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 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. power management 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 Memory-mapped I/O (port (computing)) 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 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 Peripheral Component Interconnect devices, and a controllers will be named OHCI, UHCI or EHCI respectively. ===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, computer keyboard, computer mouse, 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 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, computer 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==== {| |+ Standard USB connector pinout ! Pin !! Function (host) !! Function (device) |- | 1 || VBUS (4.75–5.25 V) || VBUS (4.4–5.25 V) |- | 2 || D− || D− |- | 3 || D+ || D+ |- | 4 || Ground (electricity) || Ground (electricity) |} 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. Contrary to popular belief, D+ and D− operate together; they are not separate simplex connections. USB supports three data rates. A Low Speed rate of 1.5 Mbit (192 KBytes) per second that is mostly used for Human Input Devices (HID) such as keyboards, mice and joysticks. USB has a Full Speed rate of 12 Mbit (1.5 MBytes) per second. 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. USB 2.0 added a Hi-Speed rate of 480 Mbit (60 MBytes) per second. 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 USBIF certifies devices and provides special marketing logos for each speed. 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==== {| |+ Mini USB connector pinout |- ! Pin !! Function |- | 1 || VBUS (4.4–5.25 V) |- | 2 || D− |- | 3 || D+ |- | 4 || ID |- | 5 || Ground |} 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 impossible 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 notebook Liquid crystal displays. 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 Personal Digital Assistants 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. ===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. A given segment of the bus is specified to deliver up to 500 Ampere. 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 or higher than 5 volts 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 Ampere maximum) and state how much current they need, before switching, with the host's permission, into high-power mode. 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 Grill ''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 === [[Image:Cruzertitaniumusbkey.jpg|thumb|A keydrive, a typical USB mass storage device]] USB implements connections to storage device 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 Advanced Technology Attachment (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 USB enclosure, 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. You install a drive into the enclosure and then plug the enclosure into the computer; thus functioning as a regular USB mass-storage device. Firewire technology is also commonly used with portable hard drives and some include both types of ports. Firewire tends to perform better in speed Benchmark (computing) 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 accessibility from the front or rear of the computer case. Similarly support for joysticks, keypads, tablets and other human-interface devices is progressively migrating over from MIDI, "game", and PS/2 connectors. 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. If one has the choice of connecting a keyboard to either a USB or a PS/2 port, it is important to determine whether the BIOS supports the USB keyboard when the operating system cannot enter its normal state, because on some older computers an operating system driver is needed to use a USB keyboard. Thus on some computers a USB keyboard cannot be used for troubleshooting when the OS will not start. Apple computers have used USB mice and keyboards exclusively since January 1999. Compaq included USB as early as April 1996. ==Version history== ===USB=== *USB 1.0 FDR: Came out in November 1995, the same year that Apple Computer adopted the IEEE 1394 standard known as Firewire. *USB 1.0: Came out in January 1996. *USB 1.1: Came out in September 1998. *USB 2.0: Came out in April 2000. The major feature of this standard was the addition of high-speed mode. This is the current revision. ===USB On-The-Go Supplement=== *USB On-The-Go Supplement 1.0: Came out in December 2001. *USB On-The-Go Supplement 1.0a: Came out 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 provides more power to devices, and 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 (IEEE1394) * USB Flash Drive * USB streaming * U3 == External links == * [http://www.usb.org/ USB home, including documentation] * [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.barefeats.com/usb2.html USB 2.0 vs Firewire] - benchmark using external drives on Macs * [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)] Computer buses USB th:ยูเอสบี
Universal Serial Bus
== Use of SCSI Command Set by USB == Q: I think it would be useful to mention that USB uses the SCSI command set, despite adopting a different physical architecture. This is actually highlighted in the SCSI article. As the current article stands SCSI and USB seem like completely unrelated technologies. User:Nickg 08:50, 20 Aug 2004 (UTC) A: See USB Mass Storage Rationale of SCSI over USB. == Please specify USB version == "USB has a Full Speed rate of 12 Mbit per second." Is this version 1.0 or 1.1? :no one knows. - User:Omegatron 14:36, Jan 15, 2005 (UTC) *? whaddya mean nobody knows? VERSION NUMBERS MEAN NOTHING TO THE SPEED. DO NOT PAY ATTENTION TO VERSION NUMBERS. Full speed is 12Mb. Always, has been, since .9 at least. User:SchmuckyTheCat 19:25, 15 Jan 2005 (UTC) :* update to myself, Full Speed has ''always'' been in the spec, with bandwidth testing of multiple FS devices being included as reference at 0.9 User:SchmuckyTheCat 17:11, 8 Mar 2005 (UTC) == Can someone please explain usb cable types == I know there are at least two and maybe more types of cables for USB - Could someone differentiate them? Is one type of plug/cable for USB 1 and another for USB 2? Or does the cable/plug type not matter? I know they are compatible, does one cable/plug limit the bandwidth? I have tried to find this on the internet but my searches only turn up vendors trying to sell stuff - it is overwhelming. Could someone please research this and maybe even put pictures of the USB plug types up on the page? Oh, and I know that a USB 1 hub would have to be replaced to handle the bandwidth of USB 2, but I do not know if the cables make a difference. ---- ''The link to the USB.ORG site includes access to the specs for cables which include pictures (drawings). The A end hooks to the host and the B end hooks to the device. As noted above, in the extension called On-the-go, there is also a hermaphroditic socket which will accept either the A or the B end of the mini-version of the standard cables. There is no plug/socket change for High speed (480Mpbs) but the cable spec was tightened in the 2.0 version to allow for the higher transfer rates. A 1.1 Spec. hub will work on a 2.0 system but will limit the maximum speed of any down-stream devices to 12Mbps regardless of whether the downstream devices are High-speed capable or not. - richard'' == USB A-A cables == Can someone write in detail on the topic of USB A-A cables? From what I know, there are 2 types of these: *Plain A-A cablle *A-A with electronics I don't know what the first one is used for, it costs ~$5 (nominal cost - cable and 2 terminals). The latter can be used to connect 2 computers and includes some kind of device, so that files can be copied (~$50). I have some info that files cannot be copied from computer to computer on the plain ones because USB is an asynchronous protocol, but I don't understrand the technical details. Any info on this? Any software workarounds for file transfers using the cheap cables possible? User:Helix84 13:25, 3 Nov 2004 (UTC) There shouldn't be any topic of USB A-A cables. The ones that have electronics in the middle are just another device. The electronics just expose a bulk interface on each side, one to each host controller. These usually require some special software that sends files or whatever over the bulk interface to the same software running on the other machine. These aren't deserving of their own topic, they're just another device. Those that don't have electronics in the middle are invalid according to the spec. Since the first machine you plug one into is sending 5v and the next machine you plug it into EXPECTS to send 5v, you're just likely to blow the motherboard of one or both machines. Just bad, bad, bad. Of course you occasionally run into some no-name stupid device that uses an A-A as a device cable, avoid them like the plague. If they couldn't even get the cable spec right it's unlikely the device will work well AT ALL. -- user:SchmuckyTheCat 8 Dec 2004 : At some point I was going to add a section on those little USB PC-to-PC networky things (but I confess I've never used one, never seen one, and I'm really not sure what they're officially called). That section would have a one-liner explaining why an A-A cable wouldn't work. I suspect that in addition to the "you cooked my motherboard" phenomenon, there's the unavoidable fact that the low-layer protocol handlers (built into the relevant host controllers) aren't built this way (unlike those in USB-to-go) and anyway the resistors that balance the datapair will be doubled (which isn't good). - User:John Fader 03:31, 9 Dec 2004 (UTC) : we bought a bunch of one brand and reverse engineered our own driver for a few purposes: throughput, loopback, and bit verification testing. yeah, it might be worth a few sentences of "dispelling common misperception" in the discussion of cable types. - User:SchmuckyTheCat 22:42, 8 Dec 2004 == USB 2 HS vs. FireWire == I would like to comment on USB2.0 being in direct competition with IEEE 1394. Here are a list of features that makes then resolutely different: ===Application domain=== *USB addresses needs for a wide range of devices (mouse, keyboard, modem, hard disk drives, scanner, printer...) that do not exist in IEEE 1394. *IEEE 1394 addresses needs of audio-video devices such as videorecorder, digital camera that have no real equivalent on USB. ''USB's isochronous mode does apply to streaming devices including videorecorders and audio devices such as speakers, microphones, etc. The High-speed (480Mbps) use of isochronous is directly comparable to use of IEEE 1394/Firewire for these devices - richard'' ===Communication paradigm=== *USB provides host to peripheral communication. A host computer is required in the system. *IEEE 1394 provides non-centralized networking. You may build a network with simply a VCR and a TV tuner. Sylvwild ''Yes, the system requres a host - but the USB On-the-go extension allows for the possibility of a unit that is usually used as a device to become a host for the purposes of a point to point conversation as you describe. The initial setup is determined by which end of the cable is plugged in to which unit but is switchable under software control so that the initial device may assume host and vice versa. The USB OTG sockets are hermaphroditic - can accept either the A or B end of the cable. - richard'' == "Throughput" == "USB 2.0 boasts 480Mbps throughput" I don't think so. USB 2.0 does signaling on the wire at 480 megacycles per second. The physical layer transports up to 480 million bits in one second. But that doesn't mean it has 480 Mbps ''throughput''. In fact, it pretty much guarantees the throughput is less than that. Looking at [http://www.usb.org/developers/docs/ the spec], the fastest way to transfer data seems to be with a high-speed bulk transaction with a data payload of 512. This gets you (see p.55) 53248000 bytes/second of bandwidth, or just under 426 Mbps throughput. And that's assuming you can saturate the line with 100% high-speed bulk transactions; I think you have to have other transactions going on to request all that data, which is why even the theoretical throughput is lower than 426 Mbps. Am I missing something? I haven't seen the Firewire spec, but it probably has something similar going on. I doubt it has 400 Mbps of throughput. But it does seem to perform better than USB 2.0: this should be a good indicator that USB doesn't really "boast" more "throughput". == USB 1.0, 1.1, 2.0 == "Confusingly, the USB Forum has renamed USB 1.1 to USB 2.0 Full Speed; and USB 2.0 to USB 2.0 High Speed." Are those definitely absolutely the same? Please provide a reference. I find conflicting info online. - User:Omegatron 22:39, Nov 6, 2004 (UTC) subsequent edits deleted that info, it was wrong. == Maximum cable length == I think about adding a mention of maximum cable length, for people (as I was) concerned about the distance between two equipments. There's an answer on the USB.org [http://www.usb.org/faq/ans5/ USB Info: Frequently Asked Questions]. Any comment ? - --User:Olivier Debre 09:02, 18 Jan 2005 (UTC) :Certainly this should be included. Explain why cable length matters and explain the changes in recommendations by the USB.org. (Originally it was a length, but they changed it to a signal integrity spec instead.) - User:Omegatron 21:49, Jan 26, 2005 (UTC) == interrupts and synchronization == Which way are frames initiated? Does the host/hub send out a frame to each device every ms, or do the devices send out frames every ms? Since the clocks of the devices would vary slightly I would imagine the host does. How does the USB host interrupt the CPU? Where can I find more details about these, since they may be too technical for this article? - User:Omegatron 21:47, Jan 26, 2005 (UTC) *the USB host sits on an internal bus in the PC. HCD --> bus --> CPU I don't know of any implementation of USB that isn't PCI, but maybe Mac's or other non-x86 architectures use some other internal bus, I've never had to care. On Windows the USB stack transfers URB messages between driver components, URBs are equivalent to IRPs in the rest of the Windows kernel messagaging interface. User:SchmuckyTheCat 03:58, 31 Jan 2005 (UTC) :: :-) Well, at least I know of some acronyms to look up now... - User:Omegatron 04:06, Jan 31, 2005 (UTC) :: Host controller device (USB host), Interrupt return pointer, USB Request Block - User:Omegatron 04:11, Jan 31, 2005 (UTC) *also, i gave you an incomplete answer. the bus has 10% of the traffic reserved for control purposes - so devices DO keep in synch. it also uses control to make sure everybody gets a chance to talk, but who talks is dependent on the type of traffic they want to send (see the transfer types under technical details). transfer types have priority: control, interrupt, isoch, bulk. interrupt devices can interrupt but they don't send large amounts of data, so this shouldn't interfere. isoch devices reserve bandwidth in an OS dependent fashion (on windows it's first-come first-served and no hoarding) so they can talk as much as they want in their reserved pipe. bulk devices talk when the HCD tells them too and they get whatever bandwidth is left over. they have to scale back when something else is going on. simple enough. but then: see also how the virtual 1.1 HCDs in EHCI create virtual busses on the same wire, and how the TT virtualizes 1.1 traffic in a 2.0 hub. i think i'll do a re-write of portions of the article for this, the article is currently too technical for simple concepts to come out of it. :I don't really understand all of that. One thing I was asking was, since the host and devices each have their own clocks, which clock is used to synchronize transfers between them? I assume the host clock, and the devices only send data when they are spoken to? - User:Omegatron 18:42, Jun 6, 2005 (UTC) == Types of USB connectors == Would anybody who knows this topic well care to add pictures and information about the various types of USB connectors, such as Type A, Type B, and mini USB? — User:Brim 08:28, Feb 5, 2005 (UTC) == Which information should be hidden or removed? == I simply wanted to add information that some people might want to know: USB 1.1 introduced Full Speed, or 12.5 Mbit/s transfer speed. In my first edit, the wording was poor but not factually incorrect when I said 1.1 had Full Speed. Then an unnamed editor reverted it, stating that version 2.0 also has support for Full Speed, not just 1.1. Not a valid argument at all for a revert, but a true statement nonetheless, I thought, so I simply added more information to this for clarity and said 1.1 introduced Full Speed. This was ''yet again'' reverted by the same editor. It seems like this person has an ''agenda of removing information'' from the main article. Strange, since one would think an Encyclopedia would be a place where you ''add'', as opposed to ''remove'', information (especially the ones people would likely to be interested in). As a consumer, I would like to know as a basic fact what speeds are associated with the version number of a USB port. User: SchmuckyTheCat thinks otherwise, and believes that this information must be hidden from plain view. At his/her request, I have created a section where s/he could respond, hopefully without the caps-lock on. --User:69.214.224.74 20:19, 8 Mar 2005 (UTC) : First off, please don't make a technical discussion into a personal conflict by making comments about other users and what you imagine their motives to be. Let's just stick to the article, okay. Now, it's clear that there's disagreement both about what speeds were introduced when, and how we should present this. I think we should have a "feature table" showing the different versions of USB, their features, and significant changes. Now, as to which version did what: sources ''need'' to be cited, particularly so now that there is controversy. A source in this case is ideally the version, page, and line number (and ideally a quote) from the official USB spec. When this (surely an unimpeachable) source is available, bickering without citing exact references is a waste of everyone's time. -- 20:50, 8 Mar 2005 (UTC) ::This info was wrong in the past and someone fixed it. See above. Please don't change it unless you know for certain you are right. Cite your sources to prove it. - User:Omegatron 22:38, Mar 8, 2005 (UTC) :*Full speed has always been part of the spec, probably since the version number was just an idea in a few engineers head. It was never introduced, it has always been. I'm actually interested to see why you think this is true. Is there another website or product literature saying this? I understand the user confusion, but the version number of a port doesn't correspond to speed. Like all technical interfaces, the new interface is "faster, better, cheaper" than the last but backwards compatible. You never, for instance, talk about RAM as being DDR v2.0. It's DDR266 or PC2100, and of course the faster stuff slows down in a slower slot. Unfortunately, some USB device marketers started advertising the version number as a speed indicator, which is enitrely misleading. A USB 2.0 device might only do Full Speed (12mb). What sells more, a 12mb device that says "12mb" or a 12mb device that says "USB 2.0"? A device might even do 480Mb, but the manufacturer fails to get it certified (maybe it fails other parts of the spec). They can't put the Hi-Speed logo on the box, but they can say 2.0. "Hi-Speed" is trademarked. "2.0" is not. A smart consumer should look for certification logos and declared speeds, not version numbers. User:SchmuckyTheCat 15:16, 9 Mar 2005 (UTC) :* Also, see previous discussions on this on this talk page under "Please specify version number" and "USB 1.0, 1.1, 2.0" User:SchmuckyTheCat 18:07, 9 Mar 2005 (UTC) What is the difference between USB 1.0 and 1.1? If they supported the same device speeds, someone had better go tell list of device bandwidths, which may or may not be confusing Low-Speed/Full-Speed with 1.0/1.1. (It lists 1.1 as being faster than 1.0; can someone who knows the difference go look?) What benefits did moving to 1.1 offer for devices that made up for breaking compatibility with 1.0 host controllers? User:Grendelkhan|User_talk:Grendelkhan 19:05, 2005 Mar 9 (UTC) * krikey, what a mess that list is. I will go correct. If someone writes a USB history section the difference between 1.0 and 1.1 would be: pretty much nothing. User:SchmuckyTheCat 19:44, 9 Mar 2005 (UTC) == impedance == cable has a characteristic impedance of 90 ohms. each driver should be 45 ohms ± 10%. for high-speed, each receiver shuold also be 45 ohms, standard nonreflective impedance matching. not sure if the 45 ohms are ''required'' for low/full speed devices though. - User:Omegatron 20:22, Jun 6, 2005 (UTC) :Actually it's more complicated than that: "When the full-speed driver is not part of a high-speed capable transceiver, the impedance of each of the drivers (ZDRV) must be between 28 Ω and 44 Ω, i.e., within the gray area in Figure 7-4. When the full-speed driver is part of a high-speed capable transceiver, the impedance of each of the drivers (ZHSDRV) must be between 40.5 Ω and 49.5 Ω, i.e., within the gray area in Figure 7-5." - User:Omegatron 20:36, Jun 6, 2005 (UTC)
Universal serial bus
#REDIRECT Universal Serial Bus
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