什么是USBUSB是种由Intel和Microsoft开发外设总线数据通信

什么是USBUSB是一种由l开发的外设总线的数据通讯标准。最大的特色是支持热插拔（Hotplug）和即插即用(Plug&Play)。当设施插入时，主机列举（enumerate）此设施并加载所需的驱动程序，所以使用远比PCI和ISA改变了在台式PC反面安装过多连结器和端口的状况，并供给用于计算机电话设施的接口。USB对键盘、鼠标、打印机、调制解调器、游戏杆、音频设施、CD．ROM、数字相机和其余设施供给单调连结端口，而不是对它们分别进行连结。USB除去了与PC相关的好多问题，比如要打开计算机才能安装适配卡、改换双列直插开关、配置IRQ(中止恳求等。USB和总线拓扑构造拥有高达12Mbit／s的数据传输速率。它使用一条长达5米的电缆将设施以菊化链的形式串接到一个单调端口上。在这类设计中，能够在不需要从头启动系统的状况随时增添设施，最多可达63台。该拓扑构造是分层的况下，集线器可能需要电源。USB还对某些设施供给电源，所以它们将不需要电线和电池。USB甚至同意设施就它们的电源需求与USB主机控制器进行通讯。USB速度比并行总线（ParellelBs比如EPP、LPT）与串行端口（SerialPot比如RS-232）等传统计算机用标准总线快很多。USB有两个规范，即USB1.1和2.0。USB1.1是目前较为广泛的USB规范，其高速方式的传输速率为12Mbps，低速方式的传输速率为1.5Mbps（b是的意思），1MB/s（兆字节秒）=8Mbps（兆位秒），12Mbps=1.5MB/s。USB2.0USB1.1规范演变而来的。它的传输速率达到了480MbpsMB60MB/sUSB2.0中的“加强主机控制器接口”（EHCI）USB1.1相兼容USB2.0USB1.1设施。也就是说，全部USB1.1USB2.0USB线、插优等等附件也都能够直接使用。通用串行总线UniversalSerialBus(US)准，在计算机上使用宽泛，但也能够用在机顶盒和游戏机上，增补标准（On-The-Go)使其能够用于在便携设施之间直接互换数据。USB的设计为非对称式的，它由一个主机（控制器和若干经过hub施以树形连结的设施构成。一个控制器下最多能够有5级hub，包含Hub在内，最多能够连结127个设施，而一台计算机能够同时有多个控制器。 和SPI-SCSI等标准不一 USBhub不需要终结器样，USBhubUSB能够连结的外设有鼠标、键盘、gamepad和网络零件。对数码相机这样的多媒体外设USB已经是缺省接口；USB也逐渐代替并口成为打印机的主流连结方式。2004年已经有超出1亿台USB设施；到2005年显示器和高清楚度数字视频外设是仅有的USB需要更高的传输速率。现标准中将UBS一致为USB2.0,分为高速：传输速率 25Mbps～400Mbps(480Mbps)500Kbps～10Mbps(12Mbps低速：传输速率10Kbps～100Kbps(最大1.5Mbps）标准USBImplementersForum(USBIF) 负责USB标准制定，其成员包含苹果电脑、惠普、NEC、微软和lUSBIF于2001年末宣布了规范，从前还有0.9、1.0、和On-The-GoSupplementtotheUSB2.0Specificatio的目前版本是1.0a。USB的连结器分为、B连结器是和Mini-AB的插口。技术细节技术指标目前USB支持3种数据信号速率，USB确注明其使用的速率。USB-IF设施供给基本速率（低速和全速）和高速的特别商标同意。1.5Mbit/s低速速率，主要用于人机接口设施（HumanInterfaceDevice，HID）比如键盘、鼠标、游戏杆。12Mbit/s全速速率，在USB2.0从前是以前是最高速率，后起的更高速率的高速接口应当兼容全速速率。多个全速设施间能够依据先全部的USBHub支持全速速率。480Mbit/s的高速速率。并不是全部的USB2.0高速设施插入全速hub时应当与全速兼容。而高速hub拥有所谓Transactionr能，能够隔绝全速、低速设施与高速之间数据机械和电气标准标准USB接口USB标准、B插头及其触点未按比率绘制)标准USB连结器触点触点 功能主机) 功能(施)1 VBUS (4.7－5.25V)US (4.－5.25V)D- D-D+ D+接地 接地USB信号使用分别标志为D+和号并共同工作，以抵消长导线的电磁扰乱，MiniUSB接口MiniUSBA、B连结器及其触点未按比率绘制MiniUSB连结器触点触点 功能1 VBUS V)D-D+ID接地除了第4针，其余标准和miniUSB接口功能同样。第4针成为连结到第5针，在能够悬空亦可连结到第5针。软件架构一个USB主机经过hub担当多种功能，比如路由器同时也能够是一个SD卡读卡器，USB的术语中设施（devic）指得是功能（function）。集线器（hub）的看法其实不以为是functio。直接连结到主机的hub是根（roo）hub。端点设施功能（和集线器）与管道逻辑通道）制器和被称为端点的逻辑实体连结起来。管道和比特流（比如UNIX有着同样的含义，而在USB至在正式文档中。端点（和各自的管道）在每个方向上依据0-15编号，所以一个设施功能最多有32个活动管道，16个进，16个出。出（OUT）指走开控制器，而入（入主机控制器。两个方向的端点0老是留给总线管理，占用了32个端点中的2个。在管道中，数据使用不一样长度的包传达，端点能够传达的包长度上限一般是math>2^n</math>字节，所以USB包常常包含的数据量挨次有、16、32、64、128、256、512或许024字节。一个端点只好单向（进出）传输数据，自然管道也是单向的。每个USB起码有两个端点管道：它们分别是出入方向的，编号为0，用于控制总线上的设施。依据各自的传输种类，管道被分为4类：于总线控制的0号管道。等时传输——依据有保障的速度（可能但不用定是赶快地）传输，可能有数据丢掉，比照及时的音频、视频。中止传输——用于一定保证赶快反响的设施（有限延缓），比如鼠标、键盘。批量传输——使用余下的带宽大批地（可是没相关于延缓、连续性、带宽和速度的保证）传输数据，比如一般的文件传输。一旦设施（功能）经过总线的hub主机上独一的7位地点。主机控制器经过投票分派流量，一般是经过轮询模式，所以没有明确向主机控制器恳求从前设施不可以传输数据。W为了接见端点，一定获取一个分层的配置。连结到主机的设施有且仅有一个设施描绘符（devicedescriptr，而设施描绘符有若干配置描绘符绘符有若干界面描绘符，用于描绘设施的必定方面，所以能够被用于不一样的用途：如一个相机可能拥有视频和音频两个界面。界面描绘符有一个缺省界面设置（defaultinterfacesetn可能多个代替界面设置（alternateinterfacesettings），它们都拥犹如上所述的端点描绘符。一个端点能够在多个界面和代替界面设置之间复用。HCDThehardwarethatcontainsthehostcontrollerandtheroothubhasaninterfacetowardtheprogrammerwhichiscalledHostControllerDevice(HCD)andisdefinedbythehardwareimplementer.Inpractice,thesearehardwareregisters(ports)inthecomputer.Atversion1.0and1.1thereweretwocompetingHCDimplementations.Compaq'sOpenHostControllerInterface(OHCI)wasadoptedasthestandardbytheUSB-IF.However,IntelsubsequentlycreatedaspecificationtheycalledtheUniversalHostControllerInterface(UHCI)andinsistedotherimplementerspaytolicenseandimplementUHCI.VIATechnologieslicensedtheUHCIstandardfromIntel;allotherchipsetimplementersuseOHCI.ThemaindifferencebetweenOHCIandUHCIisthefactthatUHCIismoresoftware-driventhanOHCIis,makingUHCIslightlymoreprocessor-intensivebutcheapertoimplement(excludingthelicensefees).Theduelingimplementationsforcedoperatingsystemvendorsandhardwarevendorstodevelopandtestonbothimplementationswhichincreasedcost.DuringthedesignphaseofUSB2.0theUSB-IFinsistedononlyoneimplementation.TheUSB2.0HCDimplementationiscalledtheExtendedHostControllerInterface(EHCI).OnlyEHCIcansupporthigh-speedtransfers.EachEHCIcontrollercontainsfourvirtualHCDimplementationstosupportFullSpeedandLowSpeeddevices.ThevirtualHCDonIntelandViaEHCIcontrollersareUHCI.AllothervendorsusevirtualOHCIcontrollers.某些版本的MicrosoftWindows”（"Enhanced"），2.0Linux能够列出全部的PCIUSB会分别命名为OHCI、UHCI或许EHCI。设施分类Devicesthatattachtothebuscanbefull-customdevicesrequiringafull-customdevicedrivertobeused,ormaybelongtoadeviceclass.Theseclassesdefineanexpectedbehaviourintermsofdeviceandinterfacedescriptorssothatthesamedevicedrivermaybeusedforanydevicethatclaimstobeamemberofacertainclass.AnoperatingsystemissupposedtoimplementalldeviceclassessoastoprovidegenericdriversforanyUSBdevice.Themostuseddeviceclassesare:USB人接口设施类键盘、鼠标等.USB大容量储存设施类用来做U盘,便携式硬盘多媒体读卡器数码相机MP3播放器等.USB通讯设施类("CDC")用于调制解调器及winmodems),网卡缆),,FaxUSB打印设施类打印机一类的设施USB音频设施类声卡一类的设施USB视频设施类,webcam之类的设施设施的分类取决于USB建立论坛的设施工作组。USB连结器TheconnectorswhichtheUSBcommitteespecifiedweredesignedtosupportanumberofUSB'sunderlyinggoals,andtoreflectlessonslearnedfromthevariedmenagerieofconnectorstheninservice.Inparticular:Theconnectorsaredesignedtoberobust.Manypreviousconnectordesignswerefragile,withpinsorotherdelicatecomponentspronetobendingorbreaking,evenwiththeapplicationofonlyverymodestforce.TheelectricalcontactsinaUSBconnectorareprotectedbyanadjacentplastictongue,andtheentireconnectingassemblyisfurtherprotectedbyanenclosingmetalsheath.AsaresultUSBconnectorscansafelybehandled,inserted,andremoved,evenbyasmallchild.Theencasingsheathandthetoughmouldedplugbodymeanthataconnectorcanbedropped,steppedupon,evencrushedorstruck,allwithoutdamage;aconsiderabledegreeofforceisneededtosignificantlydamageaUSBconnector.不行能将USB连结器插错，连结器不可以上下插反，从外观和kinestheticsensation能够特别清楚地进行连结whentheplugandsocketarecorrectlymated.连结器的生产特别的廉价TheconnectorsenforcethedirectedtopologyofaUSBnetwork.USBdoesnotsupportcyclicalnetworks,sotheconnectorsfromincompatibleUSBdevicesarethemselvesincompatible.Unlikeothercommunicationssystems(e.g.RJ-45cabling)gender-changersareneverused,makingitdifficulttocreateacyclicUSBnetwork.Amoderateinsertion/removalforceisspecified.USBcablesandsmallUSBdevicesareheldinplacebythegrippingforcefromthereceptacle(withouttheneedforthescrews,clips,orthumbturnsotherconnectorsrequire).Theforceneededtomakeorbreakaconnectionismodest,allowingconnectionstobemadeinawkwardcircumstancesorbythosewithmotordisabilities.Theconnectorconstructionalwaysensuresthattheexternalsheathontheplugcontactswithitscounterpartinthereceptaclebeforetheconnectorswithinareconnected.Thissheathistypicallyconnectedtothesystemground,allowingotherwisedamagingstaticchargestobesafelydischargedbythisroute(ratherthanviadelicateelectroniccomponents).Thismeansofenclosurealsomeansthatthereisa(moderate)degreeofprotectionfromelectromagneticinterferenceaffordedtotheUSBsignalwhileittravelsthroughthematedconnectorpair(thisistheonlylocationwhentheotherwisetwisteddatapairmusttraveladistanceinparallel).TheUSBstandardspecifiesrelativelylowtolerancesforcompliantUSBconnectors,intendingtominimizeincompatibilitiesinconnectorsproducedbydifferentvendors(agoalthathasbeenverysuccessfullyachieved).Unlikemostotherconnectorstandards,theUSBspecalsodefineslimitstothesizeofaconnectingdeviceintheareaarounditsplug.Thiswasdonetoavoidcircumstanceswhereadevicecompliedwiththeconnectorspecificationbutitslargesizeblockedadjacentports.Compliantdevicesmusteitherfitwithinthesizerestrictionsorsupportacompliantextensioncablewhichdoes.TheUSB1.0,1.1and2.0specificationsdefinetwotypesofconnectorsfortheattachmentofdevicestothebus:A,andB.However,themechanicallayerhaschangedinsomeexamples.Forexample,theIBMUltraPortisaproprietaryUSBconnectorlocatedonthetopofIBM'slaptopLCDs.ItusesadifferentmechanicalconnectorwhilepreservingtheUSBsignalingandprotocol.Othermanufacturersofsmallitemsalsodevelopedtheirownsmallformfactorconnector,andawidevarietyofthesehaveappeared.Forspecificationpurposes,thesedevicesweretreatedashavingacaptivecable.AnextensiontoUSBcalledUSBOn-The-Goallowsasingleporttoactaseitherahostoradevice-chosenbywhichendofthecableplugsintothesocketontheunit.Evenafterthecableishookedupandtheunitsaretalking,thetwounitsmay"swap"endsunderprogramcontrol.ThisfacilitytargetsunitssuchasPDAswheretheUSBlinkmightconnecttoaPC'shostportasadeviceinoneinstance,yetconnectasahostitselftoakeyboardandmousedeviceinanotherinstance.USBOn-The-Gohasthereforedefinedtwosmallformfactorconnectors,themini-Aandmini-B,andahermaphroditicsocket(mini-AB),whichshouldstoptheproliferationofproprietarydesigns.ThemaximumlengthofaUSBcableis5meters;greaterlengthsrequirehubs[1].常用的USB延伸线电源TheUSBconnectorprovidesasinglenominally5voltwirefromwhichconnectedUSBdevicesmaypowerthemselves.Inpractice,deliveredvoltagecandropwellbelow5V,toonlyslightlyabove4V.Thecompliancespecrequiresnomorethan5.25Vanywhereandnolessthan4.375Vattheworstcase;alow-powerfunctionafterabus-poweredhub.Intypicalsituationsthevoltageiscloseto5.Agivensegmentofthebusisspecifiedtodeliverupto500mA.Thisisoftenenoughtopowerseveraldevices,althoughthisbudgetmustbesharedamongalldevicesdownstreamofanunpoweredhub.Abus-powereddevicemayuseasmuchofthatpowerasallowedbytheportitispluggedinto.Bus-poweredhubscancontinuetodistributethebusprovidedpowertoconnecteddevicesbuttheUSBspecificationonlyallowsforasinglelevelbus-powereddevicesfromabus-poweredhub.Thisdisallowsconnectionofabus-poweredhubtoanotherbus-poweredhub.Manyhubsincludeexternalpowersupplieswhichwillpowerdevicesconnectedthroughthemwithouttakingpowerfromthebus.Devicesthatneedmorethan500mAorhigherthan5voltsmustprovidetheirownpower.WhenUSBdevices(includinghubs)arefirstconnectedtheyareinterrogatedbythehostcontroller,whichenquiresofeachtheirmaximumpowerrequirements.ThehostoperatingsystemtypicallykeepstrackofthepowerrequirementsoftheUSBnetworkandmaywarnthecomputer'soperatorwhenagivensegmentrequiresmorepowerthanisavailable(andwillgenerallyshutdowndevicesorhubsinordertokeeppowerconsumptionwithintheavailableresource).AnumberofdevicesusethispowersupplywithoutparticipatinginaproperUSBnetwork.ThetypicalexampleisaUSB-poweredreadinglight,butfans,batterychargers(particularlyformobiletelephones)andevenminiaturevacuumcleanersareavailable.Inmostcases,theseitemscontainnoelectroniccircuitry,andthusarenotproperUSBdevicesatall.Thiscancauseproblemswithsomecomputers—theUSBspecificationrequiresthatdevicesconnectinalow-powermode(100mAmaximum)andstatehowmuchcurrenttheyneed,beforeswitching,withthehost'spermission,intohigh-powermode.SomedevicesintendedforconnectiontolaptopsdrawmorepowerthanispermittedbythespecificationforasingleUSBport;toavoidrequiringanexernalpowersupply,thesedevicescomewithdualcables,andtheuserisinstructedthatthedevicemustbeplugged-intotwoUSBports.Onalaptopwithonlytwoports,thismeansonlyonesuchdevicecanbeusedatatime,unlessapoweredhubisadded.AnumberofperipheralsforIBMlaptops(nowmadebyLenovo)aredesignedtousedualUSBconnectionsinthismanner.USB-powereddevicesattemptingtodrawlargecurrentswithoutrequestingthepowerwillnotworkwithcertainUSBcontrollers,andwilleitherdisruptotherdevicesonthebusorfailtoworkthemselves(orboth).ThoseproblemswiththeabuseoftheUSBpowersupplyhaveinspiredanumberofAprilFoolhoaxes,liketheintroductionofaUSB-poweredGeorgeForemaniGrill[2]andadesktopUSBFondueSet[3].同类标准比较储存AFlashDrive,atypicalUSBmassstoragedeviceUSBimplementsconnectionstostoragedeviceusingasetofstandardscalledtheUSBmassstoragedeviceclass.Thiswasinitiallyintendedfortraditionalmagneticandopticaldrives,buthasbeenextendedtosupportawidevarietydevices.USBisnotintendedtobeaprimarybusforacomputer'sinternalstorage.BusessuchasATA(IDE)andSCSIfulfillthatrole.However,USBhasoneimportantadvantageinmakingitpossibletoinstallandremovedeviceswithoutopeningthecomputercase,makingitusefulforexternaldrives.Today,anumberofmanufacturersofferportableUSBharddrivesthatofferperformancecomparabletoconventionalATA(IDE)drives.Theseexternaldrives,calledenclosures,areoftencomposedoftranslatingdevicesthatconnecttoUSBononesideandtoconventionalIDE,ATA,ATAPI,orSCSIdrivesontheother.Youinstalladriveintotheenclosureandthenplugtheenclosureintothecomputer;thusfunctioningasaregularUSBmass-storagedevice.FireWiretechnologyisalsocommonlyusedwithportableharddrivesandsomeincludebothtypesofports.FireWiretendstoperformbetterinbenchmarktests.However,USBportsaremorecommononconsumer-levelcomputers,whichenhancestheportabilityofaUSBdrive.人接口设施(HIDs)USBhasnotcompletelyreplacedATkeyboardconnectionsandPS/2keyboardandmouseconnections,butvirtuallyallPCmotherboardsmanufacturedtodayhaveoneormoreUSBports.Asof2004,mostnewmotherboardshavemultipleUSB2.0high-speedports,thoughsomeareinternal,an