电气工程及其自动化专业外文文献英文文献外文翻译方面Word格式文档下载.doc

电气工程及其自动化专业外文文献英文文献外文翻译方面Word格式文档下载.doc

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memory

Input&

Output

unit

CPU

Data

-5A-1AHarvardtype

CPU

-5A.AconventionalPrincetoncomputer

Timer/

Counter

System

clock

External

Timing

components

SerialI/O

ROM

Reset

Prarallel

I/O

RAM

Interrupts

CPU

Power

Fig3-5A-3.Principalfeaturesofamicrocomputer

Readonlymemory（ROM）.ROMisusuallyforthepermanent,non-volatilestorageofanapplicationsprogram.Manymicrocomputersandmicrocontrollersareintendedforhigh-volumeapplicationsandhencetheeconomicalmanufactureofthedevicesrequiresthatthecontentsoftheprogrammemorybecommittedpermanentlyduringthemanufactureofchips.Clearly,thisimpliesarigorousapproachtoROMcodedevelopmentsincechangescannotbemadeaftermanufacture.Thisdevelopmentprocessmayinvolveemulationusingasophisticateddevelopmentsystemwithahardwareemulationcapabilityaswellastheuseofpowerfulsoftwaretools.

SomemanufacturersprovideadditionalROMoptionsbyincludingintheirrangedeviceswith（orintendedforusewith）userprogrammablememory.Thesimplestoftheseisusuallydevicewhichcanoperateinamicroprocessormodebyusingsomeoftheinput/outputlinesasanaddressanddatabusforaccessingexternalmemory.ThistypeofdevicecanbehavefunctionallyasthesinglechipmicrocomputerfromwhichitisderivedalbeitwithrestrictedI/Oandamodifiedexternalcircuit.TheuseoftheseROMlessdevicesiscommoneveninproductioncircuitswherethevolumedoesnotjustifythedevelopmentcostsofcustomon-chipROM[2];

therecanstillbeasignificantsavinginI/Oandotherchipscomparedtoaconventionalmicroprocessorbasedcircuit.MoreexactreplacementforROMdevicescanbeobtainedintheformofvariantswith'

piggy-back'

EPROM（ErasableprogrammableROM）socketsordeviceswithEPROMinsteadofROM。

ThesedevicesarenaturallymoreexpensivethanequivalentROMdevice,butdoprovidecompletecircuitequivalents.EPROMbaseddevicesarealsoextremelyattractiveforlow-volumeapplicationswheretheyprovidetheadvantagesofasingle-chipdevice,intermsofon-chipI/O,etc.,withtheconvenienceofflexibleuserprogrammability.

Randomaccessmemory（RAM）.RAMisforthestorageofworkingvariablesanddatausedduringprogramexecution.Thesizeofthismemoryvarieswithdevicetypebutithasthesamecharacteristicwidth（4,8,16bitsetc.）astheprocessor,Specialfunctionregisters,suchasstackpointerortimerregisterareoftenlogicallyincorporatedintotheRAMarea.ItisalsocommoninHarardtypemicrocomputerstotreattheRAMareaasacollectionofregister;

itisunnecessarytomakedistinctionbetweenRAMandprocessorregisterasisdoneinthecaseofamicroprocessorsystemsinceRAMandregistersarenotusuallyphysicallyseparatedinamicrocomputer.

Centralprocessingunit（CPU）.TheCPUismuchlikethatofanymicroprocessor.Manyapplicationsofmicrocomputersandmicrocontrollersinvolvethehandlingofbinary-codeddecimal（BCD）data（fornumericaldisplays,forexample）,henceitiscommontofindthattheCPUiswelladaptedtohandlingthistypeofdata.Itisalsocommontofindgoodfacilitiesfortesting,settingandresettingindividualbitsofmemoryorI/Osincemanycontrollerapplicationsinvolvetheturningonandoffofsingleoutputlinesorthereadingthesingleline.Theselinesarereadilyinterfacedtotwo-statedevicessuchasswitches,thermostats,solid-staterelays,valves,motor,etc.

Parallelinput/output.Parallelinputandoutputschemesvarysomewhatindifferentmicrocomputer;

inmostamechanismisprovidedtoatleastallowsomeflexibilityofchoosingwhichpinsareoutputsandwhichareinputs.Thismayapplytoallorsomeoftheports.SomeI/Olinesaresuitablefordirectinterfacingto,forexample,fluorescentdisplays,orcanprovidesufficientcurrenttomakeinterfacingothercomponentsstraightforward.SomedevicesallowanI/Oporttobeconfiguredasasystembustoallowoff-chipmemoryandI/Oexpansion.Thisfacilityispotentiallyusefulasaproductrangedevelops,sincesuccessiveenhancementsmaybecometoobigforon-chipmemoryanditisundesirablenottobuildontheexistingsoftwarebase.

Serialinput/output.Serialcommunicationwithterminaldevicesiscommonmeansofprovidingalinkusingasmallnumberoflines.Thissortofcommunicationcanalsobeexploitedforinterfacingspecialfunctionchipsorlinkingseveralmicrocomputerstogether.Boththecommonasynchronoussynchronouscommunicationschemesrequireprotocolsthatprovideframing（startandstop）information.ThiscanbeimplementedasahardwarefacilityorU（S）ART（Universal（synchronous）asynchronousreceiver/transmitter）relievingtheprocessor（andtheapplicationsprogrammer）ofthislow-level,time-consuming,detail.tismerelynecessarytoselectedabaud-rateandpossiblyotheroptions（numberofstopbits,parity,etc.）andload（orreadfrom）theserialtransmitter（orreceiver）buffer.Serializationofthedataintheappropriateformatisthenhandledbythehardwarecircuit.

Timing/counterfacilities.Manyapplicationofsingle-chipmicrocomputersrequireaccurateevaluationofelapsedrealtime.Thiscanbedeterminedbycarefulassessmentoftheexecutiontimeofeachbranchinaprogrambutthisrapidlybecomesinefficientforallbutsimplestprograms.Thepreferredapproachistousetimercircuitthatcanindependentlycountprecisetimeincrementsandgenerateaninterruptafterapresettimehaselapsed.Thistypeoftimerisusuallyarrangedtobereloadablewiththerequiredcount.Thetimerthendecrementsthisvalueproducinganinterruptorsettingaflagwhenthecounterreacheszero.Bettertimersthenhavetheabilitytoautomaticallyreloadtheinitialcountvalue.Thisrelievestheprogrammeroftheresponsibilityofreloadingthecounterandassessingelapsedtimebeforethetimerrestarted,whichotherwisewoundbenecessaryifcontinuouspreciselytimedinterruptswererequired（asinaclock,forexample）.Sometimesassociatedwithtimerisaneventcounter.Withthisfacilitythereisusuallyaspecialinputpin,thatcandrivethecounterdirectly.

Timingcomponents.Theclockcircuitryofmostmicrocomputersrequiresonlysimpletimingcomponents.Ifmaximumperformanceisrequired,acrystalmustbeusedtoensurethemaximumclockfrequencyisapproachedbutnotexceeded.Manyclockcircuitsalsoworkwitharesistorandcapacitoraslow-costtimingcomponentsorcanbedrivenfromanexternalsource.Thislatterarrangementisusefulisexternalsynchronizationofthemicrocomputerisrequired.

WORDSANDTERMS

culminationn.顶点

spiltadj.分离的

volatilen.易变的

commitv.保证

albeitconj.虽然

customadj.定制的

variantadj.不同的

piggy-backadj.背负式的

socketn.插座

B:

PLC[1]

PLCs（programmablelogicalcontroller）faceevermorecomplexchallengesthesedays.Whereoncetheyquietlyreplacedrelaysandgaveanoccasionalreporttoacorporatemainframe,theyarenowgroupedintocells,givennewjobandnewlanguages,andareforcedtocompeteagainstagrowingarrayofcontrolproducts.Forthisyear'

sannualPLCtechnologyupdate,wequeriedPLCmakersonthesetopicsandmore.

Programminglanguages

HigherlevelPLCprogramminglanguageshavebeenaroundforsometime,butlatelytheirpopularityhasmushrooming."

AsRaymondLeveille,vicepresident&

generalmanager,SiemensEnergy&

Automation.inc;

Programmablecontrolsarebeingusedformoreandmoresophisticatedoperations,languagesotherthanladderlogicbecomemorepractical,efficient,andpowerful.Forexample,it'

sverydifficulttowriteatrigonometricfunctionusingladderlogic."

LanguagesgainingacceptanceincludeBoolean,controlsystemflowcharting,andsuchfunctionchartlanguagesasGraphcetanditsvariation.Andthere'

sincreasinginterestinlanguageslikeCandBASIC.

PLCsinprocesscontrol

Thusfar,PLCshavenotbeenusedextensivelyforcontinuousprocesscontrol.Willthiscontinue"

ThefeelingthatI'

vegotten,"

saysKenJannotta,manger,productplanning,seriesOneandSeriesSixproduct,atGEFanucNorthAmerica,'

isthatPLCswillbeusedintheprocessindustrybutnotnecessarilyforprocesscontrol."

Severalvendors-obviouslybettingthattheoppositewillhappen-haveintroducedPLCsoptimizedforprocessapplication.RichRyan,manger,commercialmarketing,Allen-bradleyProgrammableControlsDiv.,citesPLCs'

sincreasingusesuchindustriesasfood,chemicals,andpetroleum.Ryanfeelstherearetwotypesofapplicationsinwhichthey'

reappropriate."

one,"

hesays,"

iswherethesizeoftheprocesscontrolsystemthat'

sbeingautomateddoesn'

tjustifyDCS[distributedcontrolsystem].Withthestartingpricetagsofchoseproductsbeingrel