Microcomputers Systems.docx

Microcomputers Systems.docx

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MicrocomputersSystems

MicrocomputerSystems

Electronicsystemsareusedforhandinginformationinthemostgeneralsense;thisinformationmaybetelephoneconversation,instrumentreadoracompany’saccounts,butineachcasethesamemaintypeofoperationareinvolved:

theprocessing,storageandtransmissionofinformation.inconventionalelectronicdesigntheseoperationsarecombinedatthefunctionlevel;forexampleacounter,whetherelectronicormechanical,storesthecurrentandincrementsitbyoneasrequired.Asystemsuchasanelectronicclockwhichemployscountershasitsstorageandprocessingcapabilitiesspreadthroughoutthesystembecauseeachcounterisabletostoreandprocessnumbers.

Presentdaymicroprocessorbasedsystemsdepartfromthisconventionalapproachbyseparatingthethreefunctionsofprocessing,storage,andtransmissionintodifferentsectionofthesystem.ThispartitioningintothreemainfunctionswasdevisedbyVonNeumannduringthe1940s,andwasnotconceivedespeciallyformicrocomputers.Almosteverycomputerevermadehasbeendesignedwiththisstructure,anddespitetheenormousrangeintheirphysicalforms,theyhaveallbeenofessentiallythesamebasicdesign.

Inamicroprocessorbasedsystemtheprocessingwillbeperformedinthemicroprocessoritself.Thestoragewillbebymeansofmemorycircuitsandthecommunicationofinformationintoandoutofthesystemwillbebymeansofspecialinput/output（I/O）circuits.Itwouldbeimpossibletoidentifyaparticularpieceofhardwarewhichperformedthecountinginamicroprocessorbasedclockbecausethetimewouldbestoredinthememoryandincrementedatregularintervalsbutthemicroprocessor.However,thesoftwarewhichdefinedthesystem’sbehaviorwouldcontainsectionsthatperformedascounters.Theapparentlyratherabstractapproachtothearchitectureofthemicroprocessoranditsassociatedcircuitsallowsittobeveryflexibleinuse,sincethesystemisdefinedalmostentirelysoftware.Thedesignprocessislargelyoneofsoftwareengineering,andthesimilarproblemsofconstructionandmaintenancewhichoccurinconventionalengineeringareencounteredwhenproducingsoftware.

Thefigure1.1illustrateshowthesethreesectionswithinamicrocomputerareconnectedintermsofthecommunicationofinformationwithinthemachine.Thesystemiscontrolledbythemicroprocessorwhichsupervisesthetransferofinformationbetweenitselfandthememoryandinput/outputsections.Theexternalconnectionsrelatetotherest（thatis,thenon-computerpart）oftheengineeringsystem.

Fig.1.1ThreeSectionsofaTypicalMicrocomputer

Althoughonlyonestoragesectionhasbeenshowninthediagram,inpracticetwodistincttypesofmemoryRAMandROMareused.Ineachcase,theword‘memory’isratherinappropriatesinceacomputersmemoryismorelikeafilingcabinetinconcept;informationisstoredinasetofnumbered‘boxes’anditisreferencedbytheserialnumberofthe‘box’inquestion.

MicrocomputersuseRAM（RandomAccessMemory）intowhichdatacanbewrittenandfromwhichdatacanbereadagainwhenneeded.Thisdatacanbereadbackfromthememoryinanysequencedesired,andnotnecessarilythesameorderinwhichitwaswritten,hencetheexpression‘random’accessmemory.AnothertypeofROM（ReadOnlyMemory）isusedtoholdfixedpatternsofinformationwhichcannotbeaffectedbythemicroprocessor;thesepatternsarenotlostwhenpowerisremovedandarenormallyusedtoholdtheprogramwhichdefinesthebehaviorofamicroprocessorbasedsystem.ROMscanbereadlikeRAMs,butunlikeRAMstheycannotbeusedtostorevariableinformation.SomeROMshavetheirdatapatternsputinduringmanufacture,whileothersareprogrammablebytheuserbymeansofspecialequipmentandarecalledprogrammableROMs.ThewidelyusedprogrammableROMsareerasablebymeansofspecialultravioletlampsandarereferredtoasEPROMs,shortforErasableProgrammableReadOnlyMemories.Othernewtypesofdevicecanbeerasedelectricallywithouttheneedforultravioletlight,whicharecalledElectricallyErasableProgrammableReadOnlyMemories,EEPROMs.

Themicroprocessorprocessesdataunderthecontroloftheprogram,controllingtheflowofinformationtoandfrommemoryandinput/outputdevices.Someinput/outputdevicesaregeneral-purposetypeswhileothersaredesignedforcontrollingspecialhardwaresuchasdiscdrivesorcontrollinginformationtransmissiontoothercomputers.MosttypesofI/Odevicesareprogrammabletosomeextent,allowingdifferentmodesofoperation,whilesomeactuallycontainspecial-purposemicroprocessorstopermitquitecomplexoperationstobecarriedoutwithoutdirectlyinvolvingthemainmicroprocessor.

Themicroprocessorprocessesdataunderthecontroloftheprogram,controllingtheflowofinformationtoandfrommemoryandinput/outputdevices.Someinput/outputdevicesaregeneral-purposetypeswhileothersaredesignedforcontrollingspecialhardwaresuchasdiscdrivesorcontrollinginformationtransmissiontoothercomputers.MosttypesofI/Odevicesareprogrammabletosomeextent,allowingdifferentmodesofoperation,whilesomeactuallycontainspecial-purposemicroprocessorstopermitquitecomplexoperationstobecarriedoutwithoutdirectlyinvolvingthemainmicroprocessor.

Themicroprocessor,memoryandinput/outputcircuitmayallbecontainedonthesameintegratedcircuitprovidedthattheapplicationdoesnotrequiretoomuchprogramordatastorage.Thisisusuallythecaseinlow-costapplicationsuchasthecontrollersusedinmicrowaveovensandautomaticwashingmachines.Theuseofsinglepackageallowsconsiderablecostsavingstoemadewhenarticlesaremanufacturedinlargequantities.Astechnologydevelops,moreandmorepowerfulprocessorsandlargerandlargeramountsofmemoryarebeingincorporatedintosinglechipmicrocomputerswithresultingsavinginassemblycostsinthefinalproducts.Fortheforeseeablefuture,however,itwillcontinuetobenecessarytointerconnectanumberofintegratedcircuitstomakeamicrocomputerwheneverlargeramountsofstorageorinput/outputarerequired.

Anothermajorengineeringapplicationofmicrocomputersisinprocesscontrol.Herethepresenceofthemicrocomputerisusuallymoreapparenttotheuserbecauseprovisionisnormallymadeforprogrammingthemicrocomputerfortheparticularapplication.Inprocesscontrolapplicationsthebenefitslffittingtheentiresystemontosinglechipareusuallyoutweighedbythehighdesigncostinvolved,becausethissortlfequipmentisproducedinsmallerquantities.Moreover,processcontrollersareusuallymorecomplicatedsothatitismoredifficulttomakethemassingleintegratedcircuits.Twoapproachesarepossible;thecontrollercanbeimplementedasageneral-purposemicrocomputerratherlikeamorerobustversionlfahobbycomputer,orasa‘packaged’system,signedforreplacingcontrollersbasedonoldertechnologiessuchaselectromagneticrelays.Intheformercasethesystemwouldprobablybeprogrammedinconventionalprogramminglanguagessuchastheonesto9beintroducedlater,whileintheothercaseaspecial-purposelanguagemightbeused,forexampleonewhichallowedthefunctionofthecontrollertobedescribedintermsofrelayinterconnections,IneithercaseprogramscanbestoredinRAM,whichallowsthemtobealteredtosuitchangesinapplication,butthismakestheoverallsystemvulnerabletolosslfpowerunlessbatteriesareusedtoensurecontinuityofsupply.AlternativelyprogramscanbestoredinROM,inwhichcasetheyvirtuallybecomepartoftheelectronic‘hardware’andareoftenreferredtoasfirmware.Moresophisticatedprocesscontrollersrequireminicomputersfortheirimplementation,althoughtheuselflargescaleintegratedcircuits‘thedistinctionbetweenminiandmicrocomputers,Productsandprocesscontrollersofvariouskindsrepresentthemajorityofpresent-daymicrocomputerapplications,theexactfiguresdependingonone’sinterpretationoftheword‘product’.Virtuallyallengineeringandscientificusesofmicrocomputerscanbeassignedtooneorotherofthesecategories.ButinthesystemwemoststudyPressureandPressureTransmitters.Pressureariseswhenaforceisappliedoveranarea.ProvidedtheforceisoneNewtonanduniformlyovertheareaofonesquaremeters,thepressurehasbeendesignatedonePascal.Pressureisauniversalprocessingcondition.Itisalsoaconditionoflifeontheplanet:

weliveatthebottomofanatmosphericoceanthatextendsupwardformanymiles.Thismassofairhasweight,andthisweightpressingdownwardcausesatmosphericpressure.Water,afundamentalnecessityoflife,issuppliedtomostofusunderpressure.Inthetypicalprocessplant,pressureinfluencesboilingpointtemperatures,condensingpointtemperatures,processefficiency,costs,andotherimportantfactors.Themeasurementandcontrolofpressureorlackofit-vacuum-inthetypicalprocessplantiscritical.

Theworkinginstrumentsintheplantusuallyincludesimplepressuregauges,precisionrecordersandindicators,andpneumaticandelectronicpressuretransmitters.Apressuretransmittermakesapressuremeasurementandgenerateseitherapneumaticorelectricalsignaloutputthatisproportionaltothepressurebeingsensed.

Intheprocessplant,itisimpracticaltolocatethecontrolinstrumentsoutintheplaceneartheprocess.Itisalsotruethatmostmeasurementsarenoteasilytransmittedfromsomeremotelocation.Pressuremeasurementisanexception,butifahighpressureofsomedangerouschemicalistobeindicatedorrecordedseveralhundredfeetfromthepointofmeas