攀枝花學院本科畢業設計（論文）外語文翻譯攀枝花學院本科畢業設計（論文）外語文翻譯1外語文獻翻譯原文：3.4.1CADHARDWAREComputersTherearetwomajortypesofhardwareusedinaCADsystem.Computerandinput/output(I/O)devices.IntheearlydaysofCAD.SomecommercialCADsystemsusedproprietarycomputers.Today,nearlyallCADsoftwarerunsonageneral-purposecomputer.DependingonthecomplexityoftheCADpackage,itmayrequireamainframecomputer,aminicomputer,orsimplyamicrocomputer.Ingeneral,themorefunctionalitiesaCADsystemprovides,themorepowerfulthecomputerneeded.Threedimensionalsolidmodelersrequiremuchmorecomputingthandotwo-dimensionaldraftingsystems；thus,theyneedmorepowerfulcomputers.Systemsthatintegrateengineeringanalysisorsimulationpackagesgenerallyaremorecomputation-intensive.ApowerfulcomputernotonlyspeedsuptheresponseofaCADsystem,butalsocansupportmultipleuserswithoutsignificantperformancedegradation.Severalotherparametersandcomponentsconcerningthecomputeralsohavetobeconsidered.CADHARDWARErandom-accessmemory(RAM)capacitypermanentdisk-storagecapacityspecialgraphicsacceleratortapebackupsRAMistheactualphysicalmemory(vs.virtualmemory)ofacomputer.Asmallmemorycapacitymeansslowprocessingduetofrequentswappingbetweenthephysicalmemoryandthevirtualmemoryondisk.BecauseCADisextremelystoragedemanding,diskcapacityisimportant.Smalldiskcapacitylimitsthesystemtothestorageofonlyafewdrawings.CADisgraphics-basedandrequirestremendousdataprocessing.Agraphicsacceleratorcandrasticallyincreasetheperformanceofthesystem.Anothercomponent,theharddisk,istypicallythemajorcostofacomputersystem.Therefore,itisnotcost-justifiabletostorealldrawingsonlineondisk.Magnetictapeisstillthemosteconomicalmediumtouseforoff-linedatastorage.ItisnecessaryforaCADsystemtohaveatapebackupsubsystem.Currently,alllevelsofcomputersareusedinCADsystems.Personalcomputersareusedinlow-cost,2-Ddraftingapplications,andwiththenewpowerofthesecomputers,heyarealsobeingusedforsome3-Dsolidmodelingapplications.EngineeringworkstationshavebeenthemainstayinCAD,andareusuallyasingle-userCADworkstationinanetwork.MinicomputersareusedinmultiuserCADsystems.MainframesareusedforlargemultiuserCADsystemstosupportrealtimesimulationandengineeringanalysis.Sometimes,inalargecorporation,alllevels攀枝花學院本科畢業設計（論文）外語文翻譯2ofCADsystemsareimplemented.Allthesesystemsarelinkedtogetherthroughahierarchicalcomputernetwork.3.4.2Input.Output(I/O)DevicesFigure3.7showsthetypicalI/OdevicesusedinaCADsystem.Inputdevicesaregenerallyusedtotransferinformationfromahumanorstoragemediumtoacomputerwhere”CADfunctions”arecarriedout.Akeyboardisthestandardinputdeviceusedtotransmitalphanumericdatatothesystem.Functionkeypadsarealsousedtomakeinputeasier.Joysticks,trackballs,andmoussearealsousedtomanipulateacursor.Theycanbeusedtopositionthegraphiccursor(e.g.crosshair)onamonitorandfeedbackthelocationofanobjectonthemonitortothecomputer.Usingthesedevicesallowsanoperatortoaddressterminallocationstointeractivelyinreachinganaccurateposition.Mouseshavebeenusedextensivelywithwindowsandpull-downmenus.Theyareeasytouseforpointing；However,usingamousetotraceacurveisnotaneasytask.Therearethreebasicapproachestoinputanexistingdrawing；(1)modeltheobjectonadrawing,(2)digitizethedrawing,or(3)scantheobject.Digitizingisusuallymucheasierthanmodeling.AdigitizerisadevicethattranslatestheX-Ylocationsonadrawingthanmodeling.AdigitizerisadevicethattranslatestheX-Ylocationsonadrawingintoadigitalandfeedsthatsignaltoacomputer.GraphicsscannersscanadrawingandconvertittoaCAD-system-readableformat.Somescannershavebuilt-incharacter-recognitionsoftwarethatcanconvertcharactersonpaperintoASCIIcodes.Anotherinputdeviceisasketchpadcalledagraphicstablet.Agraphicstabletisaspecialflatsurfaceonwhichauserdrawswithastylus.Thelocationofthestylusissenttothecomputer.Thetabletisanabsolutecoordinatedevice.Itiseasytoimplementoverlaymenusonthetabletandpulldownmenusonthescreentoimprovetheoperation.Currently.thetabletisthemostpopularinputdeviceusedinCADotherthanthekeyboard.ThestandardoutputdeviceforCADisamonitordisplay.Modernmonitordisplaysareraster-scandisplaymonitors.SimilartoaTVmonitor,anelectrongun(SonyTrinitronusesthreegunsforthreebasiccolors)sendsanelectronicbeamtothefrontofthemonitor.Easydisplaydotiscalledapixel(pictureelement).Forcolordisplays,eachpixelisrepresentedbythreecloselylocateddotswithred,green,andbluecolors.Theelectronicbeamselectsthecolorelementsandtheintensityofeachcolorelement.Theresolutionofthemonitorisdeterminedbythenumberofdifferentcolorsorgrayscalesdisplayableateachpixel,usuallymeasuredinbits,A1-bitdisplaycanturneachpixeleitherONorOFF.Agrayscalehasatleastv2bits.A1-bitdisplaycanturneachpixeleitherONorOFF.Agrayscalehasatleast2bitswhichallowthreelightintensitiesplusanOFF(2*2).Usually,displaysare4bits(16colors),8bits(256colors),16bits(thousandsofcolors),and24bits(millionsofcolors).Theelectronicbeamsweepsthescreenfromthetoptothebottomlinebyline.Becausethedotislitforonlyaveryshortperiodoftime,thesweepmustbedoneveryquicklyinordernottohaveaflickeringimage.Therefreshrateisameasureofhowmanytimesthemonitorisredrawninonesecond.攀枝花學院本科畢業設計（論文）外語文翻譯3Thecomputerdoesnotdrawdirectlyonthemonitor.Connectingthecomputertothemonitorisu-suallyaRGB(red,green,blue)cable.Threeseparatesignallinesareconnected.Onthegraphicsboardinthecomputerarecircuitstogeneratetheanalogsignalsrequiredtodrivethemonitor.Thereisalsoaframebufferthatservesasmemorytostoretheimage.Eachpixelonthescreenhasacorrespondingaddressintheframebuffer.Thedataintheframebufferareconvertedintotheappropriateanalogsignalandsenttothemonitorattherefreshrate.Separately,thecomputerwritesdirectlytotheframebuffertheimageitintendstodraw.Thegreaterthedisplaysizeandnumberofcolors,themoreframebufferisneeded.Theframebufferonthegraphicsboardismadeofrandom-accessmemory(RAM).ThecalculatorlendsupporttoestopaintthesoftwareAutoCADtomakeuscanbewiththesketchthattheastonishingspeeddrawtheengineeringsketchormachinesketchandotherwithaccuracytosophisticate.Thecalculatorpaintstopaintthedifferentplace,oneofthemwithhandicraftthepaintingswithoutthediagramdivideastepproceedof,becauseofbutwouldsometimesappearthetraditionalhandicraftthepaintinghourtheproblemoftheimpossibleemergence.Generalcircumstancebottom,usagetheAutoCADproceedspainting,controltheprecisionpaintingandnotthatdifficult,butwantaccuratethenneedthecertaintechniqueoutthediagram.Us-uallypaintoroutsingdiagramtohavenotoneedtochangetomeasuretoAutoCADsystemworthmakethemodificationwithestablish,takeitstheprovinceworthcannormalwork,buthavethespecialrequest,mustmodifytherelatedsystemtochangemeasure；Toosometimestherequestofthediagrampaperwithmakethediagramswayofdoingtheantinomy,andbetoadoptthetomakethediagrammethodtocantattaintherequestofthediagrampaper,willadoptsometechniqueiquestomodifytherelatedsystemtochangetomeasure,andcanmakethecalculatordrawtoattaintherequestofthediagrampaperwithoutputssketch.AutoCADlinetypeestablishingofcomparisonAtteachingofprocessinsidewouldusuallyrunintowhatthiskindofcircumstance,whileestablishingdiagramlayer,clearlyestablishedthepointlineationed,andconsequentlyoutputtohoweverinvestigateitsreasonforthesolidlined,andisusuallybecauseofthelinetypedtheestablishingofcomparisonisnotappropriatea