3.4.1CADHARDWARE

Computers

TherearetwomajortypesofhardwareusedinaCADsystem.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.

CADHARDWARE

random-accessmemory(RAM)capacity

permanentdisk-storagecapacity

specialgraphicsaccelerator

tapebackups

RAMistheactualphysicalmemory(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,alllevelsofCADsystemsareimplemented.Allthesesystemsarelinkedtogetherthroughahierarchicalcomputernetwork.

3.4.2Input.Output(I/O)Devices

Figure3.7showsthetypicalI/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.

Thecomputerdoesnotdrawdirectlyonthemonitor.Connectingthecomputertothemonitorisu-

suallyaRGB(red,green,blue)cable.Threeseparatesignallinesareconnected.Onthegraphicsb-

oardinthecomputerarecircuitstogeneratetheanalogsignalsrequiredtodrivethemonitor.Ther-

eisalsoaframebufferthatservesasmemorytostoretheimage.Eachpixelonthescreenhasac

-orrespondingaddressintheframebuffer.Thedataintheframebufferareconvertedintotheappropriateanalogsignalandsenttothemonitorattherefreshrate.Separately,thecomputerw-

ritesdirectlytotheframebuffertheimageitintendstodraw.Thegreaterthedisplaysizeandnu-

mberofcolors,themoreframebufferisneeded.Theframebufferonthegraphicsboardismadeofrandom-accessmemory(RAM).

ThecalculatorlendsupporttoestopaintthesoftwareAutoCADtomakeuscanbewiththesketchthattheastonishingspeeddrawtheengineeringsketchormachinesketchandotherwithaccuracytosophisticate.Thecalculatorpaintstopaintthedifferentplace,oneofthemwithhan-

dicraftthepaintingswithoutthediagramdivideastepproceedof,becauseofbutwouldsomet-

imesappearthetraditionalhandicraftthepaintinghourtheproblemoftheimpossibleemergence.Generalcircumstancebottom,usagetheAutoCADproceedspainting,controltheprecisionpain-

tingandnotthatdifficult,butwantaccuratethenneedthecertaintechniqueoutthediagram.Us-

uallypaintoroutsingdiagramtohavenotoneedtochangetomeasuretoAutoCADsystemworthmakethemodificationwithestablish,takeitstheprovinceworthcannormalwork,buthavethespecialrequest,mustmodifytherelatedsystemtochangemeasure;Toosometimestherequesto-

fthediagrampaperwithmakethediagram'swayofdoingtheantinomy,andbetoadoptthetom-

akethediagrammethodtocan'tattaintherequestofthediagrampaper,willadoptsometechnique

iquestomodifytherelatedsystemtochangetomeasure,andcanmakethecalculatordrawtoatta-

intherequestofthediagrampaperwithoutput'ssketch.

AutoCADlinetypeestablishingofcomparisonAtteachingofprocessinsidewouldusuallyrunintowhatthiskindofcircumstance,whileestablishingdiagramlayer,clearlyestablishedthepointlineationed,andconsequentlyoutputtohoweverinvestigateitsreasonforthesolidlined,andisusuallybecauseofthelinetypedtheestablishingofcomparisonisnotappropriateandcau-

sable.Shouldyousoestablishthecomparisonofwhatkindoflinetype?Howtousethelinetypethecomparisoninthepainting?

UsageAutoCADpainting,besidescontinuouslinetype(Continuous),theotherlinetypeisallfromthesolidlinesegment,blanksegment,pointseriesforortextthisconstituting.On-linetypedefinitiondocumenttheinsidehavealreadydefinedtheselinetypesthesegment'sstandardlength,andshowonheldtheactoratprintthemachine\paintingoutput,eachlengthandex-

portationcomparisonwithlinetypecomparisondirectproportion.Whenshoworprint,thepain-

tingboundarythatcustomerthatrequest,thishourestablishisoutofaccordancewiththepaintingboundaryoftheprovincedifferencebig,onheldtheactshoworpaintoutput'slinetypewouldmatchtheengineeringgraphicstowillpassthechangeslinetypecomparisonthesystemthemethodthatchangethedeal,enlargeorcontractalllinetypeseachasmallsegmentoflength,makethesketchmademeettherequest.IntheAutoCAD,implytheadjustmentlinetypethecom-

parison'sorder:Overallsituationlinetypecomparisonfactortiedepartmentexportationratioforlinetyperatiofactor,usedforingthecontrolnotcontinuouslinetype.TheLtscaleisrighttohavetheobjectvalidity,Celtscaletheprovincefortonewobjectvalidating,twolychangingthedealw-

orthallfor1.

ForusethelinetypethatAutoCAD2002painting,bigpartcontainthreekindsofforms,(su-

chas:ComparisonthatCenter,Center2,Center*2,Dashed,Dashed2,Dashed*2)thefirstisahalf,thethirdlinetypeforthestandardform,thesecondlinetypethatcomparisonthatthefirstlinety-

peisthefirstlinetypeoftwotimes,iflinetypethecomparisonestablishtoisnotsuitablefor,andwouldthenmakesomeexportationdiagramstheline(suchasthepointlineation,dottedline...etc.)changeintoasolidline.

Two,linebreadththecountofestablish

Passbywhathandicraftpainting,itspaintingwithoutsthediagramisasynchronousprocee-

dingof,nowcalculatorpainting,itspaintingwithoutsthediagramdivideastepproceeds.Wetherdrawtheconstructionengineeringdiagramormachinesparepartsdiagram,atprintcontrolthataworkthatexportationengineeringdrawing,themostisalinetoprintthedrawingthebreadth,linethetype,color,sealalinebreadthfor,andeachlygrowingtolinetypeallcontainitscowgirl,int-

heAutoCADpaintingprocessoftheoldedition,evendefinedthelinetypethewidth,itsatseethediagramareacan'talsodisplaythelinetypethewidth'sdifferentiation.(notcontainmanyrighteo-

usnesslinesthePline)Widthforcolorforcanpassingthediagramlayerorsetuppingthelineco-

mingdistinguishanalysingtheline.Whatresearchofcurrentpaintingtriesa,adoptofisthiskindofmethod.ButintheAutoCAD2002,addentitythat"Lweight"(linetypethewidthestablish)or-

der,providedthenewfunctionforcustomer,thenmakeuseoftheentitycharacteristictoproceedthecontrol,andestablishthelinebreadthforthesketch,andmakeuseofitcanatholdtheacttoupshowwithprinttooutput,controltheentity'slinebreadth.2002versionsinsideprinttoestablis-

handcanisdividedintothreepart,andforprintingmachinebelongtothesextoestablish,printt-

hestyletoestablishrespectivelyandthepageestablish.

Becauseofthemodelofbreadthlinethespaceisdifferentfromdiagrammanifestationofsp-

acepaperresult,inthemodelspacethereforetheelephantvegetableshow,butinthediagrampap-

erthespaceisthenthentoprintthewidthtoproceedtoshow,becauseofbutwewhiledrawings-

ketchshouldknowatthediagramlinethattruewidthestablishedbylinebreadthofmodelspace,andisnotequalwithobject.Whenthelinebreadthisworthtoestablishto"0",itshowwiththemi-

nimumwidth(plainbreadthofanelephant)onheldtheact,andthelinebreadthsettletootherwo-

rth,thencomeshowingwiththeworthspecificvalueofplainwidthandtrueunitofelephant.

Three,thesketchoutputtheestablishingofcomparisonWhenwedrawsketchthatcomparis-

on