Writing Data.docx

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1、WritingDataTheTDMSfileformatwasdesignedtostreamdataasquicklyaspossiblewhilestillbeingflexibleenoughtoaccommodatechangesinthenumberofchannelsandtheirsamplingratesduringtheacquisition.Datafilesthatstreamquickly,though,donotnecessarilyloadquickly.TheTDMSfileisanentirelybinaryfilethatconsistsofmultiples

2、ections,onelayeredontopoftheotherasyouwritetothefile.Thesesectionscontainbuffersofdatavaluesassignedtooneormorechannelsand/ormeta-datapropertiesattachedtooneormorelevels.ThefewersectionstheTDMSfilehas,ingeneral,thefasteritwillload.EachtimeaTDMSfileiswrittenorread,aTDMS_Indexfileiscreatedthatcontains

3、amapofthebinarysections.SubsequentreadsofthesameTDMSfileconsulttheTDMS_Indexfiletoreadallthepropertiesandtodeterminethecorrectbytepositionstoreadouteachstoredblockofeachchannel.Roughlyspeaking,iftheresultingTDMS_IndexfileissimilarinsizetoitsTDMSfile,thenthisTDMSfileis“fragmented,meaningithasmoresect

4、ionsthanitneedsandwillthereforeloadslowerthanitshould.Thereareseveralapproachesyoucanimplement,bothduringandaftertheacquisition,thatwillminimizethenumberofunnecessarysectionsandmaximizethereadingspeedoftheresultingTDMSfile.WritingTDMSFileswithMinimalFragmentationFirstoff,ifyouareacquiringdatawithNIs

5、dataacquisitionhardware,considerusingtheNI-DAQmxTDMSwritingcapability,sinceitautomaticallywritesun-fragmentedTDMSfiles.IfyouareusingNILabVIEWtoacquireyourdatachannels,youcanchoosetheVIsfromtheTDMSAdvancedpalettetowriteaminimallyfragmentedTDMSfile.IfyouareusingthestandardTDMSwritingfunctions,thefollo

6、wingtipswillhelpminimizetheTDMSfilefragmentation.WritealltheTDMSpropertieseitherbeforeorafterthedataacquisition(loop).Writepropertyvaluesfromsame-datatypepropertiesusinga2DarrayandoneTDMSwritefunction.Writeatleast1000datapointsatatimetoeachdatachannelintheTDMSfile.Ifyouhavetowrite1datapointatatime,s

7、etthechannelpropertyNI_MinimumBufferSizeequalto1000.DefragmentingTDMSFilesAftertheAcquisitionEvenifyourdataacquisitionconstraintsforceyoutocreatefragmentedTDMSdatafiles,youcanstilladdresstheissueaftertheacquisition.IfyouareusingNILabVIEW,youcanexecutetheTDMSDefragmentfunctiontorewritetheTDMSfilewith

8、minimalfragmentation.Alternatively,ifyouloadtheTDMSdatafileintoNIDIAdemandsimplyresaveit,theresultingTDMSdatafilewillbeminimallyfragmented.WritingLoad-Speed-EnhancingChannelPropertiesIfyourtargetapplicationtoreadTDMSfilesisNIDIAdem,youcandramaticallyimproveloadingspeedintoNIDIAdembycreatingthefollow

9、ingfourpropertiesattachedtoeverynumericordatetimedatachannelintheTDMSfile.IfanyoneofthesefourpropertiesismissingornotpopulatedwithavalidvaluewhenagivenTDMSdatachannelisbeingloaded,thenNIDIAdemwillautomaticallycalculateallfourpropertiesforthatchannelinordertospeedupgraphaxisauto-scaling.Iftheseproper

10、tiesarealreadycreatedandfilledwithvalidvaluesandattachedtoeachdatachannelintheTDMSfile,thenthatTDMSfilewillloadmuchfasterintoNIDIAdem,becausethatcalculationforeachchannelwillbeavoidedduringtheTDMSfileloadingprocess.Notethatthepropertynamesarecasesensitiveandmustbeinlowercase.NoneofthesefourchannelpropertiesaresetbydefaultinNILabVIEWyouneedtoaddthosepropertiesyourselftoeverywaveformchannelyoucreateintheTDMSfile.Table3.PropertiestoImproveLoadingSpeedinNIDIAdem