Critical Spacing of CAN Bus Connections (Rev. A

1 1 MinimumDistanceBetweenNodeConnectionsApp licationReportSLLA279A November2008 "messagepriorityinversion" , :2003 CANbusisadistributedparametercircuitwhos eelectricalcharacteristicsandresponsesar eprimarilydefinedbythedistributedinducta nceandcapacitance(1) ,connectors,terminators, ,thecharacteristictransmissionlineimpeda nceseenintoanycutpointinanunloadedCANbus isdefinedbyZ= L/C, ,thebusimpedanceisloweredtoZ'.Whenthebus impedanceislowered,animpedancemismatchoc cursbetweenunloadedandloadedsectionsofth ebus.(1) November2008 RevisedJanuary2009 CriticalSpacingofCANBusConnections1 SubmitDocumentationFeedbackVS120W120WS1 Loaddt = 0Z = >meters(ifCispF/m)orfeet(ifCispF/ft). Distributed Capacitance - pF/mDistance - mC = 10 pFLC = 20 pFLC = 30 pFLC = 40 pFLC = 50 ,whenS1switchesattimezerofromadominantst atetoarecessivestate,theCANdriverdiffere ntialoutputvoltage,VS, , ,d,isafunctionofthedevicelumpedloadcapac itanceCL,andthecable'sdistributedcapacit anceperunitlength,C, ,connectorcontacts,printed-circuitboardt races,protectiondevices, ,suchastheSN65 HVD233usedinthisexample, ,shielded-twisted-paircableto70 November2008 RevisedJanuary2009 SubmitDocumentationFeedback120 WLoad15 "Load200 mLoad2 Load3 Load10120WZ = ,Figure3displays10SN65 HVD233 CANtransceiversconnectedtothebuswith5inc hesof120- , ,or1 s, November2008 November2008 softhebit, ,thisreflectionlastsfor50%ofthe2- ,CLofeachCANtransceiver,boardtrace.

2 AndBergconnectoramountstoapproximately20 pFpernodeinthisexample,andthedistributed capacitanceperunitlength,C, November2008 , :AHandbookofBlackMagic, , (SLLA123) :2003 CANS pecification6 CriticalSpacingofCANBusConnectionsSLLA27 9A November2008 RevisedJanuary2009 SubmitDocumentationFeedbackAppendixAMath ematicalDevelopmentoftheNodeCapacitancea ndNodeSpacingTrade-offVS120W120WS1 Loaddt = 0Z = 120 OWLoadLoadLoadThevoltagereflectedbackfro mthemismatchisVR1=pL VJ1where,Ld > > 3 + ( 3) + ( 3) >= (A-1)0L00000Z Z => + ZZ Z > (Z + Z )Z (1 + ) >Z (1 )Z > -- -- - (A-2) L/C, ,intheformofdevicesandtheirinterconnecti on,thebusimpedanceisloweredtoZ'.Whentheb usimpedanceisloweredonasectionofthebus, ,theCANdriverdifferentialoutputvoltage,V S, , ,anattenuation(oramplification) +VJ1+VR1,whereVL0istheinitialdifferentia lvoltage,VJ1istheinputsignaldifferential voltagetransition, +VJ1+pL ,aCANdrivercreatesahigh-to-lowdifferenti alvoltagechangefromthestandardmaximumof3 Vto0V, ,VL1, ,Now,solvingforZ', , , ,theloaded-busimpedancecanbeapproximated bySLLA279A November2008 RevisedJanuary20097 SubmitDocumentationFeedback200Z CCZ == Z(C+C )C+C' (A-3)0L20L20LL20CZ = ZCC+dCZ'=ZCC+dZCC= C+Z'dCd =ZC1Z' - (A-4)L200meters (if C is pF/m) or feet (if C is pF/ft)LCd > > - (A-5) '= L/(C+C')

3 ,whereC' ,Z' , , ,onecansolveforL,fromtherelationshipZ0= L/C,asL= 'andsimplifying,C'isthedistributeddevice capacitance,CL,dividedbythedistance,d,be tweendevicesorC'= ,Now,substitutingtheminimumZ' ,8 CriticalSpacingofCANBusConnectionsSLLA27 9A November2008 RevisedJanuary2009 SubmitDocumentationFeedbackIMPORTANTNOTI CET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,modifications,enhancements,improvement s, , , ,eitherexpressorimplied,isgrantedunderan yTIpatentright,copyright,maskworkright,o rotherTIintellectualpropertyrightrelatin gtoanycombination,machine, , ,conditions,limitations, (suchaslifesupport)whereafailureoftheTIp roductwouldreasonablybeexpectedtocausese verepersonalinjuryordeath, ,andacknowledgeandagreethattheyaresolely responsibleforalllegal,regulatoryandsafe ty-relatedrequirementsconcerningtheirpro ductsandanyuseofTIproductsinsuchsafety-c riticalapplications, , "enhancedplastic.

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