ApplicationReportSLLA070D–June2002–RevisedMay2010RS-422andRS-485StandardsOverviewandSystemConfigurationsMannySoltero,JingZhang,andChrisCockril.....................................................HPA-IndustrialInterfaceUpdatedbyKevinZhang,ClarkKinnaird,andThomasKugelstadt.............................................................ABSTRACTANSITIA/EIA-422andTIA/EIA-485standards,commonlyknownasRS-422andRS-485,respectively,specifybalanceddata-transmissionschemesfortransmittingdataoverlongdistancesinnoisyenvironments.Thesestandardsarecompared,andtheirbasicdifferencesandsimilaritiesarediscussed.Techniquesforimpedancematchingtominimizelinereflectionsinseveralapplicationsarepresented,withlaboratorytestresults.Contents1Introduction..................................................................................................................32OverviewofRS-422andRS-485Standards............................................................................32.1SelectedRS-422ElectricalSpecifications......................................................................32.2SelectedRS-485ElectricalSpecifications......................................................................93FailsafeOperation.........................................................................................................123.1TheNeedforFailsafeProtection...............................................................................123.2InternalFailsafe...................................................................................................133.3ExternalFailsafe..................................................................................................134SuggestedTerminationandGroundingTechniques..................................................................134.1NoTermination...................................................................................................134.2ParallelTermination.........................................................................................
