National Instruments PCI-8430 - Page 152

RS-485Termination
Becauseeachdifferentialpairofwiresisatransmissionline,youmust
properlyterminatethelinetopreventreflections.Acommonmethodof
terminatingatwo-wiremultidropRS-485networkistoinstallterminating
resistorsateachendofthemultidropnetwork.Ifyoudaisy-chained
multipleinstrumentstogether,youneedaterminatingresistoratonlythe
firstandlastinstruments.Theterminatingresistorshouldmatchthe
characteristicimpedanceofthetransmissionline(typically100to120 ).
YoucanorderanoptionalDB-9RS-485terminationconnectorthat
containsembeddedterminatingresistorsforeasyterminationfrom
NationalInstruments.Fororderinginformation,contactNational
Instruments.
Thefollowingfigureshowsamultidropnetworkusingterminating
resistors.
MultidropNetworkUsingTerminatingResistors
Thefollowingfigureshowsafull-duplexnetworkusingterminating
resistors.
Full-DuplexNetworkUsingTerminatingResistors