National Instruments PCI-8430 - Bias Resistors

BiasResistors
AnRS-485transmissionlineentersanindeterminatestateifnonodes
aretransmittingonit.Thisindeterminatestatecancausethereceiversto
receiveinvaliddatabitsfromnoisepickeduponthecable.Topreventa
linefromreceivingthesedatabits,thetransmissionlinecanbeforced
intoaknownstatebyinstallingtwobiasresistorsatonenodeonthe
transmissionline.Doingsocreatesavoltagedividerthatforcesthe
voltagebetweenthedifferentialpairtobegreaterthan200mV,the
thresholdvoltageforthereceiver.
Thefollowingfigureshowsatransmissionlineusingbiasresistors.
TransmissionLineUsingBiasResistors
OnthePCIRS-485twoandfour-portserialcards,PXIRS-485twoand
four-portserialcards,andENETRS-485twoandfour-portserialcards,
therearefouruser-configurablebiasresistorsinfrontofeachconnector.
Theseresistorsaresocketedandpre-loadedwith620 resistors.They
areconnectedtothereceivesignalsofeachporttomaintainaknown
statewhenthebusisidle.Theconnectionsaremadeasfollows:
RXD+andCTS–arepulledupto+5V
RXD–andCTS+arepulleddowntoGND
Ratherthanusingtwo620 resistorsatonenode,youcanincreasethe
valueoftheresistorsandputthemateverynode.Forinstance,ifthere
areeightnodesinasystem,youcanuse4.7k resistorsateachnodeto
effectivelyachievethesameresult.
Ifyourapplicationneedstermination,anoptionalexternalRS-485DB-9
terminatorthatprovides120 isavailable.