National Instruments PCI-8430 - Page 153

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.FormoreinformationaboutRS-485
biasresistors,refertoUsingYourSerialHardware.