National Instruments PCI-8430 - Page 112

USB-485ProgrammaticallyControlledBias
Resistors
OntheUSB-485hardware,thereareprogrammaticallycontrolled
onboardbiasresistors.Inaddition,theUSB-485twoandfour-port
hardwarehasuser-configurablesocketedbiasresistors.Bydefault,the
USB-485hardwareusestheprogrammaticallycontrolledbiasresistors,
which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+3.3V
RXD–andCTS+arepulleddowntoGND
ThefollwingfigureshowsaUSBtransmissionlineusingbiasresistors.
USB-485TransmissionLineUsingBiasResistors
Thevaluesoftheprogrammaticallycontrolledbiasresistorshavebeen
calculatedsuchthatthereisavoltageofatleast200mVbetweenthe
differentialpair.Ratherthanusingtheprogrammaticallycontrolledbias
resistors,youcanloadcustomvaluesofresistorsintothesocketsinfront
ofeachconnectororuseexternalbiasresistors.Iftheseresistorsare
installed,makesuretheprogrammaticallycontrolledbiasresistorsare
disabled.
NoteUsingcustombiasresistorsontheUSB-485twoportmay
violatetheUSBsuspendcurrentspecification.
NoteTheseries25 resistorsinthefigureaboveareforshort-
circuitprotectionanddonotimpactserialcommunication.