AppendixB SerialPort Information
SerialHardware and Software for Windows B-6 ni.com
FigureB-4 shows a transmission line using bias resistors.
FigureB-4. TransmissionLine Using Bias Resistors
On the PCI RS-485 2- and 4-port serial cards, there are four
user-configurablebias resistors in front of each connector. These resistors
aresocketed and pre-loaded with 620 Ωresistors. They are connected to the
receivesignals of each port to maintain a known state when the bus is idle.
The connectionsare m adeas follows:
•RXD+ and CTS–arepulleduptoVCC
•RXD–andCTS+arepulleddowntoGND
Rather than using two 620 Ωresistors at one node, you can increase the
valueof the resistors and put them at every node. For instance, if there are
eight nodes in a system, you can use 4.7 kΩresistors at each node to
effectivelyachieve the same result.
620
BiasResistor
+5
A
Rx
B
620
BiasResistor
100
TerminationResistor