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Appendix B Serial Port Information
Serial Hardware and Software for Windows B-6 ni.com
Figure B-4 shows a transmission line using bias resistors.
Figure B-4. Transmission Line Using Bias Resistors
On the PCI RS-485 2- and 4-port serial cards, there are four
user-configurable bias resistors in front of each connector. These resistors
are socketed and pre-loadedwith 620 Ω resistors. Theyare 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– arepulleduptoVCC
• RXD– andCTS+arepulleddowntoGND
Rather than using two 620 Ω 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 kΩ resistors at each node to
effectively achieve the same result.
620
Bias Resistor
+5
A
Rx
B
620
Bias Resistor
100
Termination Resistor