National Instruments AT Manual de usuario Pagina 137

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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-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 CTSarepulleduptoVCC
RXDandCTS+arepulleddowntoGND
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.
620
Bias Resistor
+5
A
Rx
B
620
Bias Resistor
100
Termination Resistor
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