Appendix A Serial Port Information
PCI Serial for Windows Me/9x A-6 ni.com
Figure A-4 shows a transmission line using bias resistors.
Figure A-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 Ω 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 CTS– are pulled up to VCC
•RXD– and CTS+ are pulled down to GND
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