National Instruments 3.22E+04 用户手册

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 k

Ω resistors at each node to 

effectively achieve the same result.

620  
Bias Resistor

+5

A

Rx

B

620  
Bias Resistor

100  
Termination Resistor