Serial Troubleshooting Wizard
Termination and Bias Resistor Information
Because each differential pair of wires is a transmission line, you must
properly terminate the line to prevent reflections. A common method of
terminating a two-wire multidrop RS-485 network is to install terminating
resistors at each end of the multidrop network. If you daisy-chained
multiple instruments together, you need a terminating resistor at only the
first and last instruments. The terminating resistor should match the
characteristic impedance of the transmission line (typically 100–120 Ohms).
National Instruments offers an optional DB-9 RS-485 termination
connector that contains embedded terminating resistors for easy
The transmission line into the RS-485 port enters an indeterminate state when it is not being transmitted to. This indeterminate state can cause the receivers to receive invalid data bits from the noise picked up on the cable. To prevent these data bits, you should force the transmission line into a known state. By installing two 620 Ohm bias resistors at one node on the transmission line, you can create a voltage divider that forces the voltage between the differential pair to be less than 200 milli-Volts, the threshold for the receiver. You should install these resistors on only one node. The figure below shows a transmission line using bias resistors:
Rather than using two 620 Ohm resistors at one node, you can also 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 kilo-Ohm resistors at each node to effectively achieve the same result.