Recommendations for Making Custom Cabling for 2- and 4-Wire Transmission With RS-485

Issue Details

I am working with a Serial interface device using RS-485 protocol to use a third-party device with LabVIEW. Do NI provide some recommendations to make the connection between both devices?

Solution

1. Check the pinout of your NI serial interface device available in Connectors and Pinouts section of the NI Serial Hardware Specifications and Features document.

2. Based on the 485 mode protocol, you must properly terminate the line to prevent reflections. For this reason you must use Termination Resistors and Bias Resistors to get the right performance.

NI serial interfaces do not have any built-in 120Ohm termination resistors, but some do have built in bias resistors. Refer to the Serial Hardware Features table in the manual for details on each model.

Termination Resistors
National Instruments provides a termination adapter with the necessary 120 Ω resistors (part number 182844-01). This adapter provides the termination necessary for all 485 modes (2-wire and 4-wire).

Bias resistors
In the Serial Hardware Features table of the manual, RS-485 Socketed Bias Resistors refers to holes in the device's PCB to add your own resistors. Note the 8431 variants with 8 and 16 ports do not have this feature, whereas the 2 and 4 port versions do, so this is important to check.

 

Additional Information