Why Do I Need an Isolated External Power Supply for my NI-9870 or NI-9871 Modules?

Updated Oct 16, 2019

Reported In

Hardware

  • NI-9870
  • NI-9871

Issue Details

I noticed that the user manual of my NI-9870 and NI-9871 C Series Modules recommends using an isolated power supply to power the modules. Why is this necessary? Can I use the same power supply that I use with the controller?

Solution

We do not recommend that you use the same power supply to power both the modules and the controllers. The NI-9870 and NI-9871 modules contain serial transceivers that require more power than the controller can provide. National Instruments recommends using an isolated power supply to power these modules for safety reasons. An isolated power supply is a separate supply from the power supply powering the CompactRIO controller.

This is necessary for the NI-9870 and NI-9871 modules because the instrument you are communicating with may be on a substantially different ground plane than the module, causing a large common mode voltage. This common mode voltage may cause problems with communication. 

The controller is isolated from the chassis of CompactRIO controllers, preventing damage caused by connecting instruments on different ground planes. However, if this same ground plane is also connected to the controller via a common power supply, damage can occur.

If you wish to use the same supply to power modules and the controller, you should take precautions to ensure that there are no common mode voltage issues with devices with which you communicate. For more information about common mode voltage, see Isolation Types and Considerations when Taking a Measurement .