Why Don't I See All the Wire Modes for the RS-485 Port on My Controller or Serial Card?

Updated Jul 28, 2023

Reported In


  • Serial Interface Device
  • PXI-8433/4
  • PXI-8431/4

Operating System

  • NI Linux Real-Time

Issue Details

Why do I only see 2 Wire Auto and 4 Wire modes for the RS-485 port on my cRIO Controller, PXI Controller, or Serial Card?


The number of transceiver modes on all RS-485 ports are limited by the operating system of the Controller. All RS-435 ports are capable of the following 4 transceiver modes, except ports on Linux RT controllers: 2 Wire Auto, 2 Wire DTR, 2 Wire DTR Echo, and 4 Wire. The Linux RT controllers are capable of 2 Wire Auto and 4 Wire only. You can determine your controller's operating system by opening the controller's System Settings tab in NI MAX.

See your hardware manual for further information on the supported wire modes.