Using Digital Output or +5 Volt Lines on the NI 651x

Updated Apr 17, 2023

Reported In


  • PXI-6515
  • PXI-6514
  • PCI-6514/6515

Issue Details

  • I am using either the +5 Volt line or digital output lines on my NI 651x to perform a digital output task. I'm not seeing any response when measuring the terminals though. Why is this happening?
  • Do I need to provide an external supply to out the digital outputs on my PXI-651x card?


Yes, according to page 3-3 of the 651x manual , you need to provide a +5V - +30V power supply to the card in order to use the 5 volt lines, or perform digital outputs.
"The output lines on the NI 6515 consist of photocouplers and Darlington
arrays. To connect to these signals, connect a 5 V to 30 V power supply to
COM, connect GND to ground, and cross a load between digital output
and COM."
Schematic drawings on how to provide this external power supply can be found below (sourced from page 4-6 of user manual).
Note: The schematic diagrams for each 651x card differs slightly. Refer to the User Manual to see the exact connections for your device.

Additional Information

The 651x family uses 24V logic. This means a low signal will output 0V-4V, while the high signal will output 11V - 30V. If you want to change the low signal to output 0V exactly, you can use a Pull-down resistor