Inverted Digital Output Values on DAQ Device

Updated May 2, 2018

Reported In

Hardware

  • USB-6525

Driver

  • NI-DAQmx

Issue Details

The digital output lines on my DAQ device seem be inverted. When I set the output to high in software, it sends a digital low signal. How do I fix this?

Solution

This behavior very often occurs when dealing with sinking digital outputs. (For info on sinking vs. sourcing outputsĀ click here.)

This is generally because the device was set up with a "high-side" connection like this schematic from the USB-6525 User Guide:
This can be changed by switching to a "low-side" configuration. This can be done by connect a pull-down resistor from one of the connections to ground in parallel to the output connections while connecting the other side to external power.

Additional Information

To better understand this behavior from an electrical standpoint, it is useful to think of a sinking output as an electrically activated switch. When you set the software value to high, the switch closes allowing current to flow, and when you set the software value to low, the switch opens.

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