How do I Monitor a Switch with a Digital Input Module

Updated May 3, 2018

Reported In

Hardware

  • NI-9411

Software

  • LabVIEW Full

Driver

  • NI-DAQmx

Issue Details

I am trying to monitor a switch opening and closing using a Digital Input device, when the switch is closed it reads high as expected but when I open my switch it stays high, when I am expecting it to go low. 

Solution

In order to see the behavior you are expecting ensure that the digital input is single ended and implement a pull down resistor. 

NI DAQ products are not tested with no input and so the value read in software is floating, which is why when you open the switch the value stays high. 

With a pull down resistor the value read by the DAQ device will be low when the switch is open since it is directly connected to a ground through the resistor. The resistor must have a higher resistance than the logic circuit impedance.

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