Slow Open Contact Acquisition for NI 921x C-Series Modules

Updated Jan 30, 2019

Reported In

Hardware

  • NI-9212
  • NI-9219
  • NI-9214
  • NI-9213

Issue Details

My NI-921x has a ~1 second delay in acquisition when trying to capture open contact measurements. Where is this delay coming from?

Solution

Contact Detection mode works just like a 2-wire resistance measurement. This means that there is a constant current source applied to the channel and the ADC reads the voltage across the sensor. If there is an open circuit then there is no way for the module to tell the difference between this and an actual over current condition and therefore goes into auto-recover mode where there is no communication with the ADC, the module returns railed data while trying to recover which is interpreted by software as an Open Contact.  There is a 500 ms delay to allow for the excitation to disable and then some other smaller delays that allow for recovery routines and switches to settle.  This mode doesn't follow the normal timing modes because of the recovery routine. 

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