Slow VeriStand Loop Rates with PXIe-4330/4331

Updated Dec 7, 2017

Reported In

Hardware

  • PXIe-4330
  • PXIe-4331

Software

  • VeriStand 2014
  • VeriStand 2015
  • VeriStand 2015 SP1
  • VeriStand 2016
  • VeriStand 2017 Full

Issue Details

I am using a PXIe-4330 strain/bridge module in my VeriStand system and am seeing slow PCL rates.  If I increase the Primary Control Loop rate above 711 Hz, VeriStand throws Error -201410:
 
Why does the PXIe-4330 module limit my PCL rate to 711 Hz?  Similarly, why does the PXIe-4331 limit me to 2500 Hz?

Solution

The PXIe-4330 is running in Hardware Timed Single Point mode in VeriStand, which limits its sampling rate (and thus, the VeriStand Primary Control Loop rate) to 711.11 Hz.  Its specified maximum sample rate is 25.6 kS/s/ch, which means each individual data point takes 1/25,600 seconds (0.000039 seconds) to process.  In single-point mode, the module acquires enough samples to account for front-end filter delay which is 36 samples at the maximum sampling rate of 25.6 kS/s/ch.  

These 36 samples are then averaged and the PXIe-4330 returns a single-point data point.  Given that the ADC needs 0.000039 seconds to acquire a single data point, the 36 required data points take 0.0014 seconds to acquire, resulting in a maximum Hardware Timed Single Point acquisition speed of 711.11 Hz. 

Similarly, the PXIe-4331 is limited to a theoretical maximum of 2498 Hz in single-point mode.  

Additional Information

To allow a higher control loop rate, you can run the PXIe-4330 or PXIe-4331 in waveform acquisition mode.  This will allow a higher sample rate but will buffer waveforms into VeriStand's Primary Control loop rather than single-point data.  This is recommended only for datalogging configurations; if you are attempting to do control with information gathered by the 4330 or 4331 hardware, single-point is the recommended operation mode.  

If you are attempting to use the data from your PXIe-4330 as an input to a control loop above 711.11 Hz, upgrading to the PXIe-4331 will yield loop rates up to 2498 Hz.  

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