Unexpected Delay when Using DAQmx Analog Start Trigger

Updated Apr 18, 2019

Reported In

Hardware

  • Multifunction I/O Device

Driver

  • NI-DAQmx

Issue Details

When I use a DAQmx analog edge start trigger with an M Series DAQ device, my program appears to last 250-300ms longer than when I do not use an analog edge start trigger. Why is this happening, and is there anything that I can do to get around it?

Solution

There is a built-in delay of 250 ms that occurs in the DAQmx Start Task VI when an analog edge start trigger is used with an M Series device.  When the analog edge trigger is configured, a reference voltage must be created at that analog value onboard the DAQ device. The delay added to the DAQmx Start Task VI is necessary to allow the hardware components to settle before measurements can be taken. This reference voltage is created differently and thus takes a different amount of time to settle, depending on the type of device that is being used. For example, E-Series DAQ devices experience a much shorter delay than the 250 ms seen with the M Series devices. Certain cDAQ modules also experience less of a delay than the M Series boards. For example, the 9205 has a delay of 50 ms.

This delay only occurs once when the task is explicitly committed. This means that, for a retriggerable acquisition, the delay will not occur each time the trigger occurs; it will only be seen when the task is first committed. There is not any way to get around this delay, so it should be accounted for during application development.

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