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Waveform Timing Limitations

NI-DAQmx 18.6 Help

Edition Date: January 2019
Part Number: 370466AH-01
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NI-DAQmx 18.6 Help
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The limitation on t0 is that NI-DAQmx calculates the starting time for the task when data is read the first time. At this time, NI-DAQmx calculates the starting time for the task by reading the current system time and subtracting the number of samples acquired × dt from the system. Therefore, if you call read after the acquisition is complete, the calculated start time for the task is not accurate. This inaccuracy is reflected in the t0 returned with the waveform data.

The limitation on dt is that for certain timing types, NI-DAQmx cannot calculate the value of dt. When you use sample clock timing, NI-DAQmx calculates dt based on the rate of the clock. Because NI-DAQmx does not know the rate when handshake, implicit, on demand, or change detection timing is specified, NI-DAQmx returns dt as 0. Waveforms with a dt of 0 often do not work with the waveform analysis functions. However, you can always update the value of dt in your application if you know the expected rate of the timing source. Your ADE has an interface to update the value of dt.

Note Note   The waveform data only supports symmetric timing between samples. If your timing is not symmetric such as if each sample has a time stamp, the waveform data format cannot contain the timing information. However, you can use your ADE's analysis library to resample the data using a constant dt. You can the use the resampled data with the waveform based analysis library.


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