Finds the first level-crossing location in a waveform. You can retrieve the trigger location as an index or as a time. The trigger conditions are specified in terms of threshold level, slope, and hysteresis. Wire data to the signal in input to determine the polymorphic instance to use or manually select the instance.

Note Do not use the single-channel version of this VI for continuous multiple-channel processing.


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This VI can perform single-channel measurements in both one-shot mode (single call) and continuous mode (multiple calls with history). It also can perform multichannel measurements in both one-shot mode and continuous mode. If you want to make multiple-channel measurements in continuous mode, you either must use the multichannel version of this VI or use one instance of this VI per channel. This VI detects only the first trigger for each channel.

The single-channel version of this VI is intended primarily for continuous processing of a single channel. Generalizing this behavior to the multichannel case, typically by using this single-channel VI in a For Loop to continuously process multiple channels by indexing an array of waveforms, is a misuse of this VI.

The single-channel version of this VI maintains internal state information for a single channel only. Calling this VI to process another channel without clearing the history, using reset or restart averaging, results in an unexpected behavior of this VI because the internal state information is passed from one channel to another.

LabVIEW uses the hysteresis to prevent noise from causing a false trigger. For a rising edge slope, the signal must pass below level – hysteresis before a trigger level crossing is detected. For a falling edge slope, the signal must pass above level + hysteresis before a trigger level crossing is detected. The following graph shows how LabVIEW uses the hysteresis.

In the previous graph, the white line is the input signal. If the level is 0.5, and the hysteresis is 0.0, LabVIEW returns the green line that represents the false trigger caused by the noise. If the hysteresis is 0.15, LabVIEW returns the red line at approximately 0.125s that represents the valid trigger.

Examples

Refer to the following example files included with LabVIEW.

  • labview\examples\Signal Processing\Waveform Measurements\Basic Level Triggering of Waveforms.vi