Ai Reference Trigger Signal - National Instruments DAQ X Series User Manual

Chapter 4 Analog Input

AI Reference Trigger Signal

Use AI Reference Trigger (ai/ReferenceTrigger) signal to stop a
measurement acquisition. To use a reference trigger, specify a buffer of
finite size and a number of pretrigger samples (samples that occur before
the reference trigger). The number of posttrigger samples (samples that
occur after the reference trigger) desired is the buffer size minus the
number of pretrigger samples.
Once the acquisition begins, the DAQ device writes samples to the buffer.
After the DAQ device captures the specified number of pretrigger samples,
the DAQ device begins to look for the reference trigger condition. If the
reference trigger condition occurs before the DAQ device captures the
specified number of pretrigger samples, the DAQ device ignores the
condition.
If the buffer becomes full, the DAQ device continuously discards the oldest
samples in the buffer to make space for the next sample. This data can be
accessed (with some limitations) before the DAQ device discards it. Refer
to the KnowledgeBase document, Can a Pretriggered Acquisition be
Continuous?, for more information. To access this KnowledgeBase, go to
and enter the Info Code .
ni.com/info rdcanq
When the reference trigger occurs, the DAQ device continues to write
samples to the buffer until the buffer contains the number of posttrigger
samples desired. Figure 4-23 shows the final buffer.
Reference Trigger
Pretrigger Samples Posttrigger Samples
Complete Buffer
Figure 4-23. Reference Trigger Final Buffer
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