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Chapter 4 Analog Input
© National Instruments Corporation 4-23 NI 6238/6239 User Manual
complete description of the use of ai/StartTrigger and ai/ReferenceTrigger
in a pretriggered DAQ operation.
AI Reference Trigger Signal
Use a 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.
After 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
ni.com/info and enter the info code 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 4-14 shows the final buffer.
Figure 4-14. Reference Trigger Final Buffer
Reference Trigger
Pretrigger Samples
Complete Buffer
Posttrigger Samples