Chapter 4 | Analog Input |
The device also uses ai/StartTrigger to initiate pretriggered DAQ operations. In most pretriggered applications, a software trigger generates ai/StartTrigger. Refer to the AI Reference Trigger Signal section for a 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.
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 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
Reference Trigger
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Complete Buffer
Figure
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