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Chapter 4 Analog Input
The source also can be one of several other internal signals on your DAQ device. Refer to Device
Routing in MAX in the NI-DAQmx Help or the LabVIEW Help for more information.
You also can specify whether the measurement acquisition begins on the rising edge or falling
edge of AI Start Trigger.
Using an Analog Source
When you use an analog trigger source, the acquisition begins on the first rising edge of the
Analog Comparison Event signal.
Routing AI Start Trigger to an Output Terminal
You can route AI Start Trigger out to any PFI <0..15> or RTSI <0..7> terminal. The output is an
active high pulse. All PFI terminals are configured as inputs by default.
The device also uses AI Start Trigger to initiate pretriggered DAQ operations. In most
pretriggered applications, a software trigger generates AI Start Trigger. Refer to the AI Reference
Trigger Signal section for a complete description of the use of AI Start Trigger and AI Reference
Trigger in a pretriggered DAQ operation.
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
ni.com/info
and enter the Info Code rdcanq.
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