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Chapter 3 Analog Input
AI Reference Trigger Signal
Use Reference Trigger 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 cDAQ chassis writes samples to the buffer. After the cDAQ
chassis captures the specified number of pretrigger samples, the cDAQ chassis begins to look
for the reference trigger condition. If the reference trigger condition occurs before the cDAQ
chassis captures the specified number of pretrigger samples, the chassis ignores the condition.
If the buffer becomes full, the cDAQ chassis 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 cDAQ chassis discards it. Refer to the Can a Pretriggered Acquisition be
Continuous? document for more information. To access this document, go to
ni.com/info
and enter the Info Code rdcanq.
When the reference trigger occurs, the cDAQ chassis continues to write samples to the buffer
until the buffer contains the number of posttrigger samples desired. Figure 3-3 shows the final
buffer.
Figure 3-3. Reference Trigger Final Buffer
Using a Digital Source
To use Reference Trigger with a digital source, specify a source and a rising or falling edge.
Either PFI or one of several internal signals on the cDAQ chassis can provide the source. Refer
to the Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more
information.
Reference Trigger
Pretrigger Samples
Complete Buffer
Posttrigger Samples
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