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4-5
Routing DI Start Trigger to an Output Terminal
You can route DI Start Trigger to any output PFI terminal. The output is an active high pulse.
DI Reference Trigger Signal
Use a reference trigger (di/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 cDAQ chassis writes samples to the buffer. After the cDAQ
chassis captures the specified number of pretrigger samples, the 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 KnowledgeBase document,
Can a Pretriggered
Acquisition be Continuous?
, for more information. To access this KnowledgeBase, go to
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 4-2 shows the final
buffer.
Figure 4-2.
Reference Trigger Final Buffer
Using a Digital Source
To use DI 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
Sa
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Complete B
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Po
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ttrigger
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