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Chapter 2 Digital I/O
Using a Digital Source
To use DI Start Trigger with a digital source, specify a source and an edge. You can route many
signals to DI Start Trigger. To view the complete list of possible routes, see the Device Routes
tab in MAX. 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- or falling-edge
of DI Start Trigger.
Routing DI Start Trigger to an Output Terminal
You can route DI Start Trigger out to any PFI <0..39>, RTSI <0..7>, PXI_Trig <0..7>, or
PXIe-DSTARC terminal. The output is an active high pulse. All PFI terminals are configured as
inputs by default.
The device also uses DI Start Trigger to initiate pretriggered DAQ operations. In most
pretriggered applications, a software trigger generates DI Start Trigger. Refer to the
DI Reference Trigger Signal section for a complete description of the use of DI Start Trigger and
DI Reference Trigger in a pretriggered acquisition operation.
DI Reference Trigger Signal
Use the DI 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.
After the acquisition begins, the device writes samples to the buffer. After the device captures
the specified number of pretrigger samples, it begins to look for the reference trigger condition.
If the reference trigger condition occurs before the device captures the specified number of
pretrigger samples, it ignores the condition.
If the buffer becomes full, the 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
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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