National Instruments NI 6614, User Manual

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Chapter 2 Digital I/O
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>, 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.
Once 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, the 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
.
When the reference trigger occurs, the device continues to write samples to the buffer until the
buffer contains the number of posttrigger samples desired. Figure 2-6
shows the final buffer.
Figure 2-6. Reference Trigger Final Buffer
Reference Trigger
Pretrigger Sa mple s
Complete B u ffer
Po s ttrigger Sa mple s