National Instruments 6368, Руководство пользователя

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4-53
X Series User Manual
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.
Note (NI USB-6356/6366 Devices) You can select the buffer on the host or on the
NI USB-6356/6366 device. To enable a Reference Trigger to Onboard Memory, set
the AI Data Transfer Request Condition property in NI-DAQmx to When
Acquisition Complete.
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 document,
Can a Pretriggered Analog Acquisition be
Continuous? , for more information. To access this document, go to
ni.com/info
and enter the
Info Code
rdcanq
.
When the reference trigger occurs, the DAQ device continues to write samples to the buffer until
the buffer contains the number of posttrigger samples desired. Figure 4-32 shows the final
buffer.
Figure 4-32.
Reference Trigger Final Buffer
Reference Trigger
Pretrigger Sa mple s
Complete B u ffer
Po s ttrigger Sa mple s