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Chapter 4 Analog Input
© National Instruments Corporation 4-19 NI 6124/6154 User Manual
Using an Analog Source
When you use an analog trigger source, the acquisition begins on the
first rising edge of the Analog Comparison Event signal.
Routing AI Start Trigger to an Output Terminal
You can route AI Start Trigger out to any PFI <0..15> or RTSI <0..7>
terminal. The output is an active high pulse. All PFI terminals are
configured as inputs by default.
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.
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 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.