National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 22)

NI cDAQ-9172 User Guide and Specifications 22 ni.com
Using an Analog Source
Some C Series I/O modules can generate a trigger based on an analog
signal. In NI-DAQmx, this is called the Analog Comparison Event.
When you use an analog trigger source for ai/StartTrigger, 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/StartTrigger to any output PFI terminal. The output is an
active high pulse.
AI Reference Trigger Signal
Use a 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 NI cDAQ-9172 chassis writes samples to
the buffer. After the NI cDAQ-9172 chassis captures the specified number
of pretrigger samples, the NI cDAQ-9172 chassis begins to look for the
reference trigger condition. If the reference trigger condition occurs before
the NI cDAQ-9172 captures the specified number of pretrigger samples, the
NI cDAQ-9172 ignores the condition.
If the buffer becomes full, the NI cDAQ-9172 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 NI cDAQ-9172 chassis
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 .