© National Instruments | 4-5
NI cDAQ-9132/9133/9134/9135/9136/9137 User Manual
DI Reference Trigger Signal
Use a 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 cDAQ controller writes samples to the buffer. After the cDAQ
controller captures the specified number of pretrigger samples, the controller begins to look for
the reference trigger condition. If the reference trigger condition occurs before the cDAQ
controller captures the specified number of pretrigger samples, the controller ignores the
condition.
If the buffer becomes full, the cDAQ controller 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 cDAQ controller 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 cDAQ controller continues to write samples to the buffer
until the buffer contains the number of posttrigger samples desired. Figure 4-2 shows the final
buffer.
Figure 4-2. Reference Trigger Final Buffer
Using a Digital Source
To use DI Reference Trigger with a digital source, specify a source and a rising or falling edge.
Either PFI or one of several internal signals on the cDAQ controller can provide the source.
Refer to the Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more
information.
Using an Analog Source
Some C Series 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, the acquisition stops on the first rising or falling edge of
the Analog Comparison Event signal, depending on the trigger properties.
Reference Trigger
Pretrigger Samples
Complete Buffer
Posttrigger Samples
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