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DI Reference Trigger Signal

Use a reference trigger 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 cRIO controller writes samples to the buffer. After the cRIO

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 cRIO

controller captures the specified number of pretrigger samples, the controller ignores the

condition.

If the buffer becomes full, the cRIO 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 cRIO controller discards it. Refer to the Can a Pretriggered Acquisition be

Continuous? document for more information. To access this document, go to ni.com/info and

enter the Info Code rdcanq.

When the reference trigger occurs, the cRIO controller continues to write samples to the buffer

until the buffer contains the number of posttrigger samples desired. The figure below shows

the final buffer.

Figure 40. Reference Trigger Final Buffer

Reference Trigger

Pretrigger Samples

Complete Buffer

Posttrigger Samples

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 cRIO 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.

Routing DI Reference Trigger Signal to an Output Terminal

You can route DI Reference Trigger to any output PFI terminal. Reference Trigger is active

high by default.

64 | ni.com | cRIO-904x User Manual

National Instruments cRIO-904 Series User Manual

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