Use a reference trigger 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 cRIO 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.
To use a 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.
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.
You can route a reference trigger to any output PFI terminal. Reference Trigger is active high by default.