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 chassis writes samples to the buffer. After the cDAQ chassis captures the specified number of pretrigger samples, the chassis begins to look for the reference trigger condition. If the reference trigger condition occurs before the cDAQ chassis captures the specified number of pretrigger samples, the chassis ignores the condition.

If the buffer becomes full, the cDAQ chassis 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 chassis discards it. Refer to the Can a Pretriggered Acquisition be Continuous? document for more information. To access this document, go to ni.com/r/rdcanq.

When the reference trigger occurs, the cDAQ chassis continues to write samples to the buffer until the buffer contains the number of posttrigger samples desired. The following figure shows the final buffer.

Figure 36. 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 on an installed C Series module or one of several internal signals on the cDAQ chassis can provide the source. Refer to the Device Routing in MAX topic in the NI-DAQmx User Manual 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.

Note Depending on the C Series module capabilities, you may need two modules to utilize analog triggering. Routing DI Reference Trigger Signal to an Output Terminal You can route DI Reference Trigger to any output PFI terminal on an installed C Series module. Reference Trigger is active high by default.