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NI cDAQ-9181/9184/9188/9191 User Manual

Routing DI Start Trigger to an Output Terminal

You can route DI Start Trigger to any output PFI terminal. The output is an active high pulse.

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 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 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 chassis 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 chassis can provide the source. Refer

to the Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more

information.

Reference Trigger

Pretrigger Samples

Complete Buffer

Posttrigger Samples

National Instruments NI CompactDAQ 9184 User Manual

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