© National Instruments | 5-5
cDAQ-9185/9189 User Manual
Note Depending on the C Series module capabilities, you may need two modules
to utilize analog triggering.
Using a Time Source
To use the Start Trigger signal with a time source, specify a specific time in NI-DAQmx. Refer
to the Timestamps and Time Triggering topics in the NI-DAQmx Help for more information on
accessing time-based features in the NI-DAQmx API.
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 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 cDAQ chassis continues to write samples to the buffer
until the buffer contains the number of posttrigger samples desired. Figure 5-2 shows the final
buffer.
Figure 5-2. Reference Trigger Final Buffer
Reference Trigger
Pretrigger Samples
Complete Buffer
Posttrigger Samples
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