NI cDAQ-9178 - Page 24

NI cDAQ-9178/9174 User Guide and Specifications 24 ni.com
Routing DI Start Trigger to an Output Terminal
You can route di/StartTrigger 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 NI cDAQ-9178/9174 chassis writes samples to the buffer.
After the NI cDAQ-9178/9174 chassis captures the specified number of pretrigger samples, the
NI cDAQ-9178/9174 chassis begins to look for the reference trigger condition. If the reference trigger
condition occurs before the NI cDAQ-9178/9174 captures the specified number of pretrigger samples,
the NI cDAQ-9178/9174 ignores the condition.
If the buffer becomes full, the NI cDAQ-9178/9174 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
NI cDAQ-9178/9174 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 NI cDAQ-9178/9174 continues to write samples to the buffer
until the buffer contains the number of posttrigger samples desired. Figure 15 shows the final buffer.
Figure 15. Reference Trigger Final Buffer
Using a Digital Source
To use di/ReferenceTrigger with a digital source, specify a source and an edge. Either PFI or one of
several internal signals on the NI cDAQ-9178/9174 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.
The NI-DAQmx Help is available after installation from Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help. To view the LabVIEW Help, select Help»Search the LabVIEW Help in
LabVIEW. Alternately, to download the LabVIEW Help, go to ni.com/manuals.
You also can specify whether the measurement acquisition stops on the rising edge or falling edge of
di/ReferenceTrigger.
Reference Trigger
Pretrigger Samples
Complete Buffer
Posttrigger Samples