© National Instruments Corporation 13 NI cDAQ-9178/9174 User Guide and Specifications
trigger to cause. The NI cDAQ-9178/9174 chassis supports internal software, external digital triggering,
and analog triggering.
Three triggers are available: Start Trigger, Reference Trigger, and Pause Trigger. An analog or digital
trigger can initiate these three trigger actions. Up to two C Series hardware-timed digital input modules
can be used in any chassis slot(s) to supply a digital trigger. To find your module triggering options, refer
to the documentation included with your C Series I/O modules. For more information about using digital
modules for triggering, refer to the Digital I/O section.
AI Start Trigger Signal
Use the Start Trigger signal to begin a measurement acquisition. A measurement acquisition consists of
one or more samples. If you do not use triggers, begin a measurement with a software command. Once
the acquisition begins, configure the acquisition to stop in one of the following ways:
• When a certain number of points has been sampled (in finite mode)
• After a hardware reference trigger (in finite mode)
• With a software command (in continuous mode)
An acquisition that uses a start trigger (but not a reference trigger) is sometimes referred to as a
posttriggered acquisition. That is, samples are measured only after the trigger.
When you are using an internal sample clock, you can specify a default delay from the start trigger to
the first sample.
Using a Digital Source
To use the Start Trigger signal with a digital source, specify a source and an edge. Use the following
signals as the source:
• Any PFI terminal
• Counter n Internal Output
The source also can be one of several other internal signals on your NI cDAQ-9178/9174 chassis. 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 begins on the rising edge or falling edge of
StartTrigger.
Using an Analog Source
Some C Series I/O 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 for StartTrigger, the
acquisition begins on the first rising edge of the Analog Comparison Event signal.
Routing AI Start Trigger to an Output Terminal
You can route the Start Trigger signal to any output PFI terminal. The output is an active high pulse.
AI Reference Trigger Signal
Use 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.
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