© National Instruments Corporation 23 NI cDAQ-9178/9174 User Guide and Specifications
Digital Waveform Acquisition (Hardware-Timed Input)
You can acquire digital waveforms using hardware-timed digital modules. The DI waveform acquisition
FIFO stores the digital samples. The NI cDAQ-9178/9174 chassis samples the DIO lines on each rising
or falling edge of the di/SampleClock signal.
Digital Input Triggering
A trigger is a signal that causes an action, such as starting or stopping the acquisition of data. When you
configure a trigger, you must decide how you want to produce the trigger and the action you want the
trigger to cause. The NI cDAQ-9178/9174 chassis supports internal software, external digital triggering,
and digital 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
analog modules for triggering, refer to the Analog Input and Analog Output sections.
DI Start Trigger Signal
Use the DI Start Trigger (di/StartTrigger) 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 delay from the start trigger to the
first sample.
Using a Digital Source
To use di/StartTrigger 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
di/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 di/StartTrigger, the
acquisition begins on the first rising edge of the Analog Comparison Event signal.
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