© National Instruments | 3-5
cDAQ-9185/9189 User Manual
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 a rising or falling 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 cDAQ chassis. Refer to the
Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more
information.
Using an Analog Source
Some C Series 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 Start Trigger,
the acquisition begins on the first rising edge of the Analog Comparison Event signal.
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 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.
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