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Chapter 4
Analog Input
AI Start Trigger Signal
Note Refer to the NI 6238/6239 Specifications for the minimum allowable pulse width
and the propagation delay of PFI <0..5>.
NI 6238/6239 User Manual
Use the AI Start Trigger (ai/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.
After the acquisition begins, configure the acquisition to stop under the
following conditions:
•
When a certain number of points are 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.
Using a Digital Source
To use ai/StartTrigger with a digital source, specify a source and an edge.
The source can be any of the following signals:
•
Input PFI <0..5>
•
RTSI <0..7>
•
Counter n Internal Output
•
PXI_STAR
The source also can be one of several other internal signals on your DAQ
device. Refer to Device Routing in MAX in the NI-DAQmx Help or the
LabVIEW 8.x Help for more information.
You also can specify whether the measurement acquisition begins on the
rising edge or falling edge of ai/StartTrigger.
Routing AI Start Trigger to an Output Terminal
You can route ai/StartTrigger out to any output PFI <6..9> or RTSI <0..7>
terminal.
The output is an active high pulse.
The device also uses ai/StartTrigger to initiate pretriggered DAQ
operations. In most pretriggered applications, a software trigger generates
ai/StartTrigger. Refer to the
AI Reference Trigger Signal
4-22
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