National Instruments Data Acquisition Device NI USB-621x - Manual (Page 53)

Chapter 4 Analog Input
NI USB-621x User Manual 4-20 ni.com
The polarity of AI Hold Complete Event is software-selectable, but is
typically configured so that a low-to-high leading edge can clock external
AI multiplexers indicating when the input signal has been sampled and can
be removed.
AI Start Trigger Signal
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:
• 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 Start Trigger with a digital source, specify a source and an edge.
The source can be any of the following signals:
• (USB-6210/6211/6215 Devices) PFI <0..3>
• (USB-6212/6216 Devices) PFI <0..15>
• (USB-6218 Devices) PFI <0..3>, PFI <8..11>
• Counter n Internal Output
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 Help in version 8.0 or later for more information.
You also can specify whether the measurement acquisition begins on the
rising edge or falling edge of AI Start Trigger.
Routing AI Start Trigger to an Output Terminal
You can route AI Start Trigger out to any output PFI terminal.
The output is an active high pulse.
The device also uses AI Start Trigger to initiate pretriggered DAQ
operations. In most pretriggered applications, a software trigger generates
AI Start Trigger. Refer to the
AI Reference Trigger Signal section for a