National Instruments NI USB-621x - Manual (Page 47)

Chapter 4 Analog Input
© National Instruments Corporation 4-21 NI USB-621x User Manual
AI Hold Complete Event Signal
The AI Hold Complete Event (ai/HoldCompleteEvent) signal generates a
pulse after each A/D conversion begins. You can route
ai/HoldCompleteEvent out to any PFI <4..8> or PFI <12..15> terminal.
The polarity of ai/HoldCompleteEvent 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.
Once 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/StartTrigger with a digital source, specify a source and an edge.
The source can be any of the following signals:
• 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 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 PFI <4..8> or PFII <12..15>
terminal.
The output is an active high pulse.