Chapter 4

Analog Input

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.

© National Instruments Corporation

4-21

NI USB-621x User Manual