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AI Start Trigger Signal

You can 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, you begin a measurement with a software command. After the acquisition begins, you can 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)

Using a Digital Source

To use ai/StartTrigger with a digital source, you specify a source and an edge. The source can be an external signal connected to any PFI or RTSI <0..6> pin. The source can also be one of several internal signals on your DAQ device. Refer to Device Routing in MAX in the NI-DAQmx Help for more information.

Also, specify whether the measurement acquisition begins on the rising edge or falling edge of the ai/StartTrigger signal.

The following figure shows the timing requirements of the ai/StartTrigger source.

Using an Analog Source

When you use an analog trigger source, the acquisition begins on the first rising edge of the Analog Comparison Event signal.

Outputting the AI Start Trigger Signal

You can configure the PFI 0/AI START TRIG pin to output the ai/StartTrigger signal. The output pin reflects the ai/StartTrigger signal regardless of what signal you specify as its source.

The output is an active high pulse. The following figure shows the timing behavior of the PFI 0/AI START TRIG pin when the pin is an output.

The PFI 0/AI START TRIG pin is configured as an input by default.

When acquisitions use a start trigger without a reference trigger, they are posttrigger acquisitions because data is acquired only after the trigger. The device also uses ai/StartTrigger to initiate pretrigger DAQ operations. In most pretrigger applications, a software trigger generates ai/StartTrigger. Refer to AI Reference Trigger Signal for a complete description of the use of ai/StartTrigger and ai/ReferenceTrigger in a pretrigger DAQ operation.