Factors to Consider when Triggering

The use of triggering is a powerful enhancement to the capabilities of an image acquisition system. It allows you to efficiently capture short-duration and high-speed events by eliminating the need to continuously acquire images while waiting for the event to occur. In this case, each image would have to be transferred across the PCI bus, then processed to determine if the event had started or not. This results in increased use of system resources and limits the frame rate to that which can be continuously processed. If you can externally identify the beginning of the event, you can then acquire at high speed into the board's onboard memory for a duration just long enough to cover the entire event. This document explains the various types of triggering available for image acquisition.

Asynchronous Reset

Asynchronous reset is a feature provided by many high-end cameras. The term "asynchronous" refers to the fact that the response of the camera to an external input (the trigger) is immediate; that is, the camera "resets" and captures an image as soon as it receives a trigger. To understand this, let's think about how a triggered acquisition without asynchronous reset works:

1. Turn on the camera. It immediately begins acquiring images and outputting data.
2. Set up the frame grabber to do a triggered acquisition.
3. Start the acquisition. Since it is triggered, the frame grabber does not actually acquire images until the trigger is received.
4. Generate the trigger and supply it to the frame grabber. The camera has been continuously outputting data, so it is probably in the middle of transferring a frame when the trigger is received. The board will ignore the data until the beginning of the next frame, at which point it commences the acquisition.

There is a delay associated with this method since you must wait for the next full frame to start before the image is acquired. With asynchronous reset, the sequence is as follows:

1. Set the camera up in asynchronous reset mode; it immediately begins acquiring images and outputting data.
2. Set up the frame grabber to do a triggered acquisition.
3. Start the acquisition. Since it is triggered, the frame grabber does not actually acquire images until the trigger is received.
4. Generate the trigger and supply it to both the frame grabber and the camera. The camera immediately interrupts the acquisition in progress and captures the current image. The frame grabber begins acquiring the camera's output data as soon as it is generated.

   See Also:
   Asynchronous Reset with the IMAQ PCI/PXI-1408

   Trigger Source
   

The trigger for the frame grabber can come from an external source as a TTL input, or can be routed from another hardware device (typically a data acquisition, or DAQ, board) via the Real-Time System Integration (RTSI) bus.

Typical applications for a trigger generated by a DAQ board would involve monitoring a parameter, such as temperature or pressure, and acquiring images whenever a setpoint is exceeded or when the parameter reaches its valid operating range. External triggers often come from position encoders, strobe lights, limit switches, and so on.

See Also:
Integrating DAQ, IMAQ, and Motion
DAQ Triggering of Acquisitions

Trigger Action

The action taken by the frame grabber upon receipt of a trigger can be one of the following:

• No action (trigger disabled).
• Start a continuous acquisition.
• Acquire a single image for each trigger.
• Acquire a buffer list for each trigger.
• Acquire a single line for each trigger (used only with line scan cameras; the trigger source is normally a position encoder).

  See Also:
  Trigger Start of Acquisition
  Trigger Each Buffer of Acquisition
  

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