Chapter 4 Analog Input© National Instruments Corporation 4-37 M Series User ManualUsing an Analog SourceWhen you use an analog trigger source, the acquisition begins on the firstrising edge of the Analog Comparison Event signal.Routing AI Start Trigger to an Output TerminalYou can route ai/StartTrigger out to any PFI <0..15> or RTSI <0..7>terminal.The output is an active high pulse.All PFI terminals are configured as inputs by default.The device also uses ai/StartTrigger to initiate pretriggered DAQoperations. In most pretriggered applications, a software trigger generatesai/StartTrigger. Refer to the AI Reference Trigger Signal section for acomplete description of the use of ai/StartTrigger and ai/ReferenceTriggerin a pretriggered DAQ operation.AI Reference Trigger SignalUse a reference trigger (ai/ReferenceTrigger) signal to stop a measurementacquisition. To use a reference trigger, specify a buffer of finite size and anumber of pretrigger samples (samples that occur before the referencetrigger). The number of posttrigger samples (samples that occur after thereference trigger) desired is the buffer size minus the number of pretriggersamples.Once the acquisition begins, the DAQ device writes samples to the buffer.After the DAQ device captures the specified number of pretrigger samples,the DAQ device begins to look for the reference trigger condition. If thereference trigger condition occurs before the DAQ device captures thespecified number of pretrigger samples, the DAQ device ignores thecondition.If the buffer becomes full, the DAQ device continuously discards the oldestsamples in the buffer to make space for the next sample. This data can beaccessed (with some limitations) before the DAQ device discards it. Referto the KnowledgeBase document, Can a Pretriggered Acquisition beContinuous?, for more information. To access this KnowledgeBase, go toni.com/info and enter the info code rdcanq.
