Chapter 4 Analog Input© National Instruments Corporation 4-21 NI USB-621x User Manualcomplete description of the use of AI Start Trigger and AI ReferenceTrigger in 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 anda number 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.After 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, goto ni.com/info and enter the info code rdcanq.When the reference trigger occurs, the DAQ device continues to writesamples to the buffer until the buffer contains the number of posttriggersamples desired. Figure 4-15 shows the final buffer.Figure 4-15. Reference Trigger Final BufferReference TriggerPretrigger SamplesComplete BufferPosttrigger Samples
