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clarification regarding the AI START vi

i feel that AI START vi starts the data acquisition depending on the trigger conditions.
now ,if the trigger condition is say - Digital trigger and the number of pretrigger scans is 50, then i beleive the acquition would start only after the triggering signal is received.
if, the acquistion starts only after the triggering signal is received , how can the pretrigger samples be acquired. i mean the acquistion has not started before the trigger. so i beleive we effectively cannot acquire the pre trigger samples.
i know there is some link i am missing . is there any buffer on the daq board which acquires data continuously and transfers the data to the main memory only after the trigger is recei
ved?if yes, then what does the AI READ do?
can any body clarify this doubt.
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Santosh,

Here is a link that explains how triggering works:

http://zone.ni.com/devzone/devzoneweb.nsf/opendoc?openagent&F48B108C254C7BEA8625687000580A3F&cat=327C75ED77E1B8E5862567E80051CF83

Basically, when you set the number of pretrigger scans >0 in AI Start it sets up the trigger as a stop trigger and by default the hardware will look at PFI1/TRIG2 (if an E-Series board) for the trigger signal.
When a stop trigger is configured LabVIEW will start reading data into a circular buffer when AI Start executes. When a trigger signal on the specified trigger line is received the acquistion is stopped and you can read off the correct number of pretrigger and post trigger scans from the circular buffer.

If the number of pretrigger scans is not wired up or is 0 then th
e trigger is step up for a start trigger.
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