Start AND stop triggering

[Joey Oravec]

Hi,

I am trying to do a start-and-stop trigger (ie. record while "the button" is
held down by a user). There's a great explanation at

http://sine.ni.com/apps/we/niepd_web_display.DISPLAY_EPD4?p_guid=B123AE0CBB00111EE034080020E74861&p_node=DZ52308&p_submitted=N&p_rank=&p_answer=&p_source=External

As mentioned in many NI articles, the AI Control vi can set the # of
pretrigger scans. In the case of start/stop triggering, the parameter "min #
of pretrigger scans" specifies the minimum number of samples which must be
acquired before a the stop trigger is accepted. The example starts
immediately, then

Instead I wanted to pre-trigger in the classical sense, meaning capture X
samples before the start trigger is received, everything in the middle, an
d
end when the stop trigger is received. Is this possible? The example uses
read/search position input of AI Read, but I don't see a way to get data
from before the start trigger!

-joey

[rgharrison]

Hi Joey,

Unfortunately, this can't be done. As you know, the Start Trigger is an actual start trigger that begins the acquisition immediately. The Stop Trigger is more of a "Reference" trigger in that it provides a point at which you acquire a specific number of points before the "Reference" point and a certain number after the "Reference" point.

The problem lies in that you need a Stop Trigger to get the pre-trigger values and then you need another trigger afterwards to end the acquisition. You can't configure 2 Stop Triggers. Hope that clears things up a bit.

Ron