We need to initiate the acquisition with an external trigger (positive slope) and finish it with negative slope, freeing the computer to execute other VIs. It`s necesary to have pretrigger and posttrigger scans.
You can look at this tutorial about triggering and at the related documents: http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/3870d265312a43a3862562c20050b6e4?OpenDocument