You have to add a Marker event to your script and then specify the signal identifier on the Export Signal VI. From the Digital Waveform Generator/Analyzer Help:
Generate the waveform and generate a Marker event when a position(s) within the waveform is generated:
generate <waveform name> marker0 (<position 1>, <position 2>, ... , <position n>)
Specify each position in samples. Use the Export Signal VI/function for your driver to specify the destination terminal of the marker. Marker position is zero-based. For example, 0 refers to the first point in the waveform, 999 refers to the 1,000th point in the waveform, and so on.
Examples (assume myWfm has 1,024 samples):
Generate myWfm and generate a marker at the start of the waveform (sample 0):
generate myWfm marker0 (0)
I put the marker on 1st sample and checked the response. You were right about early triggering. Now I can compare the correct samples. Thanks very much for your help!
@Kyle: Hey, thanks for explaining in detail how to place a marker on a particular sample. I really appreciate your help.