Lincoln;
If you use digital trigger to trigger the Analog input acquisition of, at the first event of a digital TTL compliant signal at the PFI line, the acquisition will start. Unfortunately, there isn't much you can do about that.
Regarding the second question, you will need, at least, to check if any data was acquired at the output waveform array. You don't need to graph that data, but you will need to check if there is a valid data in the buffer.
Hope this helps.
Filipe