Setting the Fetch Relative To attribute to Start sets the starting fetch position to be the first sample acquired by the digitizer. If the trigger happens immediately, the first point sampled by the digitizer would equal the first pretrigger point. However, this is generally not true because the digitizer usually has to wait for a trigger. While the digitizer waits, it continues to sample data, possibly forever. As it samples data into the circular, onboard memory, the original data is eventually overwritten. As in other cases, if you attempt to fetch data that is overwritten, NI-SCOPE returns an error. A typical use of fetching relative to start is to acquire nontriggered waveforms at precise intervals. You can use the digitizer sample clock to precisely time the duration between waveforms. Suppose you want 500 points every millisecond, while sampling at 100 MS/s. You can set the Fetch Relative To attribute to start at the beginning of your program. Then, for every iteration of a loop, call a Fetch function to retrieve 500 samples, and increment the Fetch Offset attribute by 100,000 samples. Because 100,000 samples are acquired every millisecond when sampling at 100 MS/s, this program is effectively using the sample clock to precisely time the interval between acquired waveforms.