The query returns an integer.
 -2 denotes that the instrument is not triggered and there is no trigger position.
 -1 denotes the instrument is triggered outside the internal memory; namely, at this
point, users cannot set the instrument to read the data in the internal memory
starting from the trigger position.
 An integer that is greater than 0 denotes that the return value is the position in the
internal memory that corresponds to the trigger position.