HP 54645A User's And Service Manual page 108

When we pan the display (move it horizontally) back and set the delay to
15 s, the trigger point is displayed on the screen. The part of the signal we
see displayed at the center of the screen is to the right of the trigger point.
The result resembles:
Figure 4-10
The trigger event
is displayed on the
screen. Center
screen still shows
part of the signal
to the right of the
trigger.
Panning the Display to View the Start of Acquisition
Notice the signal on A2 changing as we pan through the waveform.
Panning through the waveform is a useful way to see the relationship
between the trigger point and relative locations in acquisition memory. If you
start a new acquisition after stopping and panning, the delay value to which
you panned will be used in determining the trigger location for the new
acquisition. For example, if you acquire data with a delay value of 100 s,
then stop and pan the display to 50 s, the new acquisition will position the
trigger 50 s before the time reference point.
MegaZoom Concepts
Changing the time reference position
4-25