HP 54645A User's And Service Manual page 115

Run/Stop/Single/Autostore/Erase Operation
Memory Depth/Record Length
Given that this is the case, we now have two options:
We could acquire a full 500,000 points for every acquisition, resulting in
some samples being "off screen."
We could decrease the acquired record size, only filling a fraction of the
available 500,000 points.
When continuously running, the scope will automatically decrease the record
sizes at the faster time base settings to further increase screen update rate.
Acquiring off-screen data takes time and decreases the number of waveforms
per second to the screen.
Unfortunately, this approach would result in smaller records for subsequent
pan-and-zoom explorations. To address this shortcoming, when the Stop key
is pressed, the scope reconfigures itself to take one last acquisition using all
of the available memory (500,000 points) if another trigger is found. If,
indeed, one more trigger is found within a short period of time (~ 100 ms),
the scope can use the longer record for pan/zoom. If a trigger is not found,
the shorter record must be used.
In summary:
Single acquisitions always acquire 1M of points.
Running acquisitions that are aborted by pressing Stop will usually
result in 500,000 points. To have a smaller record, the instrument would
have to be set to a sweep speed of 200 s/div or faster, and have a
trigger rate of less than 10 Hz.
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