HP 54645A User's And Service Manual page 100

MegaZoom Concepts
Display Modes
Displaying Analog Channels After the Acquisition
The two scope channels are always acquired, so you can turn them on
and look at them even if they were off during the acquisition. You can do
math functions on channels that are turned off, but you cannot make
measurements on them. If you are using cursors to make measurements
manually, be sure you have the correct channel selected; it is easy to
mistakenly measure the peak voltage of channel 1 with the Volts/Div and
Position of channel 2.
The combination of events and displayed digital channels is described in the
topic "To turn digital channels on and off (HP 54645D)" in chapter 2.
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