HP 54645A User's And Service Manual page 279

Messages
Threshold is at limit
The threshold setting is used by the HP 54645D oscilloscope digital channel
input comparators to decide whether a signal is a logic high or logic low. The
threshold must be set to a value between –6.00 V and +6.00 V inclusive.
Time/Div is at limit
The time per division must be set to a value between 50 s/div and 2 ns/div
inclusive.
Use 'Recover Screen' in Setup menu to restore the display
If you inadvertently change the Volts/Div on a given analog channel, that
analog channel will disappear from the display. If you change the
Volts/Div back to the original setting, the last scope display of that
channel re-appears on the screen. You can also choose to recover the
, then the Recover Screen softkey.
screen by pressing
Setup
10-8