HP 54645A User's And Service Manual page 63

Triggering HP 54645A/D Oscilloscopes
To select a trigger mode: Auto Lvl, Auto, Normal
When you select Single, the oscilloscope will fill acquisition memory, and
continue flowing data through the pre-trigger buffer until the auto trigger
overrides the searching and commands a trigger. At the end of the trace, the
scope will stop and display the results.
Normal mode
Use Normal trigger mode for low repetitive-rate signals. In this mode, the
oscilloscope has the same behavior whether the acquisition was initiated by
pressing Run/Stop or Single.
When the trigger event is found, the oscilloscope will fill the post-trigger
buffer and display the acquisition memory. If the acquisition was initiated
by , the process repeats. The waveform data will be
Run/Stop
scrolled onto the display as it is being acquired.
The oscilloscope begins filling the pre-trigger buffer with data. As soon as
that buffer is full, the oscilloscope will begin searching for the trigger event,
and will flash the trigger condition indicator on the status line to indicate it is
doing so. While searching for the trigger, the oscilloscope overflows the
pre-trigger buffer; the first data put into the buffer is the first pushed out
(FIFO).
In either Auto or Normal mode, the trigger may be missed completely under
certain conditions. This is because the oscilloscope will not recognize a
trigger event until the pre-trigger buffer is full. Suppose you set the Time/Div
knob to a slow sweep speed, such as 500 ms/div. If the trigger condition
occurs before the oscilloscope has filled the pre-trigger buffer, the trigger will
not be found. If you use Normal mode and wait for the trigger condition
indicator to flash before causing the action in the circuit, the oscilloscope will
always find the trigger condition correctly.
Some measurements you want to make will require you to take some action
in the circuit under test to cause the trigger event. Usually, these are
single-shot acquisitions, where you will use the Single key.
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