HP 54620A User's And Service Manual page 67

Making Analyzer Measurements
Trigger types
Pattern Trigger
In pattern trigger, you define a pattern of highs, lows, and don't care inputs
that must be recognized across the input channels during any given input
sample. The pattern may be combined with one edge on any input channel to
form the complete trigger specification.
Pattern trigger is best when there is a unique pattern that occurs across a
group of signals, and the pattern defines the events you want to capture. For
example, suppose you have a state machine that outputs a series of hex
digits, and only outputs the digit 'A' once in every sequence. You can use a
pattern trigger to capture this event.
Pattern trigger is less useful when the same pattern occurs many times, and
most of those occurrences have little to do with the events you wish to
capture. For example, suppose that this same state machine generates
hundreds of states, and the state "C" occurs several times in the sequence,
with a different sequence of states after each occurrence. A pattern trigger
will not provide a stable waveform display.
The pattern trigger occurs when the pattern is entered; that is, the trigger
condition is satisfied as soon as the input waveforms have transitioned from a
state not matching the pattern to a state matching the pattern. You can use
the single edge to further qualify the trigger condition. For example, you may
want the analyzer to trigger when a certain pattern is present and a clock
edge occurs.
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