HP 16534A User Reference page 78

The Trigger Menu
When Field
Present > When this field is active, the scope triggers on the first
transition that causes the pattern specification to be false for every input
used in the pattern trigger specification if the specified pattern has been
true for the time duration specified. If the pattern specification becomes
false before the specified duration time has elapsed, the search for a
trigger condition starts again. If the pattern specification remains true
longer than the specified duration time, the trigger point will be the
point at which the pattern specification becomes false.
The pattern duration time can be any value between 20 ns and 160 ms in
10 ns steps.
If the count set in the Count field is one, the trigger event will be the first
pattern event that meets both the pattern specification and the duration
specification. If the count is greater than one, only the first pattern event
must meet the duration specification. Once the pattern duration
specification has been met, subsequent pattern events that meet the pattern
specification can be of any duration and each such pattern event will
contribute to meeting the count specification.
Example
With a pattern specification of HXX, a duration specification of >100 ns, and
a count of 3; a pulse string with pulse widths 80 ns, 150 ns, 50 ns, 75 ns, 20
ns, 200 ns would trigger on the trailing edge of the 75 ns pulse. In this
example, the 150 ns pulse meets the duration specification and is count 1,
the 50 ns pulse is count 2, and the 75 ns pulse is count 3.
Present < When this field is active, the scope triggers on the first
transition that causes the pattern specification to be false for every input
used in the pattern trigger specification if the specified pattern has been
true for less than the time duration specified. If the pattern specification
remains true after the specified duration time has elapsed, the search for
a trigger condition starts again the next time the pattern specification
becomes true. If the pattern specification becomes false before the
specified duration time, the trigger point will be the point at which the
pattern specification becomes false.
The pattern duration time can be any value between 20 ns and 160 ms in
10 ns steps.
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