Transitional Mode Theory; 125-Mhz Transitional Mode - HP 1660CS-Series User Manual

Transitional Mode Theory

In Transitional acquisition mode, the timing analyzer samples data at regular
intervals, but only stores data when there is a transition on currently
assigned bits of a pod pair. Each time a level transition occurs on any of the
bits, all bits of the pod pair are stored. A time tag is stored with each stored
data sample so the measurement can be reconstructed and displayed later.
One issue when using transitional timing is how many transitions can be
stored. The number depends on the mode and frequency of transition
occurrence. The following overview explains the number of transitions stored
for each transitional timing mode and why.

125-MHz transitional mode

When the timing analyzer runs in the 125-MHz mode, it operates like the
state analyzer with Count Time turned on, the exceptions being that the
store qualification comes from transition detectors instead of the trigger
sequence and that the analyzer uses an internal clock.
With 4 K of memory per channel and Count Time selected, the analyzer uses
half its memory (2 K) to store time tags. Because each pod pair must store
transitions at its own rate, each pod pair must store its own set of time tags.
This is why you do not have the option of using a free pod to retain full
memory as you have in the normal state mode.
When a transition is detected after a sample with no detected transition, both
samples are stored. If the next sample also indicates a transition, only one
sample is stored. If the next sample does not indicate a transition, no sample
is stored this time and the next transition again stores two samples.
Minimum transitions stored
Sometimes transitions occur at a relatively slow rate, slow enough to ensure
at least one sample with no transitions between the samples with transitions.
This is illustrated in the figure on the next page with time tags 2, 5, 7, and 14.
When transitions happen at this rate, two cycles are stored for every
transition. This means that with 2 K of memory, 1 K (1024) samples are
stored. Subtract 1 for the starting point and you have a minimum of 1023
stored transitions.
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