With fixed sampling, there is a deterministic relation between the sampling
time at MOD-IN and the edge at the TRIGGER-OUT connector. The external
data is sampled at fixed intervals. The latency of the external pattern
through the 81150A / 81160A is deterministic. This mode shall be used
whenever the device that provides the external data pattern can either be
driven with an external clock that is provided by the 81150A / 81160A (via
TRIGGER-OUT) or it provides a clock signal together with the data, which
can be used to trigger the 81150A / 81160A via the External-In connector. Or
in other words, when the timing of pattern source can be tightly coupled
with the 81150A / 81160A, then fixed sampling is possible.
With automatic sampling, the 81150A / 81160A is monitoring the signal at
MOD-IN to detect an initial transition. After this transition occurred, the
incoming data is sampled at the data rate that is set as frequency value in
the instrument. If there are no level changes for more than 8 unit intervals,
the initial transition search is re-started. This sampling method shall be used
for pattern sources that cannot be coupled with the 81150A / 81160A based
on a clock signal, or that send the data in 'packets' separated by a period of
inactivity between the data packets.
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