Rockwell Automation FactoryTalk Historian SE 3.0 Live Data Interface User Guide - Manual (Page 32)

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FactoryTalk Historian Live Data Interface User Guide
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Trigger Method 1 (Recommended)
For Trigger Method 1, a separate trigger point must be configured. The
output point must have the same point source as the interface. The trigger
point can be associated with any point source, including the point source of
the interface. Also, the point type of the trigger point does not need to be
the same as the point type of the output point.
The output point is associated with the trigger point by setting the
SourceTag attribute of the output point equal to the tag name of the trigger
point. An output is triggered when a new value is sent to the Snapshot of
the trigger point. The new value does not need to be different than the
previous value that was sent to the Snapshot to trigger an output, but the
time stamp of the new value must be more recent than the previous value.
If no error is indicated, then the value that was sent to the trigger point is
also written to the output point. If the output is unsuccessful, then an
appropriate digital state that is indicative of the failure is usually written to
the output point. If an error is not indicated, the output still may not have
succeeded because the interface may not be able to tell with certainty that
an output has failed.
Trigger Method 2
For Trigger Method 2, a separate trigger point is not configured. To trigger
an output, write a new value to the Snapshot of the output point itself. The
new value does not need to be different than the previous value to trigger
an output, but the time stamp of the new value must be more recent than
the previous value.
Trigger Method 2 may be easier to configure than Trigger Method 1, but
Trigger Method 2 has a significant disadvantage. If the output is
unsuccessful, there is no tag to receive a digital state that is indicative of
the failure, which is very important for troubleshooting.