Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 24)

Historian Point Configuration
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The exception specification attributes are used to define exception-reporting specifications.
Exception reporting is basically a filter. Raw values that the interface receives are filtered if
they do not pass the exception-reporting specifications. If the raw values pass exception, they
are sent to the Snapshot, where they can be viewed by client applications such as
FactoryTalk Historian ProcessBook. Unless the /sn parameter is specified on the command-
line of the interface, standard exception reporting will be implemented. The interface
documentation must be consulted for exceptions to the rule.
ExcMin is the exception minimum time; ExcMax is the exception maximum time; and
ExcDev is the exception deviation dead band.
ExcDev is defined in terms of engineering units. ExcDev is related to ExcDevPercent by:
ExcDev = ExcDevPercent * Span / 100
where Span is defined by the Span Historian point attribute. If either ExcDev or
ExcDevPercent is changed, the other is automatically updated to be compatible. If both are
changed at once, ExcDevPercent takes precedence.
Raw values are said to ―pass exception‖ if:
 The difference between the new value and the last value that passed exception is greater
than ExcDev
and
the difference between the times of the new value and the last value that passed
exception is greater than ExcMin.
or
 The difference between the times of the new value and the last value that passed
exception is greater than ExcMax.
The last value that passed exception is called the ―old value.‖ The next value that passes
exception is called the ―new value.‖ The last value that was received by the interface before
the new value is called the ―previous value.‖ There will not be a ―previous value.‖ if the
interface did not receive a value between the ―old value‖ and the ―new value.‖ When a new
value passes exception, the previous value and the new value will be sent to the Snapshot.
The new value will then become the old value, and the cycle continues.
The time between exceptions can be greater than the ExcMax if no new values are received
by the interface for a point.
Note: Th e “previous value” will be sent to Historian even if it was received before ExcMin
seconds had expired. ExcMin applies only to the “new value.”
Compressing
If the compressing attribute is set to 1 (ON), then the following compression specifications
are used to filter the data that is passed from the Snapshot to the Archive.