Rockwell Automation 1756-HIST2G FactoryTalk Historian ME 2.2 - Manual (Page 235)

A • F ACTORY T ALK H ISTORIAN ME R ULE E DITOR
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Exception Maximum (ExcMax attribute)
Specifies a limit on how long the interface can go without reporting a value to
the FactoryTalk Historian ME server. After the ExcMax time period, the
interface sends the next new value to the server, regardless of whether the new
value is different from the last reported value.
For Digital or String points, only the exception maximum and exception minimum
attributes are important. FactoryTalk Historian ME ignores the exception deviation
specification for them.
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Compression Deviation (CompDev attribute)
Specifies in engineering units how much a value may differ from the previous
value before it is considered to be a significant value. As a rule of thumb, set
CompDev to the precision of the data source or hardware (instrument). Set it a
little "loose" to err on the side of collecting, rather than losing data. After
collecting data for a while, go back and check the data for your most important
tags and adjust CompDev if necessary.
Setting the CompDev attribute value too low causes too little data compression
and wastes space in the archive. Setting the value too high causes loss of useful
data. For typical flows, pressures, and levels, a typical deviation specification of
1% or 2% of Span is used. For temperatures, the typical deviation is usually 1 or 2
degrees. However, each installation is unique and these values should be tuned to
your specific application.
You can also specify Compression Deviation as a percentage of the Span
attribute. When Compression Deviation is expressed in percentage, it is known as
CompDevPercent attribute.
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Compression Minimum (CompMin attribute)
A point is archived if the elapsed time since the previous time a point was
saved is greater than or equal to the minimum time and the value has changed
by more than the deviation. For data points associated with interfaces that
send exception reports, set CompMin to 0.
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Compression Maximum (CompMax attribute)
A point is archived if the elapsed time since the previous time a point was
saved is greater than the maximum time. The recommended maximum time
See “Exception Filtering” for more information.