Rockwell Automation FactoryTalk Historian SE 4.0 Live Data Interface User Guide - Manual (Page 27)

Point Configuration Chapter 5
Rockwell Automation Publication HSELD-UM024A-EN-E–September 2013 27
The ExDesc (Extended Descriptor) is a string attribute. Typically,
this attribute is used to implement Trigger Input points.
For example: If a PI point has the ExDesc attribute "EVENT='Tag1'
Anychange," it means trigger on any change as long as the Tag1 value
of the current event is different from the value of the previous event.
The ExDesc field allows a maximum of 1023 characters.
An output point is associated with a trigger point by setting the
SourceTag attribute of the output point equal to the tag name of the
trigger point.
For more information, see "Output Points (page 29)".
For each data point, you can set the following attributes to configure
its compression testing specification:
Item Description
Compression
Deviation (CompDev)
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 then
adjust CompDev, if necessary.
Note: 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 most
flows, pressures, and levels, use a deviation specification of 1% or
2% of span. For temperatures, the deviation should usually be 1 or
2 degrees.
ExDesc
Length
SourceTag
Compression Testing