Rockwell Automation FactoryTalk Historian SE 3.0 Live Data Interface User Guide - Manual (Page 29)

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Point Configuration
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InstrumentTag
InstrumentTag contains the ItemID of the tag. The format of this field
depends on the FactoryTalk Live Data server:
Type of the tag InstrumentTag syntax
Device tags <application name>/<area name>:<data server name>:[<shortcut
name>]<tag name>
HMI tags <application name>/<area name>:<HMI server Name>:<folder
name>\<tag name>
The field must match the point defined on the FactoryTalk Live Data
server, including punctuation, spaces, and case.
Length
InstrumentTag field allows a maximum of 1023 characters.
The FTLD interface gets the $Global scope from the /FTDirectory
parameter and / from the /FTContext parameter. For additional information
on these parameters, see Command-line Parameters (page 37).
ExDesc
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.
Length
The ExDesc field allows a maximum of 1023 characters.