User Guide User Manual

Principles of Operation
12
How FactoryTalk Historian to Historian Finds Source Points
When the FactoryTalk Historian to Historian Interface loads a point, the interface must
identify the source point from which data will be collected. In other parts of this manual, this
process is referred to as mapping the interface point to a source point.
The interface uses four receiving point attributes as possible links to the source point:
InstrumentTag, ExDesc, UserInt1, and Tag. However, the FactoryTalk Historian to
Historian Interface /tn, /tnex, and /ptid parameters exclude one or more of these
attributes from mapping to the source point. For most interface instances, none of these
parameters are used. The following table summarizes the effect of these parameters on the
search for either source point tag name or point ID. The actual implementation of the search
is described below the table.
Interface parameters
Search order for attribute containing source point tag name or ID
/tn
/tnex
/ptid
No
No
No
1. Non-empty InstrumentTag attribute is source point tag name.
2. ExDesc attribute that contains STAG= specifies source point tag.
3. UserInt1 attribute greater than 0 contains source point ID.
(Point rejected if source Historian Server-level failover configured.)
4. Tag of the interface point is also source point tag name.
No
No
Yes
UserInt1 attribute contains source point ID.
(Illegal configuration if source Historian Server -level failover is
configured.)
No
Yes
No
1. ExDesc attribute that contains STAG= specifies source point tag.
2. Tag of the interface point is also source point tag name.
No
Yes
Yes
Tag of the interface point is also source point tag name.
Yes
N/A
N/A
The interface performs the following steps to find the source point:
1. If no /tn parameter and no /tnex parameter and no /ptid parameter and the
InstrumentTag attribute is not a zero-length string, the InstrumentTag value
contains the source tag name and the search ends. Otherwise, proceed to step 2.
2. If no /tn parameter and no /ptid parameter and the ExDesc attribute begins with
case-insensitive “STAG” followed by zero or more spaces followed by “=”, the
source tag name is extracted from the remainder of the ExDesc value (as described
in the following paragraph) and the search ends. Otherwise, proceed to step 3.
To extract the source tag name, the interface ignores any spaces following the “=”. If
the next character is not a double quotation mark ("), it is the first character of the
source tag name which extends to the first comma or to the end of the ExDesc
attribute. If the first non-space following the “=” is a double quotation mark, the
source tag name begins with the following character and extends to the first double
quotation mark or to the end of the ExDesc attribute. The interface extracts the
source tag name from ExDesc and the search ends.
3. If no /tn parameter and no /tnex parameter and either the UserInt1 attribute is
greater than zero or the /ptid parameter is present, the UserInt1 attribute is the