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Guidelines for Extended Tag
Properties
Chapter 4
Use the following guidelines for extended tag properties.
Guideline
Description
Use extended tag properties to define additional
information, such as limits, engineering units, or state
identifiers, for various components within your controller
project.
You can define extended tag properties for these components:
• Tag
• User-defined data type
• Add-On Instruction
Some extended tag properties support pass-through for
data structures and arrays.
Pass-through behavior is available for descriptions, state identifiers, and engineering units and is configurable in data
structures and arrays.
Pass-through behavior is not available for limits.
You can read extended properties via logic, but you
cannot write to extended properties values in logic.
• Extended properties must be used as an input operand.
• Alias tags with extended properties cannot be accessed in logic.
• Limits can be configured for input and output parameters in Add-On Instructions. However, limit extended properties
must not be defined on an InOut parameter of an Add-On Instruction.
• Limits cannot be accessed inside Add-On Instruction logic.
• If you read an extended property value in logic, it consumes memory equivalent to an equivalent program-scoped tag
of that data type. If you do not use them in logic, extended tag properties use no user memory, only extended
memory.
If an array tag uses indirect addressing to access limit
extended properties in logic, the following conditions
apply.
• If the array tag has limit extended properties that are configured, the extended properties are applied to any array
element that does not explicitly have that particular extended property configured. For example, if the array tag
MyArray has Max configured to 100, then any element of the array that does not have Max configured inherits the
value of 100 when used in logic. However, it is not visible to you that the value inherited from MyArray is configured in
the tag properties.
• At least one array element must have specific limit extended property configured for indirectly referenced array logic
to verify. For example, if MyArray[x].@Max is being used in logic, at least one array element of MyArray[] must have
Max extended property configured if Max is not configured by MyArray.
• Under the following circumstances a data type default value is used:
– Array is accessed programmatically with an indirect reference.
– Array tag does not have the extended property configured.
– A member of an array does not have the extended property configured.
Rockwell Automation Publication 1756-RM094I-EN-P - September 2015
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