Rockwell Automation 1756-L6x_L7x ControlLogix System User Manual - Manual (Page 156)

156 Rockwell Automation Publication 1756-UM001O-EN-P - October 2014
Chapter 10 Develop Applications
Extended Properties
The Extended Properties feature lets you define more information, such as limits,
engineering units, or state identifiers, for various components within your
controller project.
Pass-through behavior is the ability to assign extended properties at a higher level
of a structure or Add-On Instruction and have that extended property
automatically available for all members. Pass-through behavior is available for
descriptions, state identifiers, and engineering units and you can configure it.
Configure pass-through behavior on the Project tab of the Controller Properties
dialog box. If you choose not to show pass-through properties, only extended
properties that have been configured for a given component are displayed.
Pass-through behavior is not available for limits. When an instance of a tag is
created, if limits are associated with the data type, the instance is copied.
You must know which tags have limits that are associated with them as there is no
indication in the tag browser that extended properties are defined for a tag. If,
however, you try to use extended properties that have not been defined for a tag,
the editors show a visual indication and the routine does not verify.
Access Extended Properties in Logic
You can access limits that are defined on tags by using the .@Min and .@Max
syntax:
• You cannot write to extended properties values in logic.
• To use extended tag properties in an Add-On Instruction, you must pass
them in as input operands to the Add-On Instruction.
• Alias tags that have extended properties cannot access the extended
properties in logic.
• Limits can be configured for input and output parameters in Add-On
Instructions. However, limits cannot be defined on an InOut parameter of
an Add-On Instruction.
• Limits cannot be accessed inside Add-On Instruction logic. Limits are
only for use by HMI applications.
Component Extended Properties
Tag In the tag editor, add extended properties to a tag.
User-defined data type In the data type editor, add extended properties to data types.
Add-On Instructions In the properties that are associated with the Add-On Instruction definition, add
extended properties to Add-On Instructions.