Rockwell Automation Publication 1756-PM004D-EN-P - September 2012 77
Data Access Control Chapter 4
Constant Value Tags
Version 18 and later of the application, you can designate tags as constants to
protect them from being changed programatically via:
• the controller programming application.
• logic in the controller.
Tags that cannot be designated as constants are User-defined type members,
Add-On Instruction input and output parameters, and local tags. A check mark
in the Constant box on tag creation dialog boxes and tag editor/monitor
windows indicates a ‘constant’ designation.
FactoryTalk security is used to control who is permitted to modify values of
constants and who can modify the constant attribute of a tag. To change the value
of a constant, you must have the Tag: Modify Constant Tag Values permission.
To modify the constant attribute of a tag, you must have the Tag: Modify
Constant Property permission.
For details on setting permissions, see the FactoryTalk Security System
Configuration Guide, publication FTSEC-QS001.
For an alias tag, the default constant setting of this tag is the same as its target tag.
For all other conditions, the default value is unchecked, indicating the tag is not a
constant value tag.
When you designate an InOut parameter as a constant, it cannot be written to
within the Add-On Instruction.
You cannot pass a constant value tag as an argument to an Output parameter of an
Add-On Instruction. You cannot pass a constant tag to an InOut parameter that is
not also designated as a constant value.
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