Publication 1756-PM004C-EN-P - October 2009 79
Data Access Control Chapter 4
Constant Value Tags
In RSLogix 5000 software version 18 and later, 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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