Rockwell Automation Logix5000 Controllers I/O and Tag Data Programming Manual - Manual (Page 77)

Data access control Chapter 4
Use FactoryTalk security to control who is permitted to change values of
constants and who can change the constant attribute of a tag. To change the value
of a constant, you must have the Tag: Modify Constant Tag Values permission.
To change 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.
Tip: 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.
This section describes the various ways you can configure a constant attribute for a
tag.
Set up a constant in the New Tag dialog box
Follow these steps to configure a tag as a constant.
1. On the Controller Organizer, right-click Controller Tags and then click
New Tag.
Configure constant tags
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