Rockwell Automation Logix5000 Controllers I/O and Tag Data Programming Manual - Manual (Page 81)

Data access control Chapter 4
Dialog Box/Window Considerations
Tag Monitor You can change the value of a constant tag in the Tag Monitor if you have both standard Tag: Modify Values permission and Tag: Modify Constant
Tag Values permission. You cannot change a constant value in any of the language editors or any other tag browser. The icon in the Value
column indicates that you are changing a constant value tag’s value. Any change to the values of a constant tag is recorded in the Controller Log for
future reference.
For controller logging, see the
Logix5000 Controllers Information and Status Programming Manual , publication 1756-PM015 .
(4) If the controller is in safety-locked mode, only the safety tags are unavailable, and the standard tags will have the same behavior as
in the unlocked mode. The Constant check box is unavailable in the Tag Properties dialog box only if the tag is a safety tag.
The Constant attribute applies only to InOut parameters. The default setting of
the property is not a Constant Value.
The Constant attribute does not apply to Input, Output, EnableIn and
EnableOut Add-On Instruction parameters. It does not apply to Add-On
Instruction Local tags.
If in an Add-On Instruction, you make an InOut parameter a constant, it means
that within the Add-On Instruction, nothing can write to that parameter. The
project fails verification if this type of write is attempted.
Appropriate usage of Constant tags is monitored by logic verification.
Add-on instructions
constant value
considerations
Rock well Automati on Publication 1756- PM004E -EN-P - October 2014 81