Publication 1756-PM004C-EN-P - October 2009 85
Data Access Control Chapter 4
Add-On Instructions
Constant Value
Considerations
The Constant attribute applies only to InOut parameters. The default setting
of the property will be ‘not a Constant Value’.
The Constant attribute will not apply to Input, Output, EnableIn and
EnableOut Add-On Instruction parameters. It will not apply to Add-On
Instruction Local tags.
By denoting an InOut parameter of an Add-On Instruction as a constant, it
means that within the Add-On Instruction, that parameter cannot be written
to. The project will fail verification if this type of write is attempted.
Appropriate usage of Constant tags is monitored by logic verification.
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