Allen-Bradley GuardLogix 5570, User Manual

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Rockwell Automation Publication 1756-UM022D-EN-P - May 2017 107
Develop Safety Applications Chapter 6
Scope
A tag’s scope determines where you can access the tag data. When you create a
tag, you define it as a controller tag (global data) or a program tag for a specific
safety or standard program (local data). Safety tags can be controller-scoped or
safety program-scoped.
Controller-scoped Tags
When safety tags are controller-scoped, all programs have access to the safety
data. Tags must be controller-scoped if they are used in the following ways:
• More than one program in the project
• To produce or consume data
• To communicate with a PanelView™ terminal
• In safety tag mapping
See Safety Tag Mapping on page 116 for more information.
Controller-scoped safety tags can be read, but not written to, by standard
routines.
Tags associated with safety I/O and produced or consumed safety data must be
controller-scoped safety tags. For produced/consumed safety tags, you must
create a user-defined data type with the first member of the tag structure
reserved for the status of the connection. This member is a predefined data
type called CONNECTION_STATUS.
Program-scoped Tags
When tags are program-scoped, the data is isolated from the other programs.
Reuse of program-scoped tag names is permitted between programs.
Safety-program-scoped safety tags can only be read by or written to via a safety
routine scoped in the same safety program.
IMPORTANT Controller-scoped safety tags are readable by any standard routine. The
safety tag’s update rate is based on the safety task period.
Table 32 - Additional Resources
Resource Description
Safety Connections on page 143 Provides more information on the
CONNECTION_STATUS member
Logix5000™ Controllers I/O and Tag Data Programming
Manual, publication
1756-PM004
Provides instructions for creating user-defined
data types