Safety Tags and Parameters
The controller supports the use of both standard and safety tags in the same application.
However, the programming software operationally differentiates standard tags from safety tags.
Safety tags have the same attributes as standard tags with the addition of mechanisms that
are certified to provide SIL 2/PLd and SIL 3/PLe data integrity.
When you create a tag, you assign the following properties:
- Name
- Description (optional)
- Tag usage
- Data type
- Scope
- Class
- Style
FactoryTalk® Design Studio™
helps prevent the direct creation of invalid tags in a
safety program. Invalid tags produce validation errors.FactoryTalk® Design Studio™
can write to safety tags online directly with the Tag
Monitor when the controller is safety-unlocked, does not have a safety signature, and is
operating without safety faults.The controller does not allow writes to safety tag data from external human machine interface
(HMI) devices or via message instructions from peer controllers.
Valid Data Types
The data type defines the type of data that the tag stores, such as bit or integer.
Data types can be combined to form structures. A structure provides a unique data type that
matches a specific need. Within a structure, each individual data type is called a member.
Like tags, members have a name and data type. You can create your own structures, such as
arrays or user-defined data types.
Safety controllers contain predefined data types for use with specific instructions. Safety
tags can be composed of the following:
- All elementary data types (BOOL, SINT, USINT, INT, UINT, DINT, UDINT, LINT, ULINT, REAL, LREAL)
- Predefined types used for safety application instructions
- User-defined data types or arrays that are composed of the two preceding types
Scope
The scope of a tag determines where you can access the tag data. When you create a tag, you
define the scope as a controller tag (global data), program tag for a specific safety or
standard program (local data), or routine tag for a specific standard or safety routine.
Safety tags can be safety controller-, program-, or routine-scoped.
IMPORTANT:
Routine-scoped safety tags require that the safety
controller run firmware revision 38 or higher.
Controller-scoped Tags
Standard and safety logic can read controller-scoped safety tags, but only safety logic can write such tags.
When you create controller-scoped tags, you must manually select the tag class.
When safety tags are controller-scoped, all safety programs have access to the safety data.
Tags must be controller-scoped if they are used in multiple programs in the application.
Controller-scoped safety tags can be read, but not written to, by standard routines.
IMPORTANT:
Safety input tags are readable by any standard
routine, but the update rate is based on the execution of the safety task. These tags are
updated at the beginning of the safety task execution, which differs from standard tag
behavior.
Program-scoped Tags
External communication devices can read program-scoped safety tags, but only local safety
routines within the safety program can write such tags.
When you create program-scoped tags, the class is automatically specified, depending on
whether you created the tag in a standard or a safety program.
Routine-scoped Tags
Routine-scoped tags are only accessible to the routine. The JSR instruction is used to pass
data in and out of the routine parameters. Only safety controllers with revision 38 or later
support routine-scoped safety tags.
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