Safety Tags

The controller supports the use of both standard and safety tags in the same project. 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 type
  • Data type
  • Scope
  • Class
  • Style
  • External Access
  • If the tag value is a constant
The
Studio 5000 Logix Designer®
 application helps prevent the direct creation of invalid tags in a safety program. If invalid tags are imported, they cannot be verified.
IMPORTANT: You cannot create a standard alias tag of a safety tag. Instead, standard tags can be mapped to safety tags using safety tag mapping. See Standard Tags in Safety Routines.
The Logix Designer application can write to safety tags directly via 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. HMI devices can have read-only access to safety tags depending on the External Access setting.

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.
Logix controllers contain predefined data types for use with specific instructions. Safety tags can be composed of the following:
  • All primitive data types (for example, BOOL, SINT, INT, DINT, LINT, REAL)
  • 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 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 safety tags can be read by either standard or safety logic or external communication devices, but can be written by only safety logic
    or another GuardLogix controller
    .
  • Program-scoped safety tags can be read by external communication devices, but only local safety routines within the safety program can write to them.
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. When you create controller-scoped tags, you must manually select the tag class.
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:
  • Multiple programs in the project
  • In safety tag mapping
  • To produce or consume data
Controller-scoped safety tags can be read, but not written to, by standard routines.
IMPORTANT: Safety input tags
and safety consumed 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.
Provide Feedback
Have questions or feedback about this documentation? Please submit your feedback here.
Normal