Allen-Bradley GuardLogix 5570

108 Rockwell Automation Publication 1756-UM022D-EN-P - May 2017
Chapter 6 Develop Safety Applications
Class
Tags can be classified as standard or safety. Tags classified as safety tags must
have a data type that is permitted for safety tags.
When you create program-scoped tags, the class is automatically specified,
depending upon whether the tag was created in a standard or safety program.
When you create controller-scoped tags, you must manually select the tag class.
Constant Value
When you designate a tag as a constant value, it cannot be modified by logic in
the controller, or by an external application such as an HMI. Constant value
tags cannot be forced.
The Logix Designer application can modify constant standard tags, and safety
tags provided a safety task signature is not present. Safety tags cannot be
modified if a safety task signature is present.
External Access
External Access defines the level of access that is allowed for external devices,
such as an HMI, to see or modify tag values. Access via the Logix Designer
application is not affected by this setting. The default value is read/write.
Table 33 - External Access Levels
For alias tags, the External Access type is equal to the type configured for the
base target tag.
External Access Setting Description
None Tags are not accessible from outside the controller.
Read Only Tags can be browsed or read, but not written to from outside the
controller.
Read/Write Standard tags can be browsed, read, and written to from outside the
controller.