Rockwell Automation 1756-SPESMNRMXT GuardLogix Controllers User Manual - Manuel (Page 96)

96 Rockwell Automation Publication 1756-UM020I-EN-P - August 2012
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.
RSLogix 5000 software 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 RSLogix 5000 software is not
affected by this setting. The default value is read/write.
Table 28 - 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 may be browsed or read, but not written to from outside the
controller.
Read/Write Standard tags may be browsed, read, and written to from outside the
controller.