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Rockwell Automation Publication 1769-UM022C-EN-P - June 2018
Chapter 9
Develop Safety Applications
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 26 - External Access Levels
For alias tags, the External Access type is equal to the type configured for the
base target tag.
Produced/Consumed Safety
Tags
To transfer safety data between Compact GuardLogix controllers, you use produced
and consumed safety tags. Produced and consumed tags require connections. The
default connection type for produced and consumed tags is unicast.
Produced and consumed safety tags are subject to the following restrictions:
• Only controller-scoped safety tags can be shared.
• Produced and consumed safety tags are limited to 128 bytes.
• Produced/consumed tag pairs must be of the same user-defined data
type.
• The first member of that user-defined data type must be the predefined
CONNECTION_STATUS data type.
• The requested packet interval (RPI) of the consumed safety tag must
match the safety task period of the producing GuardLogix controller.
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.
Table 27 - Produced and Consumed Connections
Tag
Connection Description
Produced
A GuardLogix or Compact GuardLogix controller can produce (send) safety tags to other
GuardLogix or Compact GuardLogix controllers.
The producing controller uses a single connection for each consumer.
Consumed
GuardLogix or Compact GuardLogix controllers can consume (receive) safety tags from other
GuardLogix or Compact GuardLogix controllers.
Each consumed tag consumes one connection.
