Rockwell Automation 1756-L7x GuardLogix 5570 Controllers - Manual (Page 86)

86 Rockwell Automation Publication 1756-UM022A-EN-P - November 2012
Chapter 6 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 27 - 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 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 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.
Table 28 - Produced and Consumed Connections
Tag Connection Description
Produced A GuardLogix controller can produce (send) safety tags to other 1756 or 1768 GuardLogix
controllers.
The producing controller uses a single connection for each consumer.
Consumed GuardLogix controllers can consume (receive) safety tags from other 1756 or 1768 GuardLogix
controllers.
Each consumed tag consumes one connection.