Allen-Bradley 1756-CNB User Manual page 24

24 ControlLogix Redundancy System Overview
Parameter
Number of Projects
Controller Properties
Task Structure
I/O
Task Watchdog Time
Minimizing Scan Time
Data Integrity
Produced and Consumed
Tags
Message (MSG)
Instructions
Publication 1756-UM523F-EN-P - December 2006
RSLogix5000 Project
Criteria
Only one RSLogix 5000 project for the pair of redundant controllers. The project automatically
crossloads to the secondary controller when the secondary controller synchronizes with the
primary controller.
• ControlLogix5555, ControlLogix5561, ControlLogix5562, or ControlLogix5563 controller.
• Redundancy enabled.
• Only one task at the highest priority.
• If more than one task, all tasks periodic.
• Outputs that require a bumpless switchover are in the highest priority task.
• Requested packet interval (RPI) less than or equal to 375 milliseconds. (Larger RPIs may
produce a bump at switchover.).
Watchdog time ≥ (2 * maximum_scan_time) + 150 ms
where:
Maximum_scan_time is the maximum scan time for the entire task when the secondary
controller is synchronized.
• A few large programs instead of a lot of small programs.
• No unused tags.
• Arrays and user-defined data types instead of individual tags.
• User-defined data types as compact as possible.
• Code as compact as possible.
• Code runs only when you need it.
• Data grouped by how often you need it.
• DINT tags instead of SINT or INT tags.
Special treatment for:
• Bit Shift Left (BSL) and Bit Shift Right (BSR) instructions.
• FIFO Unload (FFU) instructions.
• logic that is scan dependent.
If you want a controller in another chassis to consume a tag from the redundant controller, use
a comm format of None. In the I/O configuration of the consuming controller, select a comm
format of None for the remote CNB module (the CNB that is physically in the redundant
chassis). This comm format of None is only available over a ControlNet network.
For any MSG instruction from a controller in another chassis to a redundant controller, cache
the connection.
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