Rockwell Automation Publication 1756-UM535D-EN-P - November 2012 173
Program the Redundant Controller Chapter 7
Use this table to when using MSG instructions during a switchover.
Table 30 - MSG Instruction Behavior During a Switchover
If the MSG instruction
is
Then
From a redundant controller In a redundant controller, any MSG instruction that is in progress during a switchover experiences an error.
(The ER bit of the instruction turns on.) After the switchover, normal communication resumes.
To a redundant controller For any MSG instruction from a controller in another chassis to a redundant controller, cache the connection:
Properties of the Message to the Redundant Controller
Configured Message Instructions
If the MSG instruction
originates from a
redundant controller
Then
During a switchover The message instructions status bits are updated asynchronously to the program scan. Consequently, you
cannot crossload your message instructions status bits to a secondary controller.
During a switchover, any active message instructions become inactive. When this occurs, you will need to
reinitialize the execution of your message instructions in the new primary controller.
During qualification The scrolling display changes from CMPT for compatible to Qfng for qualifying.
• If a configured message is cached, the primary controller automatically establishes a connection with no errors.
• If a configured message is uncached or unconnected, the primary controller receives Error 1 Extended Error 301, No Buffer
Memory.
If the message is targeted to a
redundant controller
Then
During the erroring out of a message All backplane communication ceases. This stoppage allows the redundant controller to receive the message
instruction required to perform a switchover or any diagnostics.
Important: If any of your messages are active during a switchover, you can expect one of these things to happen:
• Cached and connected messages cause the message instruction to pause for 7.5 seconds because the
initiating controller has not received a response from the targeted controller. For cached messages, the
message instruction tries to execute three more times, each attempt followed by a pause of 7.5 seconds.
If, after 30 seconds pass, the targeted controller does not respond to the initiating controller, then the
switchover errors out with connected time out Error 1 Extended Error 203.
An example of a connected message would be CIP data table read-and-write messages after a
connection has been established.
• Uncached messages error out after 30 seconds if you have just initiated them because the initiating
controller never received a reply to the forward-open request. The error is Error 1F Extended Error 204,
an unconnected time out.
Examples of uncached messages would include CIP generic messages and messages captured during the
connection process.
During qualification Cached messages run with no errors. A connection has been established.
Connected, but uncached, messages or unconnected messages error out with Error 1 Extended Error 301, No
Buffer Memory.
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