Rockwell Automation Publication 1756-UM535D-EN-P - November 2012 159
Program the Redundant Controller Chapter 7
Program to Maintain Data
Integrity
When programming your redundant controllers, there are some instructions and
techniques that may cause data loss or corruption when used. These instructions
and techniques include the following:
• Array (File)/Shift Instructions
• Scan-dependent Logic
Array (File)/Shift Instructions
Interruptions to Array (File)/Shift Instructions by an a higher priority task and
then switchover can result in an incomplete data shift and corrupted data.
These Array (File)/Shift instructions may result in corrupt data in the event of a
switchover:
• Bit Shift Left (BSL)
• Bit Shift Right (BSR)
• FIFO Unload (FFU)
If Array (File)/Shift Instructions are used, these system behaviors may result:
1. If a higher priority task interrupts one of the Array (File)/Shift
instructions, the partially-shifted array values are crossloaded to the
secondary controller.
2. If a switchover occurs before the instruction completes its execution, data
remains only partially shifted.
3. After a switchover, the secondary controller starts its executing at the
beginning of the program. When it reaches the partially-executed
instruction, it shifts the data again.
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