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Buffering critical data, Scan-dependent logic – Rockwell Automation 1756-RMxx ControlLogix Enhanced Redundancy System User Manual User Manual

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Rockwell Automation Publication 1756-UM535D-EN-P - November 2012

Chapter 7

Program the Redundant Controller

Buffering Critical Data

If you cannot place Array (File)/Shift instructions in the highest-priority task,

consider using a buffer with Copy File (COP) and Synchronous Copy File (CPS)

instructions to maintain the integrity of the array of data.

The programming example shown here shows the use of a COP instruction to

move data into a buffer array. The BSL instruction uses the data in that buffer

array. The CPS instruction updates the array tag and maintains data integrity

because it cannot be interrupted by a higher priority task. If a switchover occurs,

the source data (that is, the array tag) remains unaffected.

Figure 46 - Using a Buffer to Maintain Data During Shift

For more information about BSL, BSR, FFU, COP, and CPS instructions see the

Logix5000 Controllers General Instructions Reference Manual, publication

1756-RM003

.

Scan-dependent Logic

If you program a lower priority task so that one instruction is dependent on

another instruction that occurs elsewhere in your program, your programming

may be disrupted by a task interrupt and switchover. The disruption can occur

because the lower priority task may be interrupted by the higher priority task and

then a switchover may occur before the lower priority task is completed.

When the lower priority task is executed from the beginning by the new primary

controller after the switchover, the dependent instruction may not execute at the

most recent value or state.