Rockwell Automation Publication 1756-UM535D-EN-P - November 2012 149
Program the Redundant Controller Chapter 7
Crossloads and Scan Time
It is important to plan for controller crossloads because the length of the
crossloads affects the scan time of your program. A crossload is a transfer of data
from the primary controller to the secondary controller and may occur at the end
of each program or at the end of the last program in a task.
The scan time of your program or phase is a total of the program execution time
and the crossload time. The diagram below demonstrates this concept.
Figure 38 - Crossload and Scan Time
Estimate the Crossload Time
The amount of time required for a crossload is primarily dependent upon the
amount of data being crossloaded. During a crossload, any tag that has been
written to during the program execution is crossloaded. Even if a tag has not
changed, but has been rewritten during the program execution, it will be
crossloaded.
In addition to the time required to transfer tag value changes, the crossload also
requires a small amount of overhead time to communicate information about the
program being executed.
Program Scan Time
Execution of Program Crossload
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