Rockwell Automation Publication 1756-UM535D-EN-P - November 2012 151
Program the Redundant Controller Chapter 7
Equation for Estimating Crossload Times
After you have either the size of the last data transfer or the maximum size of data
transferred, use this equation to estimate your controllers’ crossload time for each
program.
1756-L6x Controllers
Crossload time per sync point (ms) = (DINTs  0.00091) + 0.6 ms
1756-L7x Controllers
The following equations apply when a 1756-L7x controller is paired with a
redundancy module in both chassis in a redundancy system.
Table 24 - Crossload Times for 1756-L7x Controllers
Controller Paired with Redundancy Module Crossload Time
1756-L7x 1756-RM2/A Crossload time per sync point (ms) = (DINTs * 0.000275) + 0.54 ms
1756-RM/B Crossload time per sync point (ms) = (DINTs 0.00043) + 0.3 ms
1756-RM/A Crossload time per sync point (ms) = (DINTs  0.00091) + 0.6 ms
Where DINTs is the size of the data transferred measured in 4-byte words.
A sync point is a mechanism that the primary controller uses to keep the
secondary controller in sync. By default, at the end of each program scan, the
primary controller sends the secondary controller the sync point and the
secondary controller responds by moving its execution pointer to match the
primary controller.
The default for phases is not to send a sync point.
Beginning in revision 16.05x, the option exists to manipulate the sync points
for faster program execution.
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