Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 99)

1 Publication 1785-UM024B-EN-P - January 2003
Appendix
C
Data Table Crossloading
Chapter Objectives This appendix provides you with reference information on the data
table crossloading feature.
Data Table Crossloading Data table crossloading is a feature you can enable to minimize the
bump in system outputs when a processor switchover occurs. This
feature is most useful when identical PLC-5 processors are used in the
backup system.
To minimize a bump in system outputs, it is often important for the
secondary processor’s data table values to closely match the primary
processor’s data table values. This is true in control applications where
output values are dependent upon other data table variables. If the
data table values between the processors are different, there may be a
bump in the system outputs when the secondary processor takes over
from the primary processor after a switchover.
In the PLC-5 ControlNet backup system, each processor has its own
data table, and values are not automatically exchanged between the
PLC-5 processors. During operation, each processor simultaneously
receives the same set of input data, executes its program logic, and
places program logic results in its own data table. Over time, the data
table values may diverge between the primary processor and the
secondary processor due to such things as internal timer differences
or asynchronous events such as CIO instruction completion.
Once backup system qualification is completed, there is no automatic
data table crossloading from the primary processor to the secondary
processor in the default PLC-5 ControlNet backup system. However,
there may be applications where it is highly desirable to be able to
easily and automatically crossload data table data from the primary
processor to the secondary processor. To meet this need, there is a
feature called On-Demand data table crossloading that greatly
simplifies the transfer of data table information from the primary
processor to corresponding data table locations on the secondary
processor.