Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 119)

Publication 1785-UM024B-EN-P - January 2003
Application Guidelines E-7
Data table values can become outdated because the data table
crossload is performed one at a time from low data table file number
to high data table file number. The data crossloaded from the low data
table file number becomes outdated while the higher number data
table file numbers are crossloaded.
Once the qualifying PLC-5 has transitioned into Run mode, the state
data table crossload begins. The state data table crossload tries to
bring some of the state information into the qualifying PLC-5 while it
is running.
Thus, for certain data table state information such as timer
accumulators which may have become outdated during the complete
crossload, the state data table crossload will update the timer
accumulator while the PLC-5 is running. This gets the timer
accumulators synchronized and they remain that way because both
programmable controllers are executing the timers simultaneously.
Once the state data table crossload is complete, the qualifying PLC-5
should be in the same process state as the primary PLC-5.
Additionally, once the qualifying PLC-5 is in the same process state as
the primary PLC-5, and all the state variable of the qualifying PLC-5
closely match the primary PLC-5 state variables, the qualifying PLC-5 is
ready to assume control of the process if the primary PLC-5 should
fault or fail.
When the qualifying PLC-5 becomes the secondary PLC-5, it should
transition into different process states the same time as the primary
PLC-5. It may not always happen, because the state variables on the
secondary PLC-5 closely match the primary’s, they may not be exact.
Additionally, common events to both programmable controllers may
occur at different times.
EXAMPLE
When a ControlNet I/O read is done by both the
primary and secondary programmable controllers,
they may not be performed simultaneously. One of
the CIO instructions may finish slightly earlier than
the other. If the process state transition depends on
the data from the CIO instruction, one of the
programmable controllers may transition to a
different process state before the other. In most
cases, the late PLC-5 will eventually transition into
the same state as the early PLC-5, but not always.
The late PLC-5 may diverge into another process
state and not match the other PLC-5.