1747-6.22, Backup Scanner User Manual

Switchover Considerations 8-159
Divergence The synchronization of program execution in both processors limits
divergence between the two systems by ensuring that the resulting
output data in both processors is always identical.
Because the program scans of the processors are not synchronized, it
is possible that the two separate systems read the same input at
different times. Eventually, different input values are read, thus
different decisions are made and the internal states of the two systems
diverge. If the two systems diverge, the secondary system generates a
“bump” when it takes control over the process.The “bump” time is a
function of how fast the inputs of the machine or the process are
changing, as well the amount of data to send between the primary and
secondary units.
One possible method to avoid or reduce the bump is the “loosely
synchronized backup” approach. This method requires special user
programming techniques. With this approach, primary and the
secondary SLC 5/0x execute the same program and are synchronized
at a waiting point once per program scan.
To limit divergence, we recommend that the primary and secondary
processors execute the same programs.
Forcing I/O Backup communication functions provided by the SLC 500 backup
system do not transfer the force tables from the primary to the
secondary processors. Therefore, if the SLC 500 backup system has
forces set in the primary processor, the forces do not transfer to the
secondary processor. For this situation, if a switchover occurs, the
secondary processor becomes active, but the forces set in the original
primary processor are not carried over.
!
ATTENTION: Set the forces in the secondary
processor first. Then set the forces in the primary
processor. Likewise, when removing forces, remove
forces in the secondary processor before removing the
forces in the primary processor. If a switchover occurs
and forces were not set in the secondary processor first,
the secondary processor does not recognize the forcedon
input or output.
Publication 1747-6.22