Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 137)

Publication 1747-UM010B-EN-P - September 2003
Switchover Considerations 8-3
Data Table Transfer Time on HSSL
Data table transfer time refers to the amount of time it takes to transfer
critical data from the primary system to the secondary system. This
time is dependent on the amount of data to be sent between the two
systems, the number of remote I/O chassis, and the number of remote
I/O block transfers that are being executed.
The 1747-BSN module is capable of transferring up to 100K Word per
second (10 msec per K Word). This data rate is 2M bits/second.
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