User Manual

1 Publication 1747-UM010B-EN-P - September 2003
Chapter 8
Switchover Considerations
When planning programs for the SLC 500 backup system, you must
first consider that the program scans of the two processors are not
synchronized. This means that the program in the primary processor is
not executing the exact same instruction at the same time as the
program in the secondary processor. In addition, you must consider:
• timing
• divergence
• I/O forces
• Data Highway Plus switching
• remote I/O switching
• special sections of the data table
• data integrity
This chapter describes the switchover considerations listed above, as
well as switchover diagnostics. It also describes how to detect a
possible fault in the secondary module in order to avoid switchover
problems in the backup system.
Timing Requirements
Switchover from one processor to another takes less than 50 ms.
TIP
Total switchover time is a factor of the 1747-BSN
module delay and one program scan of the secondary
processor. The time required for one program scan is
added because the secondary processor technically
has not assumed control until it has completed one
scan of its program and remote I/O after a
switchover.