1747-6.22, Backup Scanner User Manual

Switchover Considerations
Chapter 8
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 diagnostic. 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.
Note: 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.
When a failure occurs in the primary system, the remote outputs
remain in the state set by the primary processor prior to switchover.
while the secondary processor assumes control of the process.
Important: You must program the backup system to account for the
following time requirements:
• input signal update time
• time-out on remote I/O link
• data table transfer time over the HSSL
Publication 1747-6.22