Backup system theory of operation -8, Backup system theory of operation – Rockwell Automation 1747-BSN Backup Scanner Module User Manual
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Publication 1747-UM010B-EN-P - September 2003
4-8 Operating Your SLC 500 Backup System
Backup System Theory of Operation
A redundant system using the 1747-BSN can be initially defined as an
“Asynchronized Data Transfer” system. While the Input Image Table is
automatically acquired from the RIO link by the secondary system, the
Data Table is transferred by an application program written by the
user. Try to minimize the amount of data to be transferred from the
primary to the secondary processor because the program logic
generates the same outputs based on the same inputs.
All timer and counter values must be transferred at least once because
the primary and backup processors may have started at different times
and are not synchronized. Therefore, timer and counter data may be
different in the two processors. For example, after a faulted processor
is repaired and reinstalled as a backup, the timer and counter
accumulated values, as well as the control words, in the two
processors are different.
The number of data table words to be transferred from a primary to a
secondary processor is dependent on the user program and/or user
architecture.
Startup Sequencing
When the system is powered up for the first time, the following
sequence must be followed:
1. Disconnect both the primary and secondary SLC 5/0x from DH+
or DH485.
2. Download programs containing the desired DH+ or DH485
node address to each processor. Leave both SLC 5/0x in
program mode, and power down both systems.
3. Apply power to the system that is intended to be primary.
4. Apply power to the system that is intended to be secondary.
5. Change the primary SLC processor to Run mode.
6. Change the secondary SLC processor to Run mode.