1747-6.22, Backup Scanner User Manual

Overview 1-39
Data table values are transferred from the primary to the secondary
system with M0 and M1 files and the HSSL. You copy the data to and
from M files via your ladder program.
You do not have to transfer data table values if not necessary for your
application.
For detailed information about data transfer from the primary to the
secondary system, refer to Chapter 5, “Operating Your SLC 500
Backup System”.
Switchover
Should a fault occur in the primary processor, control switches to the
secondary system in less than 50 ms (maximum). When a switchover
occurs, the outputs in the remote I/O maintain their last state until
they come under the control of the secondary processor.
The program scans of the two processors are not synchronized. This
means that the secondary processor may be in a different place in the
scan cycle than the primary processor. This manual explains the
switchover process and provides guidelines for developing programs
for your SLC 500 backup system. (For more information about
switchover, refer to “Switchover Considerations” on page 8-1.)
Role of the 1747-BSN Module
As an integral part of the backup system, the 1747-BSN modules
enable high-speed communication between the two SLC processors
and permit the secondary processor to assume control of the process.
In addition, the 1747-BSN module provides:
• high-speed transfer of the data table values from the primary to
the secondary system (HSSL), to ensure that the secondary
system’s data table is a copy of the primary system’s (2K words
maximum per BSN module pair)
• exchange of information on the status of the primary and
secondary systems (LSL)
• automatic transfer to the secondary system of the remote input
(discretes and analog values, etc.)
• transfer of control from the primary processor to the secondary
processor when one of the following conditions occur:
— power failure or power loss
— major processor fault
— 1747-BSN module fault, including communications timeout
in the RIO link
— change in the primary processor’s mode from:
• RUN to PROGRAM (manual switchover)
• REM RUN to REM PROG
Publication 1747-6.22