Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 37)

Publication 1747-UM010B-EN-P - September 2003
Overview 1-23
Although data transfer between the primary and secondary processors
is done as fast as possible, there is no guarantee that a switchover
from the primary to the secondary will be bumpless. The data rate is
2 Mbits/second.
The local status link (LSL) is a 57.6 KBaud serial link provided for
exchanging status between the 1747-BSNs that are in the same chassis.
Data Transfer
During normal operation, the primary system sends remote input and
retentative data table data to the secondary system so that in the event
of a switchover, the secondary system (which becomes the new
primary system) has the same data.
Remote I/O data is automatically transferred over the High-Speed
Serial Link. This transfer is independent of the application program.
Each 1747-BSN is capable of transferring up to 2 KWords of the
SLC 500 data table. This capacity is increased as the number of
1747-BSN modules per local chassis is increased in a redundant
system. For example, a system with eight 1747-BSNs in each chassis is
capable of transferring up to 16 KWord of synchronizing data table
information.
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 it is not necessary for
your application.
For detailed information about data transfer from the primary to the
secondary system, refer to Chapter 7.
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 Chapter 8, Switchover Considerations.)