Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 163)

Publication 1747-UM010B-EN-P - September 2003
Programming Techniques 9-21
Another potential problem with block transfer instructions is that
during switchover, the secondary scanner may skip one or more
remote adapters. This may happen because the “new” primary
scanner (previously the secondary) was in a different place in its
remote I/O chassis scan.
Message Instructions
When programming, you must ensure that when a switchover occurs,
that any messages which were running in the primary processor are
subsequently enabled in the secondary processor. You can do this by
conditioning message instruction rungs in both the primary and
secondary processors using the primary/secondary bit of the
1747-BSN status word. If necessary, you can pass the results of a
message addressed to the primary processor over the HSSL to the
secondary processor.
Refer to the SLC 500 Instruction Set Reference Manual, publication
number 1747-RM001, for more information on the Message
instruction.
PID Control Files
When using PID instructions, you can send all or part of these control
files, based on how you back up your PID instructions and the total
amount of data to be sent by the 1747-BSN modules. Keep in mind
that the control files contain such things as your setpoint, gains, and
words that are used by the PID instruction for internal storage and
should not be manipulated unnecessarily.
PRI_SEC
EN
DN
ER
MSG
Read/Write Message
Type
Peer-To-Peer
Read/Write Read
Target Device 500CPU
Local/Remote Local
Control Block ?
Control Block Length 14
Setup Screen
MSG