Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 55)

1 Publication 1785-UM024B-EN-P - January 2003
Chapter
4
Using I/O in Your ControlNet Hot Backup
System
Chapter Objectives This chapter describes special considerations when using I/O in a hot
backup system.
Considerations when Using
I/O For scheduled connections, a special connection type has be
established called a redundant connection. A redundant connection
allows both processors to make identical scheduled connections and
only the outputs from the primary processor are used. The outputs
from the secondary processor are discarded.
ControlNet I/O (CIO) instructions must be managed differently in the
PLC-5 ControlNet Hot Backup system:
• with ControlNet I/O write instructions, the output should only
be sent from the primary processor. Use a simple ladder
program example described in this chapter to manage this.
• with ControlNet I/O read instructions, both PLC-5s can perform
the read instruction, or the primary PLC-5 can perform the read
instruction and the data can be crossloaded from the primary
PLC-5 to the secondary PLC-5.
HMI devices have specific requirements including:
• outputs sent from the backup system to the HMI device must
come from the primary PLC-5. There are specific procedures that
that we describe later in this chapter that instruct the HMI device
to determine the primary PLC-5. When the HMI device "knows"
which is the primary PLC-5, it can then gather the outputs
from it.
• HMI inputs must be sent to both the primary and secondary
PLC-5 programmable controllers in the Hot Backup system. This
chapter describes how to do this.
Finally, some of the I/O in the PLC-5 does not switch over when the
PLC-5 programmable controllers perform a switchover. The resident
chassis I/O and the I/O on the remote "blue wire" channel are not
backed up. Also, ControlNet scheduled connections not configured to
be the redundant connection type are not backed up. When a
switchover occurs, the new secondary PLC-5 continues to control the
outputs of the non-redundant outputs