Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 60)

Publication 1785-UM024B-EN-P - January 2003
4-6 Using I/O in Your ControlNet Hot Backup System
With CIO read instructions, there are two possibilities:
• either both programmable controllers can perform the read, or
• the primary PLC-5 can perform the read and then the data can
be crossloaded from the primary to the secondary PLC-5
Considerations when Using CIO Instructions
Be aware of certain considerations when using CIO instructions for an
I/O module that has redundant connections configured for that
module.
Writing different configuration data to the I/O module than is used by
the redundant connections must be prevented. If different module
configuration data is written to the I/O module than was for the
redundant connection, the redundant connection will not reopen
should the redundant connection ever close. If the I/O module
configuration data is intentionally altered, you must first alter the
configuration data for the redundant module connections. This means
you must first alter the configuration data for the redundant module
connections and then do a CIO write to the same module. One way to
ensure that the CIO instruction uses the same configuration data as
the redundant connection is for the CIO instruction to use the same
configuration data from the redundant connection’s data table
location.
Human Machine Interface (HMI) Considerations
HMI devices can be used to:
• gather data from the programmable controllers and display it to
users, or
• gather input data from users and write it to the programmable
controllers
With these HMI considerations, we are assuming that the HMI devices
initiate the data transfers - meaning the HMI solicits requests to read
or write data to the PLC-5 programmable controllers. Additionally
these considerations apply not only to the ControlNet channel, but to
any other channel where both the primary and secondary PLC-5 are
each different nodes on the channel, such as DH+.
If the HMI device supports scheduled redundant connections on
ControlNet as described above, it also supports the PLC-5 Hot Backup
System. HMI that support scheduled but not redundant connections
should not use scheduled connections in the Hot Backup System. This
is because only one of the programmable controllers owns the
connection and if that PLC-5 were to fail, the other PLC-5 could not
pick up communication with the HMI device.