Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 59)

Publication 1785-UM024B-EN-P - January 2003
Using I/O in Your ControlNet Hot Backup System 4-5
OOV is done via a round-robin poll that runs in the background of the
primary PLC-5.
There are some possibilities where an OOV conflict may occur. For
example, if one or both of the special peer-to-peer handshake
connections between the programmable controllers were to fail, each
PLC-5 would become a primary PLC-5. In this case, one or both of the
programmable controllers would indicate OOV conflict.
Other situations where OOV conflict can occur is in high noise level
environments or with an improper network installation. Under these
conditions, it is possible for the redundant connection between the
primary PLC-5 and an output module or adapter to be corrupt. When
this happens, the adapter turns control of the output over to the
secondary PLC-5 without notifying the primary PLC-5 that it has lost
the connection. This puts the system in a state where the secondary
PLC-5 controls some of the outputs and the primary PLC-5 controls the
remaining outputs. This system state may not be acceptable in certain
applications.
The PLC-5 takes no action if there is a non-zero value in word 8 of the
CBI file for a primary PLC-5. Your application must poll word 8 of the
CBI file to determine if an OOV conflict exists. If a non-zero value is
found in word 8 of the CBI file for a primary PLC-5, the application
will notify the system operator to cycle power on one of the
programmable controllers which restores total output control to the
other PLC-5.
ControlNet I/O Instructions
ControlNet I/O (CIO) instructions are managed differently in the
ControlNet Hot Backup system. With CIO write instructions, the
output data must only be sent from the primary PLC-5. To do this, you
must add a condition to the rung with the CIO write instructions so
that the rung will only be true when the PLC-5 is the primary
programmable controller. Word 3, bit 2 in the CBI file indicates
whether the PLC-5 is primary or not.
EXAMPLE
In an application with 20 redundant connections, the
primary PLC-5 polls these redundant connections
sequentially until all of the redundant connections
have been polled. For each redundant connection
that indicated the primary processor is not the owner
of the outputs for that connection, the value in word
8 of the CBI file increments. Word 8 of the CBI file is
updated after every complete polling cycle. The
value in word 8 of the CBI file for a secondary PLC-5
will be -1 (0xFFFF Hex).