Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 89)

Switchover Considerations
Chapter 6
6-4
Backup communication functions provided by the PLC-5 backup system
do not transfer the force tables from the primary to the secondary
processors. Thus, if the PLC-5 backup system has forces set in the primary
processor, the forces do not transfer to the secondary processor. For this
situation, if a switchover occurs, the secondary processor becomes active,
but the forces set in the original primary processor are not carried over.
ATTENTION: Set the forces in the secondary processor first
and then in the primary processor. Likewise, when removing
forces, remove forces in the secondary processor before
removing the forces in the primary processor.
If a switchover occurs and forces were not set in the secondary
processor first, the secondary processor would not recognize the
forced-on input or output.
In the Data Highway Plus link, one station on the link has control of the
link at a time. The station with this control is referred to as the station that
has the “token.” In the PLC-5 backup system, only the primary processor
is connected to the link. But, the secondary processor communicates with
the secondary 1785-BCM module, making this processor think that it is on
the Data Highway Plus network.
The secondary processor can be programmed through an access-point
address which corresponds to the processor node address plus one (n+1).
The secondary processor becomes part of the link only after a switchover.
It is possible that the primary processor could have possession of the token
during a switchover from the primary to the secondary processor. If this is
the case, the token could be lost even though passage of the token from one
station to the next is done as quickly as possible.
If the token is lost, all the stations on the link have an internal watchdog
timeout (250 ms), and the nodes assume the token is lost.
Forcing I/O
Data Highway Plus Switching