Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 138)

Publication 1747-UM010B-EN-P - September 2003
8-4 Switchover Considerations
secondary processor becomes active, but the forces set in the original
primary processor are not carried over.
Data Highway Plus
Switching You can aggregate a 1747-BSN with the communication channel
configured as DH+ network to the system. In this situation, both
primary and secondary processors must be set to the same node
address. During normal operation, the other equipment in the DH+
network recognize the primary processor in its real node address and
the secondary processor in the node address which follows.
When a switchover occurs, the new primary SLC 5/04 assumes its real
node address in the DH+ network and the new secondary processor
assumes the next node address.
The DH+ smart switch is designed to provide remote programming
capability for the secondary processor with the following limitations:
•
The secondary SLC 5/04 can be accessed only by one station in
the primary link each time. If more than one station tries to
access the secondary SLC 5/04 at the same time, the
communication performance with this node is degraded
significantly.
•
The secondary SLC 5/04 only communicates with the secondary
1747-BSN. If a third device is connected to this link, it will not
work properly.
•
The secondary SLC 5/04 only answers to commands. It cannot
start a MSG instruction.
•
The primary SLC 5/04 cannot send messages through DH+ to
the secondary SLC 5/04.
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. If the real node address is 5, the primary is the node 5 and
the secondary is node 6. If the real node address is 77, the primary is
node 77 and the secondary is node 0.
ATTENTION
!
Set the forces in the secondary processor first. Then
set the forces 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 does not recognize the
forced-on input or output.