Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 57)

Publication 1747-UM010B-EN-P - September 2003
Operating Your SLC 500 Backup System 4-5
The 1747-BSN backup module provides remote programming
capability for your secondary processor. This means that even with
the programming device directly connected to the primary processor,
the secondary SLC 500 processor memory can be programmed and/or
monitored.
The primary 1747-BSN module provides an access point for a
programming device to access the secondary processor. Using the
1747-BSN module, you can communicate from a programming device
across the backup modules to the secondary processor. You must
assign both the primary and the secondary processor the same node
address in the DH+ link. The 1747-BSN module reserves the
subsequent node address as an access-point address to the secondary
processor; that is, this address is accessed by a programming device in
order to program the secondary processor.
This means that if you set both processors node addresses to n, the
programming device can communicate with the secondary processor
with the node address n+1. Therefore, do not use node address n+1
for any other device on the DH+ network.
For example, a primary processor with node number 16 on DH+
network has the same address switch setting as the secondary
processor node.
However, in this situation, the terminal addressed to station 16
attaches to the primary processor and the terminal addressed to
station 17 attaches to the secondary processor.
IMPORTANT
We recommend assigning the same node address (n)
for both processors (primary and secondary) in a
redundant system. You must also reserve the next
node address (n+1) because this address is the
access-point address to the secondary processor.