Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 77)

Operating Your PLC-5 Backup System
Chapter 5
5-13
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.
For example, a primary processor with node number 16 on Data Highway Plus
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: It is recommended to assign 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.
Figure 5.8 shows how the programming device sees the secondary processor
which is not physically connected to the Data Highway Plus link.
Important: The DH+ connection to the secondary processor is meant for
remote programming only. Also, only one terminal at a time may connect to the
secondary processor. No DH+ messages can be sent to or from the secondary
processor.
Figure 5.8
Access Point to the Secondary Processor (DH+)
n n
PLC-5 PLC-5
Node address = n + 1 Node address = n
BCM BCM HSSL
DH+
Programming
Device 11060I
When the system is started, each PLC-5 processor executes a BTR instruction to
its 1785-BCM module to obtain the system status word, which will indicate
whether it is in the primary or secondary mode. If the processor belongs to the
primary system, it will execute the primary (transmitter) program; if the
processor is secondary, it will execute the secondary (receiving) program. How the Backup System
Operates