Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 91)

Switchover Considerations
Chapter 6
6-6
Table 6.A
DH+ Switchover Scenarios
If the new primary is: the following could occur causing this result
Polling the link Message packet from another station
disrupted The source station turns on error bit in its MSG
instruction (You provide programming at the
source station to regain synchronization with the
receiving station. You can do this by monitoring
the message instruction error bit as a condition
for re-transmitting the message.)
Reply packet disrupted Same as above
Token pass packet disrupted The DH+ link times out and must rebuild. (250
ms watchdog timeout) + (50 ms x number of
stations on the link).
Listening to the link New primary processor takes the
token The new primary processor has nothing in its
active node table. The DH+ link times out and
must rebuild. (250 ms watchdog timeout) + (50
ms x number of stations on the link).
New primary processor receives a
message from another station The link continues working normally
Important: In the recommended configuration, set both PLC-5 processors
to the same station address. The backup modules permit only one of the
two processors to be connected to the Data Highway Plus at a time. In this
configuration, remote PLC processors and operator stations communicate
to the processor that currently has primary status. Note that the secondary
processor can also be accessed through the primary PLC-5 processor node
address plus one (n+1).
Important: If you are communicating with the primary processor via a
programming terminal, do not place it in Remote Program mode. If you
do, a switchover will occur and the new primary can go into Remote
Program mode, shutting the system down.
The communications protocol for the remote I/O link is a master-slave
half-duplex type. This means that the master station (processor) sends a
command message addressed to a slave (remote I/O adapter), and the slave
responds with a reply message addressed to the master. This is also called
two-way alternating communication.
When the backup system is operating properly, the relays of the primary
1785-BCM are closed and the relays of the secondary 1785-BCM are open.
Should a failure occur in the primary system, the relays of the primary
1785-BCM module open and the relays in the secondary 1785-BCM
module close.
Programming
Remote I/O Switching