Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 140)

Publication 1747-UM010B-EN-P - September 2003
8-6 Switchover Considerations
Remote I/O Switching 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.
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
retransmitting the message.
Reply packet disrupted Same as above
Token pass packet disrupted The DH+ link times out and
must rebuild. (250ms
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 SLC 500
processors to the same station address. The backup
modules permit only one of the two processors to be
connected to the DH+ at a time. In this configuration,
remote PLC processors and operator stations
communicate to the processor that currently has
primary status. The secondary processor can also be
accessed through the primary SLC 500 processor
node address plus one (n+1).