Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 142)

Publication 1747-UM010B-EN-P - September 2003
8-8 Switchover Considerations
disconnected from the link and the remote adapters have completed
their response to the last poll from the primary scanner.
In addition, in the event of an HSSL break, the carrier-detect circuit
prevents the secondary system from taking control of the link along
with the primary system. The figure below shows the block diagram
of the remote I/O switch.
1747-BSN
HSSL link to
primary
1747-BSN
Remote I/O
link
Smart-switch interface
Secondary processor
I/O scanner/adaptor
Carrier-detect circuit
Copy of remote I/O
data including
block transfers