Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 141)

Publication 1747-UM010B-EN-P - September 2003
Switchover Considerations 8-7
When the communication channel is configured as RIO, the primary
1747-BSN emulates a Series B 1747-SN module with all the features
that it provides to the user. Additionally, the secondary 1747-BSN
“spies” on the RIO link to acquire all the input data and use it to
emulate the 1747-SN functionality to the secondary SLC 5/0x.
In the secondary system, the 1747-BSN simulates the entire scanner
operation to the secondary SLC 5/04 with the following characteristics:
•
Real discrete input data is acquired by the secondary BSN from
the RIO network and transferred to the secondary SLC 5/04
through the input (I) file.
•
Real BTR input data is acquired by the secondary BSN from the
RIO network and transferred to the secondary SLC 5/04 through
the M1 file. All the timing necessary for doing a BTR command
in the SLC 5/04 is simulated by the secondary BSN.
•
The discrete output data sent by the secondary SLC 5/04 through
the output (O) file is stored in the output image table for using
in case of a switchover.
•
The BTW output data sent by the secondary SLC 5/04 through
the M0 file is stored into the secondary BSN and all the timing
for the BTW operation is simulated by the secondary BSN.
•
In case of RIO communication errors seen by the BSN in the
primary system (remote rack errors, BTR or BTW errors, etc.),
the secondary BSN simulates the same errors for the secondary
SLC 5/04. This simulation is done in order to assure that the
secondary SLC 5/04 knows the real status of the remote devices.
•
If a switchover occurs, all the input and output data inside the
BSN is ready for the operation as the new primary scanner.
When the backup system is operating properly, the relays of the
primary 1747-BSN are closed and the relays of the secondary
1747-BSN are open. Should a failure occur in the primary system, the
relays of the primary 1747-BSN module open and the relays in the
secondary 1747-BSN module close.
The smart switch interface also includes a carrier-detect circuit to
monitor the activity of the remote I/O link. The purpose of this circuit
is to detect an idle remote I/O link before the secondary relays are
closed. This guarantees that the former primary system was really
IMPORTANT
The smart-switch interface communicates with the
secondary processor only when the 1747-BSN
module belongs to the secondary system. When the
module is primary, the smart switch only monitors
the remote I/O link to obtain input/output data.