Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 61)

Publication 1747-UM010B-EN-P - September 2003
Operating Your SLC 500 Backup System 4-9
Power-up Sequencing To determine the primary and secondary systems, use the following
powerup sequence:
1. Apply power to the system that is intended to be primary.
2. After the primary SLC 5/0x has been powered-up and is
operating correctly, apply power to the system that is intended
to be secondary.
If both systems are powered-up at the same time, they use a
“talk-and-listen algorithm” to determine which system is going to be
primary and which is going to be secondary. The RIO and DH+ relays
remain open until only one of the systems assumes primary status.
This ensures that only one of the SLC 5/0x processors takes control
over the RIO and DH+ links.
Processor Mode Change
Sequencing The 1747-BSN does not control the operating mode of the SLC 5/0x in
either the primary or the backup systems. If a processor mode change
is required, it should be done primarily in the secondary system. After
that, if required, the processor mode change should also be done in
the primary processor.
If the primary operating mode is changed from Run mode to Program
mode, the secondary system takes control.
Fail-over Sequencing When the primary system fails, the communications relays are opened
immediately. The primary 1747-BSN ceases its data transfer function
and sends switching commands to the secondary 1747-BSN. After all
the RIO communications terminate, the secondary communication
channel relays are closed. Then the former secondary SLC is ready to
take control of the system.
Since the secondary’s input image table has been loaded on-line with
real data from the Smart Switch, the application program does not
have to delay the use of its input image table until the completion of
at least one complete RIO scan. The secondary’s (now primary) input
image table always has valid data.
ATTENTION
!
Use the processor mode change key to change the
primary operating mode. Unexpected operation or
system shutdown could occur if you use a
programming terminal to change the primary
operating mode from Remote Run to Remote
Program.