Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 136)

Publication 1747-UM010B-EN-P - September 2003
8-2 Switchover Considerations
When a failure occurs in the primary system, the remote outputs
remain in the state set by the primary processor prior to switchover,
while the secondary processor assumes control of the process.
Input Signal Update Time
It is possible for the secondary processor to read different input
conditions immediately after the switchover. To avoid a sudden
change in the outputs during switchover, all of the input signals must
be present for an amount of time equal to the sum of the following:
•
program scan
•
I/O scan
•
input module delay
•
1747-BSN switchover time
If these conditions are not met, outputs may be assigned different
states in the two processors. This can cause a sudden change of
operation when switchover occurs.
Time-out on Remote I/O Link
The remote I/O chassis are updated one at a time. The total remote
I/O update is 10 ms per chassis (typical) at 57.6K baud.
To keep the I/O chassis from faulting and losing control of the I/O,
the switchover in the remote I/O link is accomplished in less than
50 ms. (The remote I/O link switchover starts when the remote I/O
relays in the primary 1747-BSN module open and ends when the
remote I/O relays in the secondary 1747-BSN module close.)
This switchover time is less than the 100 ms for the remote I/O
adapter watchdog timeout. This permits continuous control of the I/O
from the backup system processors.
IMPORTANT
You must program the backup system to account for
the following time requirements:
•
input signal update time
•
time-out on remote I/O link
•
data table transfer time over the HSSL