Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual User Manual
Page 93
Switchover Considerations
Chapter 6
6-8
Figure 6.1
Block Diagram of Remote I/O Switch
1785-BCM
Copy of remote I/O
data including block
transfers
Secondary processor
I/O scanner/adapter
Smart-switch interface
Remote I/O link
Carrier-detect
circuit
11106I
HSSL link
(to primary
1785-BCM
To determine the maximum time it takes for the secondary module to close
its relays after it detects a switchover, consider the following:
the primary scanner has just requested a 64-word BTR from the remote
module at 57.6 kbaud
the remote adapter starts to respond
switchover is triggered
You can calculate the maximum time for the I/O link switchover
as follows:
1.
remote I/O communication rate = 57.6 kbits/s = 1 byte/.14 ms
2.
transfer time = 64 x 2 x .14 = 18 ms
3.
HSSL protocol overhead/BCM delay = 12 ms
4.
1785-BCM module remote I/O watchdog timeout = 20 ms
Maximum time = 18 + 12 + 20 = 50 ms
Note that this time is less than the 100 ms remote I/O adapter watchdog
timeout, and therefore allows for continuous control of the I/O from the
backup system processors.