Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 123)

1 Publication 1785-UM024B-EN-P - January 2003
Appendix
F
Comparison to the 1785-BCM System
Chapter Objectives This appendix provides you with a comparison between the
ControlNet PLC-5 backup system and the 1785-BCM backup system.
Comparison to the BCM
System Like the BCM scheme, the ControlNet PLC-5 backup system makes
use of a primary and a secondary PLC-5, where both the primary and
secondary PLC-5s simultaneously execute their control logic.
However, these two backup schemes differ in 3 major ways:
• The ControlNet PLC-5 backup system offers the capability of
synchronizing each control program scan with the scheduled
input data, while the BCM backup system does not offer any
form of program synchronization. With synchronized program
execution and scheduled inputs, the primary and secondary
systems should not diverge to any great degree. Thus the
ControlNet PLC-5 backup system eliminates the need for
massive crossloading of data and elaborate application program
development to eliminate divergence of the primary and
secondary systems.
• The ControlNet PLC-5 backup system eliminates the need for the
massive wiring required in the BCM backup system. The 1794
FLEX I/O Adapters on the ControlNet channel are able to listen
to data transmissions from both PLC-5s, as well as determine
which PLC-5 has primary control. Also, the ControlNet
producer/consumer architecture allows for input data to be
broadcast to and shared by both PLC-5s. There is no special
wiring required to implement ControlNet PLC-5 backup system
as opposed to a standalone system, other than connecting the
second PLC-5 onto the ControlNet network.
• The ControlNet PLC-5 backup system offers a simple mechanism
to crossload data. The BCM backup approach requires
complicated block transfer application programming in order to
crossload data.