Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 109)

1 Publication 1785-UM024B-EN-P - January 2003
Appendix
D
Program Synchronization
Chapter Objectives This appendix provides you with reference information on the
program synchronization feature.
Program Synchronization Both processors in a ControlNet PLC-5 backup system execute their
own program logic. You can configure the ControlNet PLC-5 backup
system to have either Asynchronous program scans or Synchronous
program scans. Asynchronous mode allows the program scans on the
two processors to run as if they were stand-alone processors. When a
program scan on a processor is finished, housekeeping is performed,
and the next program scan is immediately started. The program scan
on one processor is completely independent of the program scan on
the other processor.
Synchronous mode provides both program scan synchronization as
well as housekeeping synchronization. When a program scan on a
processor is finished, housekeeping is delayed until a synchronization
point is reached. This synchronization point is the same for both
processors. Housekeeping is then performed at the same time on both
processors. By having housekeeping performed at the same time,
both processors should be updating the Input Image Table with the
same input values. When housekeeping is finished, the program scans
then immediately start and the cycle repeats itself. Synchronous mode
allows the inputs used by the two backup processors to closely match
each other, as well as forcing each program scan to begin at the same
time.
Synchronous mode offers only one synchronization point per program
scan. After the synchronization point is passed, the two control
programs run asynchronously until the next synchronization point
during the next housekeeping.
When the ControlNet backup system is configured in Asynchronous
mode, the Sync LED is off. When the ControlNet backup system is
configured in Synchronous mode, the Sync LED is either solid green
or solid red.