Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 46)

Publication 1785-UM024B-EN-P - January 2003
3-20 Installing and Configuring Your ControlNet PLC-5 Backup System
Synchronous and Asynchronous Program Scanning
One of the most important considerations when implementing a
backup system is the effect of divergence on the system. Divergence
occurs when the primary and secondary controllers are running their
applications and scanning the I/O asynchronously to each other. This
mode of operation can cause a condition where one of the controllers
is several program and I/O scans ahead of (or behind) the other.
Because of this condition, the logic in each controller could resolve
differently, thus creating differences (i.e., divergence) in the I/O and
data tables.
As divergence between the primary and secondary controllers
increases, the possibility of a bump in the process when a switchover
occurs also increases. This could be critical in some backup
applications.
Asynchronous program scanning allows both the primary and
secondary controllers to operate as fast as they possibly can, without
attempting to synchronize with each other. This mode of operation
should be used when the divergence of programs, I/O tables, and
data tables is not a critical consideration. Asynchronous program
scanning has no effect on program scan time. You should select this
mode of operation when the primary is programmed to run the
normal application, and the secondary is programmed to perform a
different task (e.g., a safety shutdown, a clean and wash operation,
etc.).
Synchronous program scanning forces the primary and secondary
controllers to synchronize their program and I/O scanning. Both
controllers start their program scan at the same time, and both gather
input data at the same time. This mode of operation should be used
when minimizing the divergence of programs, I/O tables, and data
tables is a critical and necessary consideration. Synchronous program
scanning impacts program scan time in both controllers (i.e., scan
times will be longer), although this impact is not significant in most
cases.