Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 110)

Publication 1785-UM024B-EN-P - January 2003
D-2 Program Synchronization
Determining Which Mode to Use
Determining whether to select Asynchronous mode or Synchronous
mode depends on your application. If the program logic is completely
different between the two processors, it is appropriate to select
Asynchronous mode. Also, if the program scan time frequently bumps
out from a small value to a very large value so that the configured
Maximum Program Scan Time is too large for the application, it would
be appropriate to select Asynchronous mode.
On the other hand, if the program logic between the two processors is
identical or very similar, and if the configured Maximum Program Scan
Time is small enough for the application, it is advantageous to select
Synchronous mode. Synchronous mode minimizes data table
divergence between the two processors, resulting in bumpless
switchovers.
Using Synchronous Mode
There are two considerations to keep in mind when using
synchronous mode. First, synchronous mode requires you to
determine the Maximum Program Scan Time. There is no formula to
determine the maximum program scan time for a particular program;
you must determine this value through running the application in its
real environment.
Second, in Synchronous mode, each program scan is always
lengthened to be equal to the maximum program scan time. Thus,
even when a particular program scan takes only a fraction of the
maximum program scan time, the program scan waits until the
synchronization point is reached before continuing with
housekeeping. Depending on your application and program logic, this
may affect overall system throughput.
Determining the Maximum Program Scan Time
When configuring Synchronous mode, you must enter the Maximum
Program Scan Time (in ms). The Maximum Program Scan Time is a
time that should not be exceeded by the program scan. The program
scan is the time it takes the processor to execute the logic program
once, perform housekeeping tasks, and then start executing logic
again.