Rockwell-automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual de usuario descargar pdf (Pagina 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.

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Rockwell-automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual de usuario Pagina 110