Our customer has some Modicon Quantum PLCs, which are flashed for (and programmed with) Modsoft (legacy 984 ladder). There are several MSTR blocks in the programs. As far as I can read from available documentation, up to four (4) MSTR may be executing simultaneously.

My question is: Does anybody know what happens, if I (incorrectly) design the program to attempt to run five (or more) MSTR at the same time?

Possible answers to the question could be:

- The first four in execution order run, afterfollowing do not run.

- The first four invoked run, afterfollowing not.
- The fifth MSTR is placed in a queue, and will execute as soon as (at least) one of the first four is finished.

- The fifth MSTR will kick out one of the first four prematurely.

I would be happy to know if someone has any experience about what happens if the "max four MSTR rule" is violated.

 

The technical apps guys in Canada did a test a few years back with multiple MSTRs all running simultaneously. Result, over a period of time, all were serviced equally.

Because the MSTR contains ability to notify you of errors and completion you can schedule them so that the completion of one starts the next. Once started, the MSTR will complete.

Some users create scheduling logic.

But, for 5 MSTRs, just enable all.