Call time of the F-runtime group and max cycle-time of the f-runtime

I have inherited a distributed safety system S7 414F-3PN/DP project from a colleague who has left the company.

Using the S7fcotib.xls spreadsheet I have calculated a Max. cycle time for the F-runtime of 5ms yet I have to set a value of 200ms in order to prevent the CPU from going to stop. Why is there such a large discrepancy and is this unusual?

The F-runtime is called in OB35 every 50ms is this an issue if the max-cycle time of the F-runtime is set at 200ms?

Should there be some kind of relationship between the call time of the F-runtime group and the max cycle time of the F-runtime? Like the call time should be greater than the max cycle time?

All I see in the safety manuals is that you have to set the max cycle time to a value where it does not cause the CPU to go into STOP which seems crazy if this figure is greater than the call time of the F-runtime group.

OK so with some investigation on PLCSim and reading the s7fcotib.xls spreadsheet more closely, I think I have sorted it out.

The 5ms runtime of the F-Runtime is the total time taken to execute all the safety program instructions from start to finish.

The 50ms execution interval of OB35 is the time between calls to run the safety program. If the interval was too short such the safety program does not complete execution before OB35 is called again then the CPU will STOP unless OB80 is programmed.

The 200ms max. cycle time of the F-Runtime is the maximum time allowed between the beginning of one scan of the safety program and the next. If this value is less than the execution interval of OB35 the CPU will STOP because the safety program execution will not start until the next execution of OB35. In reality the max. cycle time of the F-Runtime needs to be sufficiently large to allow for the extension of OB35s execution interval due to higher priority services.

So the bottom line is that:

• The execution interval of OB35 must be greater than the runtime of the F-Runtime (safety program), but not so great as to extend the runtime of OB1 excessively.
• The max. cycle time of the F-runtime must be greater than the execution interval of OB35 by a sufficient amount as to allow for an extension in the interval. It should also be small enough so as not to extend the SIF overall response time beyond the maximum allowable response time of the SIF.

 Hi

JPG123

OK so with some investigation on PLCSim and reading the s7fcotib.xls spreadsheet more closely, I think I have sorted it out.

The 5ms runtime of the F-Runtime is the total time taken to execute all the safety program instructions from start to finish.

The 50ms execution interval of OB35 is the time between calls to run the safety program. If the interval was too short such the safety program does not complete execution before OB35 is called again then the CPU will STOP unless OB80 is programmed.

The 200ms max. cycle time of the F-Runtime is the maximum time allowed between the beginning of one scan of the safety program and the next. If this value is less than the execution interval of OB35 the CPU will STOP because the safety program execution will not start until the next execution of OB35. In reality the max. cycle time of the F-Runtime needs to be sufficiently large to allow for the extension of OB35s execution interval due to higher priority services.

So the bottom line is that:

• The execution interval of OB35 must be greater than the runtime of the F-Runtime (safety program), but not so great as to extend the runtime of OB1 excessively.
• The max. cycle time of the F-runtime must be greater than the execution interval of OB35 by a sufficient amount as to allow for an extension in the interval. It should also be small enough so as not to extend the SIF overall response time beyond the maximum allowable response time of the SIF.

 Thanks to share your experience.

for more information about f-cycle time,f-montoring time and response time, please refer to  Safety Engineering in SIMATIC S7 (S7 Distributed Safety and S7 F/FH Systems),appendix A: Monitoring and Response Times of F-Systems.

Also:

S7-F/FH systems- configuring and programming

S7-Distributed safety-configuring and programming

S7-300 failesafe signal modules

Regards

hosein