Siemens SINUMERIK 840D sl Function Manual page 195

In order to reduce the possible resulting computational load, machine data
● MD10098 $MN_PROFISAFE_IPO_TIME_RATIO
(factor, PROFIsafe communications clock cycle)
can be used to modify the ratio between the PROFIsafe and interpolation clock cycle.
To achieve a sufficiently fast response time for PROFIsafe communication, the PROFIsafe
cycle time must not be set to longer than 25 ms. The selected PROFIsafe clock cycle is
displayed in the machine data:
● MD10099 $MN_INFO_PROFISAFE_CYCLE_TIME
(PROFIsafe communication clock cycle)
If the PROFIsafe cycle time is longer than 25 ms, an alarm will be displayed the next time the
control runs up:
● Alarm: 27200 "PROFIsafe cycle time %1 [ms] is too long"
PROFIsafe clock cycle and DP cycle time
The PROFIsafe clock cycle should be parameterized longer than the DP clock cycle time
displayed by STEP7: HW Config. Otherwise, the load (time) on the PLC user program is
increased as a result of unnecessary OB40 interrupts.
Note
The PROFIsafe clock cycle must be parameterized, so that the following applies:
12 ms < PROFIsafe cycle < 25 ms.
PROFIsafe clock cycle overruns
Even if the parameterized software operates error-free in normal operation, run time
fluctuations in the PLC operating system (e.g. processing diagnostic alarms) can mean that
the processing of the OB40 interrupt was not able to be completed before the start of the next
PROFIsafe clock cycle.
In this particular case, the NCK attempts, up to a limit of 50 ms after the last correctly processed
PROFIsafe clock cycle, to initiate an OB40 interrupt. The repeated attempts to initiate the OB40
interrupt are no longer executed in the PROFIsafe clock cycle but in the IPO clock cycle.
After the 50 ms limit value is exceeded, Alarm 27253 "PROFIsafe communication error F-
master component %1, error %2" is output and the configured stop response (Stop D or E) is
initiated. PROFIsafe communication processing is stopped. This means that the
communication to F-modules, type F-DO or F-DI/DO is interrupted. PROFIsafe drivers of type
F-DI or F-DI/DO F-modules that have been stopped, output failsafe values (0) as F-net data
towards the SPL.
Further, an attempt is still made to initiate the OB40 interrupt and to maintain PROFIsafe
communications.
The time up to initiating the next OB40 interrupt is displayed in the following NCK machine
data:
● MD10099 $MN_INFO_PROFISAFE_CYCLE_TIME
(PROFIsafe communication clock cycle)
Safety Integrated
Function Manual, 12/2017, 6FC5397-4BP40-6BA1
Connecting sensors and actuators
8.2 Connecting I/O via PROFIsafe
195