Siemens Sinumerik 840D sl Function Manual page 217

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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 fail--safe 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:
S MD10099 $MN_INFO_PROFISAFE_CYCLE_TIME
(PROFIsafe communication clock cycle)
If the PROFIsafe clock cycle is continuously exceeded and just not sporadically,
then the following alarm is displayed:
S Interrupt: 27256 "PROFIsafe actual cycle time %1 [ms] > parameterize cycle
time"
Distribution of the computational load on the NCK side
MD10095 $MN_ SAFE_MODE_MASK, bit 3 lists an operating mode with which
the computational load can be distributed using the PROFIsafe driver to the IPO
clock cycles, the lie in a PROFIsafe clock cycle.
If this operating mode is activated, with MD13307 $MN_PROFISAFE_IPO_
RESERVE (default value = 0), users have the option of reserving part of the IPO
clock cycles so that in these clock cycles no PROFIsafe driver is computed.
This can make sense, especially for many active PROFIsafe devices, as the
PROFIsafe processing on the PLC side makes this time necessary.
Example:
MD PROFISAFE_IPO_TIME_RATIO = 8
MD PROFISAFE_IPO_RESERVE = 3
=> in 5 of the 8 IPO cycles per PROFIsafe cycle, PROFIsafe drivers are computed
in the remaining 3 IPO clock cycles, no PROFIsafe computation is performed on
the NCK side.
© Siemens AG 2015 All Rights Reserved
SINUMERIK 840D sl/SINAMICS S120 SINUMERIK Safety Integrated (FBSI sl) - - 10.2015 Edition
Connecting Sensors/Actuators
7.2 Connecting I/O via PROFIsafe
7-217