Safety
Fail-Safe Systems
7-10 A5E00085588-03
7.4.2 Calculation of the Minimum Monitoring Times
7.4.2.1 Monitoring the F Cycle Time
The monitoring time is assigned parameters at the MAX_CYC input parameter of
the F_CYC_CO fail-safe blocks.
To ensure monitoring is not triggered when there is no fault, MAX_CYC must be
greater than the maximum cycle time TCImax of the relevant cyclic interrupt OB:
MAX_CYC > TCImax
TCImax is at least as large as the configured cycle time TCI of the cyclic interrupt
OB. In the FH system, the maximum disabling time for priority classes > 15 (TP15)
at updating must also be taken into consideration. Thus the following
approximation formulae apply:
TCImax ≈ TCI + MIN(TCiR, 2500) In the F system
TCImax ≈ MAX (TCI; TP15) + MIN(TCiR,
2500)
In the FH system with cyclic interrupt OB with
special handling
TCImax ≈ TCI + TP15 + MIN(TCiR, 2500) In the FH system with cyclic interrupt OB
without special handling
Note the following:
Time Description Where to Find it?
TCI Configured cycle time of the
cyclic interrupt OB
HWCONFIG
CPU properties, "Cyclic Interrupt, Execution"
TP15 Maximum disabling time for
priority classes > 15
HWCONFIG
CPU Properties, "H Parameters"
TCiR CiR Synchronization Time:
-From the CiR-Object
parameters in STEP7
-Summarize all CiR-Object
synchronization times of the
simultaneously changing DP
buses and place total here.
If CiR is not used, enter 0.
Properties of the CiR_Object in HWCONFIG.
For additional information, refer to section 4.8.4
"Configuration in Run (CiR)".
"Cyclic Interrupt OB with Special Handling" is an H parameter of the CPU in the S7
FH system. The parameter contains the number of the cyclic interrupt OB that is
called separately by the operating system when the standby is updated, after all
the interrupts have been locked. Usually the number of the cyclic interrupt OB with
the highest priority is entered, to which F-blocks of the Safety Program are
assigned in CFC.
Note
To activate the monitoring of the maximum disabling time for priority classes > 15,
you must assign this parameter a value in HWCONFIG (CPU properties, "H
Parameters" tab).
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