Siemens SIMATIC PCS 7 Operating Manual page 38

Configuring S7F/FH hardware
6.2 Adapting CPU parameters (fault-tolerant F system)
Calculation
Once all the parameters have been set, the values are calculated by pressing the
"Recalculate" button.
If the F-signal modules are configured in safety mode in HW Config, the "Max. disabling time
for priority classes > 15" parameter is calculated using the following formula:
● T
(determined for each DP master system)
● T
where:
● T
● T
● T
● T
● T
● T
● T
Calculating the parameters may cause one of the following messages to appear:
● "The F-monitoring times cannot be adhered to due to the configuration specified. Times
which can no longer be used were output."
if:
"Maximum disabling time for priority classes > 15" < "Minimum I/O retention time" + 50
ms
● "Due to the configuration specified, monitoring times have been calculated to ensure that
the F-monitoring time can be adhered to. It cannot be guaranteed that each attempt at
coupling and updating will be successful."
if:
T
The "Maximum communication delay" and "Maximum scan cycle time extension" are
calculated from the "Maximum disabling time for priority classes > 15" by multiplying this
setting once by 4 and once by 10.
Note
When determining the shortest F-monitoring time configured on an F-signal module, none of
the monitoring times set in PROFIsafe PA devices are taken into account.
38
(DP master system) = T
P15
in ms = 0.7 x size of DBs in work memory in bytes/1024 + 75
P15_UP
= Shortest F-monitoring time configured on an F-signal module
PTO
= Target rotation time of the PROFIBUS line
TR
= Call time of the cyclic interrupt OB
CI
= Runtime of the cyclic interrupt OB
PROG
= DP switchover time
DP_ST
= Slave switchover time
SLAVE_ST
= Time for copying the DBs of the user program (UP)
P15_UP
(DP master system) < T
P15
- (2 x T + T + T
PTO TR CI PROG
P15_UP
+ T + T )
DP_ST SLAVE_ST
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB