Monitoring Times And System Response Times - Siemens SIMATIC PCS 7 Operating Manual

7.11

Monitoring times and system response times

A fail-safe system must switch the process to a safe state within a defined response time in
the event of a system error (e.g. CPU failure, F-signal module failure, communications
failure, program execution error).
To achieve this, a variety of monitoring times have been implemented to ensure that the
affected functions are brought to a safe state if this time is exceeded.
What is the purpose of monitoring times and system response times?
For the S7-400 F/FH, there are essentially two different types of monitoring/error response:
● If the monitoring times of the fail-safe program are exceeded, the affected F-program
switches off and the inputs/outputs of the F-signal modules are passivated.
● If the monitoring time for PROFIsafe communication between the F-CPU and F-signal
modules is exceeded, the inputs/outputs of the affected F-signal modules are passivated
and the corresponding substitute values are forwarded to the CPU.
Both the availability and the safety of the F/FH system must be taken into account when
configuring monitoring times.
● Availability:
The monitoring times must be set sufficiently high to prevent time monitoring from being
triggered when no errors are present.
● Safety:
The monitoring times must be set sufficiently low to prevent the process safety time from
being exceeded.
The rule of thumb is that a non-redundant S7 F/FH system supports shorter monitoring times
and, therefore, shorter response times in the event of an error. For a redundant system,
longer monitoring times have to be configured to allow for switchover of the components
(PROFIBUS, interface modules) and the coupling/updating of the H-CPU.
Calculation
The Excel file "s7ftimea.xlsm" is used for the purpose of calculating the monitoring and
response times below. You can find this file on the Internet by pointing your browser to the
following link: (https://support.industry.siemens.com/cs/ww/en/view/22557362)
The Excel file consists of the following sheets:
● max. runtime F-SG (Lib V1_3)
Here, you will find the runtimes of the F-blocks from F-library V1_3 in the various types of
CPU. You can use these to estimate the runtime of your F-program during the
configuration stage of your project.
● max. runtime F-SG (Lib V1_2)
Here, you will find the runtimes of the F-blocks from F-library V1_2 in the various types of
CPU. You can use these to estimate the runtime of your F-program during the
configuration stage of your project.
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
Configuring the safety program
7.11 Monitoring times and system response times
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