Safety Programming Guideline
Entry ID: 109750255, V1.0, 10/2017
2 Configuring Fail-Safe Controllers
2.1 Selecting the suitable F-CPU
Selecting the F-CPU depends on the following factors:
Runtime of the safety program
PROFIsafe communication time
Response time of the safety function
Number of required inputs and outputs
Number of connected I/O devices
Estimate of the response time
If you already have a rough idea of the automation system you want to use, you
can estimate the response time of your safety program using the SIMATIC STEP 7
Reaction Time Table or go through various scenarios to select the suitable F-CPU:
https://support.industry.siemens.com/cs/ww/en/view/93839056
Figure 2-1: Reaction time wizard of the SIMATIC STEP 7 Reaction Time Table
Influence of the safety program's cycle time on the standard user program
A long cycle time of the safety program slows down the response time of your
safety functions, but allows more time for processing the standard user program.
A short cycle time of the safety program increases the response time of your safety
functions, but allows less time for processing the standard user program.
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