Defining The Program Structure - Siemens SIMATIC PCS 7 Operating Manual

7.2.1

Defining the program structure

In addition to considering the standard scenario, you need to answer the following questions
when drafting a safety program:
● Which parts of the user program need to be fail-safe?
● Which response times do you need to achieve?
You will need to split your F-program into various cyclic interrupt OBs (OB 30 to OB 38) in
accordance with these requirements.
You will improve performance if you program parts of the program which are not needed for
the safety functions in the standard user program.
In terms of dividing your program between the standard user program and the safety
program, please remember that the standard user program is easier to change and
download to the F-CPU. Changes to the standard user program do not usually need to
undergo acceptance and approval.
Rules governing program structure
When drafting a safety program for S7 F/FH Systems, you need to observe the following
rules:
● F-runtime groups with F-blocks can only be assigned to cyclic interrupts OB 30 to OB 38.
● A chart can contain both F-blocks and standard blocks; they need to be inserted in
separate runtime groups in this case. You are not permitted to use these charts as F-
block types.
● In the safety program, access to the F-signal modules is only permitted via the F-channel
driver (F_CH_xx).
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
Configuring the safety program
7.2 Creating the safety program
81