Siemens SIMATIC S7 System Manual page 115

Description of Program Structure
The safety program is accessed by calling F-CALL from the standard user program. F-CALL
is called in an OB, preferably in a time interrupt OB (e.g., OB 35).
The advantage of time interrupt OBs is that they interrupt the cyclic program execution in OB
1 of the standard user program at fixed time intervals. That is, the safety program is called
and run at fixed time intervals in a time interrupt OB.
Once the safety program is executed, the standard user program resumes.
Structure of the Safety Program in F-Runtime Groups
To make it easier to handle, a safety program is formed from one or two "F-runtime groups."
An F-runtime group is a logical construct made of several related F-blocks.
An F-runtime group in the safety program for S7 Distributed Safety consists of:
• One F-CALL F-call block
• One F-program block (an F-FB/F-FC assigned to the F-CALL)
• Additional F-FBs or F-FCs programmed using F-FBD or F-LAD, as needed
• One or more F-DBs, as needed
• F-I/O DBs
• F-blocks of the
• F-blocks from custom F-libraries
• F-system blocks
• Automatically generated F-blocks
If the user divides his safety program into two F-runtime groups, portions of the safety
program (one F-runtime group) can be executed in a faster priority class, thereby achieving
faster safety circuits with short response times.
Safety Engineering in SIMATIC S7
System Manual, 04/2006, A5E00109529-05
7.3 Structure of the Safety Program in S7 Distributed Safety
Distributed Safety
F-library (V1)
Programming F-Systems
7-5