Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 97
4.2.3.6 System Function Blocks (SFB) and System Functions (SFC)
Preprogrammed Blocks
You do not need to program every function yourself. S7 CPUs provide you with preprogrammed
blocks that you can call in your user program.
Further information can be found in the reference help on system blocks and system functions
(Jumps to Language Descriptions and Help on Blocks and System Attributes).
System Function Blocks
A system function block (SFB) is a function block integrated on the S7 CPU. SFBs are part of the
operating system and are not loaded as part of the program. Like FBs, SFBs are blocks "with
memory." You must also create instance data blocks for SFBs and download them to the CPU as
part of the program.
S7 CPUs provide the following SFBs:
• For communication via configured connections
• For integrated special functions (for example, SFB29 "HS_COUNT" on the CPU 312 IFM and
the CPU 314 IFM).
System Functions
A system function is a preprogrammed function that is integrated on the S7 CPU. You can call the
SFC in your program. SFCs are part of the operating system and are not loaded as part of the
program. Like FCs, SFCs are blocks "without memory."
S7 CPUs provide SFCs for the following functions:
• Copying and block functions
• Checking the program
• Handling the clock and run-time meters
• Transferring data sets
• Transferring events from a CPU to all other CPUs in multicomputing mode
• Handling time-of-day and time-delay interrupts
• Handling synchronous errors, interrupts, and asynchronous errors
• Information on static and dynamic system data, for example, diagnostics
• Process image updating and bit field processing
• Addressing modules
• Distributed I/O
• Global data communication
• Communication via non-configured connections
• Generating block-related messages