FC Block Interface

A Function (FC) block in Siemens S7 PLCs is a reusable code block that processes input parameters via IN, IN/OUT, and OUT interfaces, using temporary local memory without an associated instance data block.

Overview of FC Blocks

A Function (FC) is a code block designed to perform specific operations on input values, such as mathematical calculations or control logic, and can be called multiple times in a program for reuse . Unlike Function Blocks (FBs), FCs do not have an instance data block (DB); they rely on the local data stack for temporary data during execution. This means that any data calculated within an FC is not retained after the block finishes unless explicitly stored in global memory locations like M memory or a global DB .

FC Interface Types

FCs use three main interface types to handle parameters:

• IN: Input-only parameters; the FC reads the value but cannot modify the caller's data.
• IN/OUT: Input and output parameters; the FC can read and modify the caller's data directly.
• OUT: Output-only parameters; the FC writes results back to the caller .

Parameter Passing Methods

Parameters can be passed in two ways:

• Call-by-value: The actual parameter value is copied into the FC's input parameter (used for simple data types like INT, DINT, REAL). Modifications inside the FC do not affect the original variable .
• Call-by-reference: The FC receives a reference (pointer) to the actual parameter (used for complex data types like STRUCT, ARRAY, STRING). Changes inside the FC directly modify the caller's data . For structured or complex data types, using IN/OUT with call-by-reference is recommended to avoid unnecessary memory usage and copying overhead .

Memory and Data Handling

• Temporary data: Stored on the local stack during FC execution; cleared after the block finishes.
• Permanent storage: To retain results, assign outputs to global memory or a global DB.
• Elementary data types (E, A, M, P, L) are always passed as copies, not pointers .

Best Practices

• Use FCs for operations that do not require persistent memory between calls.
• For repeated control tasks requiring memory retention, consider using FBs with instance DBs.
• Optimize parameter passing: simple types via call-by-value, complex types via call-by-reference to reduce memory usage . This structure allows FCs to be efficient, reusable, and suitable for frequently recurring tasks in S7-1200 and S7-1500 PLC programs.