How to Use Function Blocks in Siemens PLC Programming

In Siemens PLC programming, use Function Blocks (FB) to create reusable code modules with internal memory. You declare an FB, instantiate it as an instance data block, and call it in your program to execute its logic with stored states.

Syntax

A Function Block (FB) in Siemens PLC is declared with a name and contains code and internal variables. To use it, you create an instance data block (DB) that stores its data, then call the FB in your program with the instance DB as a parameter.

Key parts:

• FUNCTION_BLOCK FB_Name: Declares the function block.
• VAR_INPUT: Input variables to the FB.
• VAR_OUTPUT: Output variables from the FB.
• VAR: Internal variables that keep state.
• END_FUNCTION_BLOCK: Ends the FB declaration.
• In the main program, instantiate the FB with a data block, e.g., MyFB_Instance : FB_Name;
• Call the FB with MyFB_Instance(); to execute it.

structured_text

FUNCTION_BLOCK FB_Timer
VAR_INPUT
  Start : BOOL;
  PT : TIME;
END_VAR
VAR_OUTPUT
  Q : BOOL;
  ET : TIME;
END_VAR
VAR
  StartTime : TIME;
  Running : BOOL;
END_VAR

IF Start AND NOT Running THEN
  StartTime := TIME();
  Running := TRUE;
END_IF

IF Running THEN
  ET := TIME() - StartTime;
  IF ET >= PT THEN
    Q := TRUE;
    Running := FALSE;
  ELSE
    Q := FALSE;
  END_IF
ELSE
  ET := T#0s;
  Q := FALSE;
END_IF
END_FUNCTION_BLOCK

Example

This example shows how to declare a simple timer function block and use it in a main program. The FB counts elapsed time when started and sets an output when the preset time is reached.

structured_text

FUNCTION_BLOCK FB_Timer
VAR_INPUT
  Start : BOOL;
  PT : TIME;
END_VAR
VAR_OUTPUT
  Q : BOOL;
  ET : TIME;
END_VAR
VAR
  StartTime : TIME;
  Running : BOOL;
END_VAR

IF Start AND NOT Running THEN
  StartTime := TIME();
  Running := TRUE;
END_IF

IF Running THEN
  ET := TIME() - StartTime;
  IF ET >= PT THEN
    Q := TRUE;
    Running := FALSE;
  ELSE
    Q := FALSE;
  END_IF
ELSE
  ET := T#0s;
  Q := FALSE;
END_IF
END_FUNCTION_BLOCK

PROGRAM Main
VAR
  Timer1 : FB_Timer;
  StartSignal : BOOL := TRUE;
  PresetTime : TIME := T#5s;
END_VAR

// Call the function block
Timer1(Start := StartSignal, PT := PresetTime);

// Outputs
// Timer1.Q will be TRUE after 5 seconds
// Timer1.ET shows elapsed time
END_PROGRAM

Output

After 5 seconds, Timer1.Q = TRUE and Timer1.ET >= T#5s

Common Pitfalls

Common mistakes when using function blocks in Siemens PLC include:

• Not creating an instance data block for the FB, which causes loss of internal state.
• Calling the FB without passing the instance, leading to errors.
• Confusing FUNCTION (stateless) with FUNCTION_BLOCK (stateful).
• Not initializing internal variables properly, causing unexpected behavior.

Always remember that each FB instance keeps its own data, so you must declare separate instances for multiple uses.

structured_text

(* Wrong: Calling FB without instance *)
PROGRAM Main
VAR
  Timer1 : FB_Timer;
END_VAR

// Incorrect call - missing instance
FB_Timer(Start := TRUE, PT := T#5s);
END_PROGRAM

(* Right: Call with instance *)
PROGRAM Main
VAR
  Timer1 : FB_Timer;
END_VAR

Timer1(Start := TRUE, PT := T#5s);
END_PROGRAM

Quick Reference

Concept	Description
FUNCTION_BLOCK	Declares a reusable block with memory
VAR_INPUT	Inputs to the function block
VAR_OUTPUT	Outputs from the function block
VAR	Internal variables to keep state
Instance Data Block	Stores data for each FB instance
Calling FB	Use instance name with parameters to run FB

Key Takeaways

Function blocks in Siemens PLC store internal state and require instance data blocks.

Declare FBs with inputs, outputs, and internal variables using structured text syntax.

Always create and use an instance of the FB to preserve data between calls.

Call the FB in your program by passing inputs and reading outputs from the instance.

Avoid calling FBs without instances and confusing FBs with stateless functions.