Functions and function blocks (FUNCTION, FUNCTION_BLOCK)

Program organisation units (POUs)

IEC 61131-3 distinguishes three kinds of program organisation unit:

A function always returns the same result for the same inputs. A function block keeps its variables between calls — for that an instance is declared. On Siemens a function is an FC, a block an FB, and instance memory lives in an instance DB or as a multi-instance inside another FB.

Interface

FUNCTION_BLOCK FB_Motor
VAR_INPUT
    Start, Stop, Feedback : BOOL;
    FeedbackTime : TIME := T#3S;
END_VAR
VAR_OUTPUT
    Run, Fault : BOOL;
END_VAR
VAR
    tFeedback : TON;
END_VAR

Call: Motor1(Start := bStart, Stop := bStop, Feedback := bFb); — each motor has its own instance Motor1, Motor2, …

Rules for reusable blocks

• One instance, one call per cycle. Calling the same instance elsewhere in the program breaks the timers and edges inside it. The code reviewer reports this.
• The block does not touch global variables or I/O directly — everything goes through the interface. Otherwise it cannot be reused or tested.
• A fault latches until explicitly acknowledged (Reset) and does not clear itself when the cause disappears.
• Safe default state: on a fault or loss of communication the block's output goes to a state that creates no hazard.
• Temporary variables (VAR_TEMP) do not keep their value — never use them for memory between cycles.

Tools

• Function Block Generator — motor and valve blocks with feedback monitoring and acknowledgement.
• PLC Code Reviewer — repeated instance calls, undeclared variables, unused outputs.