Standard function blocks

Standard function blocks

The Function Blocks defined in IEC 61131-3 second annex (normative) — every implementation, including matiec, must provide them. They split into four families:

Bistables

State elements with S / R set / reset inputs. Behaviour depends on which input has priority.

• SR — Set-dominant bistable. S1 (set) wins over R.
• RS — Reset-dominant bistable. R1 (reset) wins over S.

Edge detectors

One-shot pulse on a BOOL transition.

• R_TRIG — Rising-edge trigger.
• F_TRIG — Falling-edge trigger.

Counters

Hold an INT count, advanced by edges on the count input.

• CTU — Up counter.
• CTD — Down counter.
• CTUD — Up-and-down counter.

Timers

Measure elapsed time in TIME units.

• TON — Timer-on-delay.
• TOF — Timer-off-delay.
• TP — Pulse timer.

Common patterns

Every standard FB follows the same use pattern:

1. Declare an instance variable in the calling POU’s VAR section: myInstance : <FB_TYPE>;.
2. Call the instance every cycle (or conditionally), passing the inputs as named parameters: myInstance(IN := x, PT := T#100ms);.
3. Read the outputs through the instance: IF myInstance.Q THEN ... END_IF;.

The full pattern with declaration is documented at Function and FB calls > FB instances vs. function calls.