Timers, counters and edges (TON, TOF, TP, CTU, R_TRIG)

All blocks below are standard IEC 61131-3 function blocks. They are in the CODESYS Standard library, TwinCAT Tc2_Standard and TIA Portal (on Siemens as IEC timers/counters TON, CTU etc. with an instance in a DB).

Timers

ET (elapsed time) shows the elapsed time. Typical uses:

• TON — debouncing and filtering short signals, feedback monitoring ("motor did not confirm start within 3 s"), alarm delays on level.
• TOF — fan run-on after stop, holding a signal after it drops.
• TP — coil pulse, fixed-length command.

tFeedback(IN := Run AND NOT Feedback, PT := T#3S);
IF tFeedback.Q THEN
    Fault := TRUE;   (* no feedback within 3 s *)
END_IF;

Counters

Counters respond to an edge, not a level: a held input counts once. Counting is limited by the width of CV (usually INT/WORD) and by the pulse having to last longer than the PLC cycle. For high-frequency encoders and flow meters use high-speed counter modules.

Edges

Internally the block just remembers CLK from the previous cycle. Manual equivalent:

StartPulse := Start AND NOT StartPrev;
StartPrev := Start;

Timer and edge pitfalls in the scan cycle

• One timer instance used in two places: the second call overwrites the first one's state; the timer never completes or fires at the wrong time.
• Conditionally called timer (IF x THEN t(IN := ...)): while the condition is false the block is not called and never sees IN going FALSE — its output freezes.
• An edge on a signal that changes twice within one cycle is missed — the process image only captures the state at the start of the cycle.
• R_TRIG on an output computed later in the same cycle: the edge appears one cycle later than expected.
• Using a timer to measure short intervals precisely: resolution is limited by the cycle time.

Tools

• Function Block Generator — motor and valve with a TON for feedback monitoring.
• PLC Code Reviewer — repeated instance call rule (PLC-W011).
• Interlock Matrix Builder — generates interlock logic in ST.