SCL Timer Functions: TON, TOF, TP, TONR

Siemens S7-1200/1500 supports IEC 61131-3 timer function blocks. Each timer instance requires its own instance data (either a named instance or multi-instance in an FB).

TON — On-Delay Timer (Einschaltverzögerung)#

Output Q goes TRUE after the input IN has been continuously TRUE for the duration PT.

Syntax:

#TON_Instance(IN := #Start_Condition,
              PT := T#5s);

#Delayed_Output := #TON_Instance.Q;
#Elapsed := #TON_Instance.ET;

Parameters:

Behavior:

• IN goes TRUE → timer starts counting
• ET reaches PT → Q goes TRUE
• IN goes FALSE before PT → timer resets, Q stays FALSE
• IN goes FALSE after Q is TRUE → Q goes FALSE, ET resets

S5 equivalent: SD (Einschaltverzögerung) — exact match.

TOF — Off-Delay Timer (Ausschaltverzögerung)#

Output Q goes TRUE immediately when IN goes TRUE. When IN goes FALSE, Q stays TRUE for the duration PT.

Syntax:

#TOF_Instance(IN := #Motor_Running,
              PT := T#10s);

#Fan_Output := #TOF_Instance.Q;  // Fan runs 10s after motor stops

Behavior:

• IN goes TRUE → Q goes TRUE immediately
• IN goes FALSE → timer starts counting
• ET reaches PT → Q goes FALSE
• IN goes TRUE before PT → timer resets, Q stays TRUE

S5 equivalent: SA (Ausschaltverzögerung) — exact match.

TP — Pulse Timer (Impulstimer)#

Output Q goes TRUE for exactly the duration PT after a rising edge on IN. The output runs for the full duration regardless of the input state.

Syntax:

#TP_Instance(IN := #Trigger,
             PT := T#2s);

#Pulse_Output := #TP_Instance.Q;  // TRUE for exactly 2 seconds

Behavior:

• Rising edge on IN → Q goes TRUE, timer starts
• ET reaches PT → Q goes FALSE
• IN state during timing has NO effect on Q (differs from S5 SI!)
• New rising edge while timer is running is ignored (NOT retriggerable, differs from S5 SE!)

S5 equivalent: SI and SE are approximately TP, but with important differences:

• S5 SI is "cut short" when input goes FALSE → TP is not
• S5 SE is retriggerable → TP is not
• For exact S5 behavior, use S_PULSE (for SI) or S_PEXT (for SE)

TONR — Accumulating On-Delay Timer (S7-1500 only)#

Accumulates time across multiple input pulses. The timer does NOT reset when IN goes FALSE — it pauses and continues when IN goes TRUE again.

Syntax:

#TONR_Instance(IN := #Condition,
               R  := #Reset,
               PT := T#60s);

#Total_Output := #TONR_Instance.Q;
#Accumulated := #TONR_Instance.ET;

Parameters:

Behavior:

• IN TRUE → timer counts up
• IN FALSE → timer pauses (ET keeps its value)
• IN TRUE again → timer continues from where it paused
• ET reaches PT → Q goes TRUE
• R pulse → ET resets to 0, Q goes FALSE

S5 equivalent: There is NO S5 equivalent. S5 SS (speichernde Einschaltverzögerung) is sometimes confused with TONR, but SS does NOT accumulate — it starts once and runs to completion. TONR accumulates across multiple pulses.

Availability: TONR is available on S7-1500 only (not on S7-300, S7-400, or S7-1200).

Timer Summary Table#

Declaration Example (FB with Multi-Instance)#

VAR
    #Delay_Timer : TON;      // On-delay instance
    #Fan_Timer : TOF;         // Off-delay instance
    #Pulse_Timer : TP;        // Pulse instance
    #Accum_Timer : TONR;      // Accumulating instance (S7-1500 only)
END_VAR

Part of the SCL Reference. See also: S5 Timer Types to S7 Equivalents. Maintained by PLCcheck.ai.

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