Reusing S7 SCL Timers Multi-Instance FB Pattern for Belt

Problem Statement: Monitoring 20 Elevator Belts with One Timer Block

A common S7 SCL question is: "Can a single timer instance drive 20 independent belt-rotation watchdogs?" The short answer is no. A TIMER or IEC timer instance is a binary state container — it can only hold one running value, one elapsed value, and one output bit at a time. Calling the same timer from twenty parallel logic branches creates scan-race conditions, residual values from the previous branch, and unpredictable STAT outputs.

This article walks through three field-proven patterns for reusing timer logic across many monitored devices:

1. Multi-instance Function Block (SFB4 / TON) — the canonical Siemens answer.
2. Counter-based emulation using OB35 100 ms pulses — the original poster's working solution.
3. IEC_TON / TP inside a re-instantiable FB on S7-1200/1500 in TIA Portal — the modern equivalent.

All three patterns share one principle: the timer state must live in its own background DB (instance DB), one per monitored device, never shared.

Why a Single S5TIME / IEC Timer Cannot Be Reused

Each call to S_ODT, S_PULSE, S_PEXT, or S_ODTS writes to a fixed word-addressable timer word (T0..T2047 on S7-300/400, T0..T511 on S7-1200). The timer word contains the BCD-encoded time base, the time value, and the output bit. Two concurrent invocations of the same timer word overwrite each other every scan.

According to the SIMATIC S7-1200 manual on Timer Operation (IEC Timers), each IEC timer block (TP, TON, TOF, TONR) is a system function block (SFB3 / SFB4 / SFB5) that requires a dedicated instance DB holding the running elapsed time ET and the output Q. Reusing one instance DB is not supported and produces undefined behavior on overflow of the internal time accumulator.

Three documented options exist:

1. Pass the timer as an in-out parameter and instantiate 20 separate instance DBs at the call site.
2. Create a re-instantiable FB and instantiate it 20 times (multi-instance pattern).
3. Replace the hardware/software timer with a software counter driven by a fixed clock OB.

Prerequisites

• STEP 7 V5.x (S7-300/400) or TIA Portal V16+ (S7-1200/1500).
• SCL compiler enabled (Options → Package Manager → SCL).
• For pattern 1/2: at least 20 unused timer words in the system data, or 20 unused IEC timer instance DBs.
• For pattern 3: OB35 (cyclic interrupt) configured to 100 ms in the CPU properties (Hardware → Cyclic Interrupts).
• Familiarity with FB/DB creation and multi-instance capability of the CPU (any S7-300/400/1200/1500 supports multi-instances).

Pattern 1: Multi-Instance FB with SFB4 (S_ODTS / TON)

The cleanest pattern is to wrap the timer inside a re-instantiable FB. Each call to the FB automatically allocates a new instance DB holding the timer state.

FB definition (S7-300/400 SCL)

FUNCTION_BLOCK FB_BeltWatchdog
VAR_INPUT
  Run         : BOOL;   // motor running feedback
  Pulse       : BOOL;   // one pulse per motor revolution
  Preset_ms   : INT;    // timeout in milliseconds
END_VAR
VAR_OUTPUT
  Alarm       : BOOL;   // TRUE if no pulse within Preset_ms
END_VAR
VAR
  TonInst     : SFB4;   // IEC "TON" – multi-instance capable
  ET_ms       : INT;    // elapsed time in ms (sampled)
  xEdge       : BOOL;   // rising edge memory
END_VAR
BEGIN
  // TON: output Q high when IN has been TRUE for PT duration
  TonInst(IN := Run AND NOT Pulse,
          PT := DINT_TO_TIME(INT_TO_DINT(Preset_ms)),
          Q  := Alarm);
END_FUNCTION_BLOCK

OB1 call site

DATA_BLOCK DB_Belt_01 FB_BeltWatchdog;   BEGIN END_DATA_BLOCK;
DATA_BLOCK DB_Belt_02 FB_BeltWatchdog;   BEGIN END_DATA_BLOCK;
... (repeat 20 times) ...

// OB1 – cyclic
FB_BeltWatchdog.DB_Belt_01(Run := "Mot01_Run",  Pulse := "Mot01_RevPulse", Preset_ms := 8000);
FB_BeltWatchdog.DB_Belt_02(Run := "Mot02_Run",  Pulse := "Mot02_RevPulse", Preset_ms := 8000);
...

Each DB_Belt_NN is a true instance DB and contains its own SFB4 state. The timer word is allocated automatically by the SFB4 instance — no manual T-number management required. This is the method Siemens officially recommends in the SiePortal "Timer in SCL" thread and in the S7-1200/1500 system manual.

Pattern 2: OB35 Counter Emulation (Original Working Solution)

When the CPU has no spare SFB4 instances, or you want a fully software-defined timer independent of T-numbers, replace the timer with a down-counter clocked by OB35. The original poster's FB_CHECKTURN_ELEVATOR uses exactly this technique. Cleaned up and ported to modern SCL:

OB35 configuration (100 ms)

Open Hardware → CPU Properties → Cyclic Interrupts. Set OB35 execution time to 100 ms. The OB35 priority (default 12) pre-empts OB1, guaranteeing the 100 ms tick is consistent regardless of OB1 scan time.

OB35 – clock flag generator

// OB35 (priority 12, 100 ms)
"Clock_100ms" := NOT "Clock_100ms";   // 50% duty, 100 ms period

FB_CHECKTURN_ELEVATOR (SCL, S7-300/400-compatible)

FUNCTION_BLOCK FB_CHECKTURN_ELEVATOR
VAR_INPUT
  PV        : WORD;    // preset in 100 ms units (e.g., 80 = 8 s)
  CV_I      : WORD;    // retained counter value (in)
  PULS      : BOOL;    // motor revolution pulse
  RUN       : BOOL;    // motor running feedback
  BITM_I    : BOOL;    // edge memory (retained)
END_VAR
VAR_OUTPUT
  STAT      : BOOL;    // alarm
  CV_O      : WORD;    // retained counter value (out)
  BITM_O    : BOOL;    // edge memory (out)
END_VAR
VAR
  CV        : INT;
  xEdge     : BOOL;
END_VAR
BEGIN
  // Load retained value or reset on stop / pulse
  IF NOT RUN OR PULS THEN
    CV := WORD_TO_INT(PV);
  ELSE
    CV := WORD_TO_INT(CV_I);
  END_IF;

  // 100 ms rising-edge detection
  xEdge  := "Clock_100ms" AND NOT BITM_I;
  BITM_O := "Clock_100ms";

  IF xEdge THEN
    IF RUN THEN CV := CV - 1; END_IF;
    IF CV < 0 THEN CV := 0; END_IF;
  END_IF;

  STAT := (CV = 0);
  CV_O := INT_TO_WORD(CV);
END_FUNCTION_BLOCK

Why the retained values matter

Because OB35 is a separate priority class, the FB's read-modify-write of CV must use the value persisted in the instance DB at the end of the last call. Inputs CV_I / BITM_I and outputs CV_O / BITM_O are passed by the caller so the values survive OB35 restarts. The example shown by the original poster implements exactly this retention through the instance DB.

Pattern 3: Modern IEC Timer in TIA Portal (S7-1200/1500)

On S7-1200 firmware V4.0+ and S7-1500, the IEC timer blocks are first-class citizens of the SCL language. The cleanest reuse pattern uses the multi-instance FB approach with a system IEC timer:

FUNCTION_BLOCK FB_BeltWatchdog_1200
VAR_INPUT
  Run         : BOOL;
  Pulse       : BOOL;
  PresetTime  : TIME;   // e.g., T#8s
END_VAR
VAR_OUTPUT
  Alarm       : BOOL;
END_VAR
VAR
  Ton_Inst    : IEC_TIMER;   // TON, TP, TOF, or TONR
  Ton_DB      : IEC_TON_DB;  // background DB type, allocated by SCL
END_VAR
BEGIN
  Ton_Inst(IN := Run AND NOT Pulse,
           PT := PresetTime,
           Q  := Alarm);
END_FUNCTION_BLOCK

Compile and call the FB twenty times — TIA Portal automatically generates twenty distinct instance DBs of type FB_BeltWatchdog_1200, each containing its own IEC_TON_DB. No manual T-number bookkeeping, no OB35 needed, no CPU priority tuning.

Parameter Reference: Timer-Type Selection

Verification Procedure

1. Offline compile. In STEP 7 / TIA Portal, compile the SCL source. Expect 0 errors. Warnings about "timer not used" should be ignored if the timer is hidden inside the FB.
2. Cross-reference. Run PLC → Monitor/Modify → Cross References on the FB name. Confirm 20 unique instance DBs, each referencing exactly one timer word or one IEC_TON_DB.
3. Online force PULS low. With the motor running, force PULS = FALSE on one belt. Verify that STAT transitions TRUE within PV × 100 ms (OB35 pattern) or PresetTime (IEC timer pattern).
4. Online force PULS high. Verify that STAT clears on the next pulse and the counter reloads PV.
5. Load distribution check. In OB35 mode, set PV to a known value (e.g., 50 = 5 s) and observe CV_O in the instance DB decrementing once every 100 ms with "Clock_100ms" toggling.
6. OB35 overrun check. In TIA Portal: Online → Diagnostics → Cycle Time. The OB35 measured time must be < configured time (100 ms) or the system raises OB80 (time error) and skips ticks. With 20 belts, OB35 runtime is typically < 1 ms, well within budget.
7. Restart test. Power-cycle the CPU (or STOP→RUN). Verify retained values CV_O / BITM_O persist when using the OB35 pattern with the instance DB marked Non-Retain = No (default for IDBs).

Common Pitfalls and Diagnostics

Migration: S5TIME to IEC_TIMER

Legacy S7-300/400 code frequently uses S_ODT(T_NO := T12, ...) with S5TIME values. When porting to S7-1200/1500:

• Replace S5TIME with TIME (1.0 ns resolution; T#8s literal).
• Replace T_NO parameter with a multi-instance variable of type IEC_TIMER.
• Convert BI (BCD out) to ET (elapsed TIME) — BI is removed in IEC blocks.
• Confirm CPU firmware ≥ V4.0 (S7-1200) or V1.8 (S7-1500) for full IEC_TON support.

Field Notes and Recommendations

For plants of 20 to 50 monitored belts, the multi-instance FB pattern (Pattern 1 or 3) is the lowest-maintenance solution. It uses one timer SFB per FB instance, all allocated automatically, and the FB is fully re-usable across projects. Reserve Pattern 2 (OB35 counter) for very small CPUs where SFB4 instance count is constrained, or where the time base must be exact and independent of SFB4 scheduling jitter.

Avoid mixing patterns: do not call Pattern 1 and Pattern 2 against the same alarm tag, or the alarm semantics (latched vs. pulse) will differ between belts.

Document the time base — "PresetTime = T#8s means 8.0 s ± 1 OB35 tick" — in the FB header comment. Field engineers are frequently tripped up by OB35 pattern quantisation when an SFB TON would deliver 1 ms precision.

FAQ