Converting S7-300 SCL Timers to S7-1200 IEC_TIMER Migration Guide

Converting S7-300 SCL Timers to S7-1200: IEC_TIMER Migration Guide

Engineers porting legacy S7-300 SCL (Structured Control Language) blocks to an S7-1200 CPU under TIA Portal frequently encounter a hard compile error when they try to reuse the original S_ODT, S_PULSE, or S_OFFDT calls. The legacy TIMER data type that S7-300/400 accepted as a VAR_INPUT or VAR_INOUT parameter does not exist in the S7-1200/1500 system data-type catalogue, and the legacy timer-function-block calls are replaced by IEC 61131-3 standard timer instances (TP, TON, TOF, TONR) wrapped in an IEC_TIMER / IEC_LTIMER structure. This reference walks through the conversion, explains the architectural reasoning, provides drop-in SCL syntax, and lists the verification steps that confirm the migrated timer behaves identically.

Architectural Differences: S7-300 Timers vs S7-1200 IEC Timers

The S7-300/400 family exposes timers as a fixed resource pool. Each CPU has a maximum number of timer words (for example, CPU 315-2 PN/DP has 256), and a timer was referenced by a 16-bit integer index plus the legacy TIMER type. The original SCL snippet in the source material demonstrates that pattern:

// S7-300 SCL legacy form (STEP 7 V5.x)
#TimeTS5 := S_ODT(
    T_NO := #IntTimer,        // word-type index into the CPU timer pool
    S    := NOT(#resetT),     // start input
    TV   := #TimeIntT,        // preset time as S5TIME
    R    := #resetT,          // reset input
    BI   := #TimeT,           // current time (BCD output)
    Q    := #B_I_Q);          // timer status

The S7-1200 family removes the timer pool entirely. Every timer is now a software instance of an IEC 61131-3 system function block (TP, TON, TOF, TONR) stored either inside an FB's static area (multi-instance) or inside a dedicated instance DB (single instance). The CPU firmware enforces the limit by RAM usage, not by a fixed word count. The IEC_TIMER and IEC_LTIMER data types are thin wrappers around the timer instance:

The S_ODT call in the source material maps cleanly onto an IEC TON (on-delay timer). The reset behaviour of S_ODT requires the TOF or the rising-edge reset idiom; engineers should not pick the first IEC timer that "looks similar" without mapping the functional behaviour.

IEC 61131-3 Timer Type Mapping

The S7-300/400 legacy timer family has five members. The S7-1200/1500 IEC family has four. The mapping below preserves the input/output behaviour of the original.

Prerequisites

• TIA Portal V16 or later (V17 / V18 / V19 / V20 supported per the Siemens TIA Portal Help).
• STEP 7 S7-SCL option package, version 5.3 SP5 or later, installed on the source machine for migration import.
• Target S7-1200 CPU with firmware 4.2 or later (firmware 4.4+ recommended for full IEC_LTIMER support on all variants).
• Source project archive (\*.s7p, \*.zap) exported from STEP 7 V5.x or STEP 7 Professional V11-V15.

S7-300 SCL Source Syntax (Original)

The original snippet declares an IntTimer input as the legacy TIMER data type and calls S_ODT with named parameters:

VAR_INPUT
    IntTimer : TIMER;          // legacy word-index into CPU timer pool
    TimeIntT : S5TIME;         // preset value in S5 BCD format
    resetT   : BOOL;
END_VAR
VAR_TEMP
    TimeTS5  : S5TIME;
    TimeT    : WORD;
    B_I_Q    : BOOL;
END_VAR

The TIMER type is a pointer-sized word; it identifies a slot in the timer area of the CPU's system memory. This pattern is illegal in S7-1200 SCL and will be flagged at compile time with error 0xE0D8 (Unknown data type) or 0xE002 (Parameter type mismatch), depending on the SCL parser version.

S7-1200 SCL Target Syntax (Converted)

Replace the VAR_INPUT timer declaration with an IEC_TIMER (or IEC_LTIMER) declared inside the FB's VAR_STAT section as a multi-instance, or declare a globally visible DB of type IEC_TIMER for a single-instance approach. The call site changes from the legacy FB form to the IEC FB form.

Multi-Instance Form (Recommended for FBs)

FUNCTION_BLOCK "FB_OnDelay"
{ S7_Optimized_Access := 'TRUE' }
VAR
    resetT : BOOL;
    Start  : BOOL;
    PT     : TIME;            // preset, e.g. T#5s
END_VAR
VAR_STAT
    InstTON : IEC_TIMER;      // multi-instance: lives inside FB instance DB
    ET_Out  : TIME;
    Q_Out   : BOOL;
END_VAR
BEGIN
    // edge-triggered start input, equivalent to S_ODT behaviour
    InstTON(IN := NOT(#resetT) AND #Start,
            PT := #PT);

    // optional level-sensitive reset (preserves S_ODT R input behaviour)
    IF #resetT THEN
        InstTON.IN := FALSE;
    END_IF;

    #Q_Out := InstTON.Q;
    #ET_Out := InstTON.ET;
END_FUNCTION_BLOCK

Single-Instance DB Form

VAR
    sODT_DB : "DB_OnDelay_Inst";  // instance DB of type IEC_TIMER
END_VAR
BEGIN
    sODT_DB.IN := NOT(#resetT);
    sODT_DB.PT := T#5s;
    sODT_DB();                     // explicit FB call
END_VAR

Step-by-Step Conversion Procedure

1. Open the legacy project in TIA Portal. Use Project > Migrate project. Confirm the S7-SCL option 5.3 SP5+ is installed; otherwise the SCL sources are imported as plain text and lose their block attributes.
2. Locate the FB containing S_ODT. Open the SCL editor. The migration tool rewrites the legacy FB call to S_PULSE / S_ODT / S_OFFDT as a comment and inserts a stub IEC_TIMER instance.
3. Replace the TIMER declaration in VAR_INPUT. Delete the line and add the timer as a multi-instance in VAR_STAT, or as a separate instance DB if it must be addressable from outside the FB.
4. Convert the TV argument. Legacy S5TIME (BCD format, e.g., S5T#5s) becomes TIME (e.g., T#5s). For variables, change the type to TIME at the calling site.
5. Map the FB call. Replace S_ODT with TON, S_PULSE with TP, S_ODTS with TONR, and S_OFFDT with TOF. Wire IN :=, PT :=, read Q and ET as outputs.
6. Re-implement the R input. The legacy R is level-sensitive. Use a guarded IF #resetT THEN InstTON.IN := FALSE; END_IF; block, or wrap the start signal with an F_TRIG on resetT to produce a single-cycle reset.
7. Set the optimisation attribute. Mark the FB as Optimised block access (default in new TIA Portal projects). The IEC_TIMER fields are then symbolic; access by offset is no longer required.
8. Compile and resolve errors. Expected residual errors: 0xE0D8 (unknown type - leftover TIMER), 0xE002 (call parameter mismatch), 0x8010 (parameter assignment missing).
9. Download to the PLC. Use the fully qualified symbolic path in the watch table to confirm InstTON.ET increments and InstTON.Q latches after PT.

Why VAR_INPUT ... : TIMER Fails on S7-1200

The TIMER type exists only as a legacy alias for a 16-bit word in the timer area of the S7-300/400 CPU. S7-1200/1500 firmware has no timer area, so the parser rejects the type. The diagnostic message returned by the SCL compiler in TIA Portal V17/V18/V19 is:

Error 0xE0D8: Unknown data type 'TIMER'.
   In FB 'FB_OnDelay', VAR_INPUT section, line 4.

The supported equivalent at an FB boundary is the fully-qualified IEC system data type IEC_TIMER (32-bit time resolution, ms) or IEC_LTIMER (64-bit long time resolution, ns). For a single-instance DB created from the System > Standard Library > IEC Timer panel, the visible fields are the same as those listed in the architecture table above.

Multi-Instance vs Single-Instance DB Configuration

Choose the storage mode by call site, not by preference.

Common Migration Errors and Resolution

Verification Procedure

1. Static syntax check. Right-click the FB → Compile > Software (only). Resolve every error and warning before download.
2. Symbolic watch table. Create a watch table with "FB_OnDelay_DB".InstTON.IN, .PT, .ET, .Q. Force IN high with a rising-edge trigger and observe ET incrementing at OB1 cycle rate.
3. Trace recording. Open Traces > New trace, add the four IEC_TIMER signals, and confirm Q rises exactly at ET >= PT minus one scan.
4. Online-Diff against legacy. If the legacy program is still available, run it on a PLCSIM instance of the S7-300 in parallel and compare trace records sample-for-sample. Drift greater than one OB1 cycle indicates a wiring mistake.
5. Reset behaviour test. Hold resetT high for 5 seconds, release, confirm Q is false at ET = 0, then re-trigger and confirm Q rises after PT as in the original.

Cross-Platform Notes: S7-1500 Compatibility

The exact same conversion applies to S7-1500, with one additional option: IEC_LTIMER supports nanosecond resolution and exposes a STATE byte in the watch table for diagnostics. If the migrated FB will be reused on both S7-1200 (firmware 4.2+) and S7-1500, declare IEC_TIMER only and verify each target's firmware supports the data type. The S7-1200 firmware 4.2 baseline supports IEC_TIMER; firmware 4.4 adds full IEC_LTIMER support on selected CPUs. Refer to the Siemens Industry Online Support entry for your specific CPU article number before relying on IEC_LTIMER.

Edge Cases and Field-Proven Caveats

• S5TIME range: Legacy S5TIME tops out at 9990 seconds (2h 46m 30s). TIME on S7-1200 covers 24 days 20h 31m 23s 647ms. If the original program relied on the S5 range as a safety clamp, replicate the clamp with an IF PT > T#9990s THEN PT := T#9990s; END_IF; guard.
• BCD BI output: The legacy BI returned the residual time as a BCD word for display on operator panels. Replace with InstTON.ET rendered as a TIME/HMI string. HMI tags must change type from WORD to TIME at the WinCC panel side.
• Optimised block access: With Optimised block access on, the IEC_TIMER fields can no longer be addressed by absolute offset (e.g., DB1.DBX0.0). Any third-party HMI driver relying on absolute offsets must be updated to symbolic addressing.
• Retentivity: TON and TOF are non-retentive by default; TONR retains the elapsed time across CPU restarts. If the legacy S_ODT was expected to remember its state across a STOP/RUN transition, declare the timer instance with the Set in IDB retentive attribute, or switch to TONR.
• Multi-instance naming clash: Two FBs cannot declare a static of the same name with different timer types. TIA Portal V17+ enforces unique-instance names across the project; pre-V15 an old compiler accepted the collision and produced silent overlap.

FAQ