TIA Portal V13: Resolving Address Not Occupied by a Tag Warning

Problem Description

When compiling STL, FBD, or LAD logic in TIA Portal V13 / STEP 7 V13 against an S7-300 (or compatible S7-400) CPU, the compiler emits the diagnostic "The address is not occupied by a tag". The message is treated as a warning, not an error, so the program still loads, runs, and exchanges I/O — but the message pollutes the compile output, blocks clean builds in CI pipelines, and frequently indicates that the FB/FC is touching local stack (L-stack) memory that was never explicitly declared in the block interface.

Typical triggers observed in the field:

• Direct L 0.0, L 20.0 operand references inside an FC or FB without a matching Temp declaration.
• Migrated legacy STEP 7 V5.x code that used absolute L-stack addresses for scratch storage.
• Code pasted from a third-party library where the original block interface was stripped.
• Use of the old OPN %DB10 / DBW0 pair without an explicit L %DB10.DBW0 prefix in the network.

Symptom signature in the inspector window:

[Warning] Block FC100 / Network 7
  The address is not occupied by a tag. 
  Operand: L0.0
  Block: FC100 - "Calc_Phase"

Why the Warning Exists

The TIA Portal V13 compiler enforces IEC 61131-3 symbolic consistency on the S7-300/400 platform. Every memory operand in a code block should resolve to a declared symbol in one of three interface sections:

When the compiler encounters an operand such as L0.0, it has no symbol to map to, so it raises the warning. The address is technically valid on the CPU — the L-stack is always present and addressable from 0.0 through the configured local data size (default 256 bytes on S7-300 CPUs) — but the absence of a declaration means the compiler cannot type-check, optimize, or warn about overlap with neighboring temporary variables. This is the exact failure mode the warning is designed to catch.

Affected Versions and Platforms

Confirmed in TIA Portal V13.0, V13.1, and the V13.1.4 service pack — the warning is generated even when the project option IEC checks is deselected at the block properties. It is not a per-block toggle.

Root Cause Analysis Flow

1. Open the Inspector > Compile tab and double-click the warning line. TIA Portal highlights the offending network and operand.
2. Right-click the block in the project tree and select "Go to > Block interface".
3. Switch to the Temp row. Confirm whether any Temp variable is defined.
4. If empty, the absolute operand is the cause. If a temp exists, verify the symbol is used (not the absolute address) in the code.
5. For migrated V5.x code, compare the V5.5 AWL source against the TIA import. The importer does not generate Temp declarations for raw L-stack references.

Solution 1 — Declare a Temp Variable (Recommended)

The cleanest fix is to give every L-stack operand a symbolic home. Open the block interface, scroll to Temp, add a new row, and select a type that matches the operand width.

1. Open the FC/FB editor in TIA Portal V13.
2. In the interface pane, click the Temp section header.
3. Insert a new row, e.g. Name: PhaseFlag, Data type: Bool.
4. Compile the block. The compiler assigns the variable to offset 0.0 by default.
5. Replace every literal L 0.0 reference in the code with the symbol #PhaseFlag.
6. Recompile — the warning disappears.

// Before (raises warning)
A   L 0.0
=   L 0.0

// After (clean compile)
A   #PhaseFlag
=   #PhaseFlag

Solution 2 — Use the AT Construct in SCL

For blocks authored in SCL, the AT overlay lets a derived view reinterpret the byte layout of an existing variable. This is the only way on S7-300/400 to obtain a sliced view, and the overlay is local to the block — it cannot be exported.

FUNCTION_BLOCK "FB_TempOverlay"
VAR
    RawWord : WORD;    // Static, persisted in instance DB
END_VAR
VAR_TEMP
    AT_View  : AT  %DB."FB_TempOverlay".RawWord : ARRAY[0..1] OF BOOL;
END_VAR
BEGIN
    // AT_View[0] corresponds to bit 0 of RawWord
    AT_View[0] := TRUE;
END_FUNCTION_BLOCK

Reference documentation for the AT construct and the S7-300/400 limitations is the Siemens FAQ "How do you program the overlapping of tags with the keyword AT in the TIA Portal?" (entry ID 57132240).

Solution 3 — Apply the Global Message Filter

If the warning is deemed acceptable (e.g. during migration cleanup), it can be suppressed globally through the message filter. TIA Portal V13 does not expose a per-message toggle, so this silences every warning of the same category.

1. Compile the block.
2. In the Inspector > Compile tab, right-click the warning row.
3. Select "Filter messages of this type" — or click the yellow exclamation mark icon in the toolbar to open the full filter dialog.
4. Uncheck "Warning" to hide all warnings, or expand the category and deselect the specific Address rule.
5. Confirm with OK and recompile.

Solution 4 — Restructure Absolute L-Stack Usage

For long-standing legacy blocks where the original programmer intended raw scratch memory, restructure into a typed temp block:

FUNCTION "FC_Scratch" : VOID
VAR_TEMP
    tByte0  : BYTE;      // offset 0.0 - 0.7
    tByte1  : BYTE;      // offset 1.0 - 1.7
    tWord0  : WORD;      // offset 2.0 - 3.7
    tBool0  : BOOL;      // bit inside tByte0 if needed
        bFlag AT %LW2 : BOOL;  // overlay view in SCL only
END_VAR
BEGIN
    // tByte0, tByte1, tWord0 are now symbolic
END_FUNCTION

This pattern is the recommended migration path for AWL code imported from STEP 7 V5.5. After the restructure, no L-stack address is referenced literally, and the IEC check can be re-enabled.

Solution 5 — Replace OPN DB / DBW0 Pairs

A second common warning in migrated code is the use of OPN %DB10 followed by L DBW0. Replace with a fully-qualified operand:

// Legacy pattern (raises warnings in TIA V13)
OPN   %DB10
L     DBW0
T     %MW100

// TIA-clean replacement
L     %DB10.DBW0
T     %MW100

The fully-qualified form is the only one the V13 STL editor accepts without warning. The DB must be present in the project, compiled without error, and not in "Unlinked" state.

Verification

1. Right-click the program blocks folder → Compile > Software (rebuild all).
2. Open Inspector > Compile. The warning list should show 0 entries for the affected block.
3. Download the project to the S7-300 CPU (or PLCSIM) and perform an online → Compare to confirm block consistency.
4. Force the affected tag in a watch table and toggle the value. The CPU should accept the write without SF / BF diagnostic LEDs.
5. Open Online & Diagnostics > Diagnostic buffer and verify no OB121 (programming error) or OB122 (I/O access error) entries appeared during the test cycle.

Edge Cases and Field Caveats

Multi-instance FBs. Inside a multi-instance FB, the compiler reserves static memory in the parent instance DB. Absolute L-stack addressing inside such a block almost always indicates a bug — the developer likely intended a Static member. Replace with #Instance.MyVar where MyVar is declared under Static.

Library blocks compiled against a different firmware. If a library FC was compiled on TIA V15 and is being used in a V13 project, the warning may be persistent in V13 even though V15 suppresses it. Compile the library from source in V13 to regenerate the block with a V13-compatible symbol table.

Know-how-protected blocks. If you cannot edit the block interface, the message filter is the only static option. Alternatively, replace the protected block with a copy whose interface you can edit.

Optimized vs. standard block access. The S7-300/400 platform uses standard access only. There is no optimized bit/byte/word overlay in the same sense as S7-1200/1500, so the AT construct is the sole method to project a typed view onto the same memory.

Local data size overflow. If the S7-300 CPU is configured with a local data size smaller than the temp footprint of a deeply nested call chain, the CPU will enter STOP with OB121 (length error when reading/writing local data). The compiler warning is the pre-emptive indicator — address it before deployment, not after the first STOP.

Troubleshooting Matrix

Best Practices Going Forward

• Author new logic in SCL where possible. The editor blocks absolute L-stack addressing at source level.
• For STL blocks, set the project option "Warn when address is not occupied by a tag" to Error via the project properties if your engineering rules require clean builds.
• When importing V5.x projects, run the "Migration cleanup" pass that converts absolute temporaries to typed symbols. This is part of the standard TIA migration wizard.
• Reserve AT overlays for the rare cases where byte-level reinterpretation is genuinely required (e.g. status word packing). Never use them as a substitute for a typed interface.