Compiling STL Source Files into S7 Blocks: Resolving VOID Errors

Compiling STL Source Files into S7 Blocks in STEP 7

Engineers migrating legacy S5 programs to S7-300/S7-400 platforms routinely encounter a single, consolidated STL source file containing dozens of network blocks (OB, FC, FB, DB, UDT). The task is to split that monolithic source back into the original block structure and compile each block into a downloadable program object. This reference covers the complete workflow: generating source from existing blocks, manually restructuring converted S5 source, diagnosing the classic VOID compile error, and verifying the final block library.

1. STL Source File Model in STEP 7

A STEP 7 STL source file is an ASCII text container that holds one or more block declarations followed by their code networks. Each block type has its own opening keyword and a closing END_ terminator that must be matched by the compiler. The file is edited either inside the STL Editor of the SIMATIC Manager or in any external text editor, then compiled back into the S7 program container.

The block declaration syntax is fixed and case-insensitive but must include a return type (or VOID for code-only blocks) followed by the begin/end envelope:

2. Prerequisites

1. SIMATIC Manager (STEP 7 V5.x) installed with the optional S5 to S7 Converter package for legacy migration paths. For TIA Portal users, the equivalent functionality is described in the official Saving blocks as external source files documentation.
2. An S7 project (S7-300, S7-400, or WinAC) containing a Sources folder in the program container.
3. Read/write access to the project path on the engineering station.
4. Familiarity with STL mnemonics: A, O, AN, S, R, =, L, T, U(, UN(, JC, JCN, CALL, BE, BEU, BEB.
5. (For S5 migration) The original S5 STL/STL source or a converted source file generated by the converter.

3. Generating Source from Existing Blocks

When you already have compiled blocks (FC, FB, DB, OB, UDT) and want to edit them as text or back them up externally, generate a single source file that contains every block. This is the reverse path of the migration problem and is useful for diff/merge, version control, and bulk find/replace operations.

1. Open the SIMATIC Manager and select the Program Blocks folder (or the S7 program container) of the target station.
2. Right-click the Sources folder and choose Insert New Object → STL Source File. Name it (e.g., PlantLibrary) and confirm with OK.
3. Open the new (empty) source file. The compiler inserts a header comment block automatically.
4. Select the blocks in Program Blocks that you want to export (Ctrl+Click for non-contiguous selections).
5. Drag the selection onto the open source editor. Each selected block is written into the source as a separate, properly delimited declaration. FBs, FCs, OBs, DBs, and UDTs are all supported.
6. Alternatively, choose File → Generate Source from the menu bar of the Program Blocks window, then pick the blocks and an output source name.
7. Save the source (Ctrl+S).

The result is a single .s7s (or .awl in some installations) text file containing every block. You can edit it directly, then recompile to overwrite the original blocks.

4. Converting an S5 Program to S7 STL Source

Siemens historically supplied the S5 to S7 Converter as an optional add-on under Start → SIMATIC → STEP 7 → S5 to S7 Converter. The converter reads an S5 program file (.s5p on disk) and produces an S7 project containing:

• An S7 program container with converted FC, FB, DB, OB equivalents.
• A single STL source file inside the Sources folder that aggregates the converted code. The source is the canonical, human-editable form of the migration and is the file engineers edit to clean up conversion artifacts before re-compilation.
• A conversion log listing remapped operand areas, changed mnemonics, and unresolved references.

Open the converted STL source in the STL editor. The first declaration looks similar to this:

FUNCTION FC 30 : VOID
BEGIN
NETWORK
TITLE = converted FC30 - motor interlock
      A   "M 0.1";
      AN  "I 1.7";
      S   "M 0.2";
END_FUNCTION

Each block in the converted source is delimited by a FUNCTION ... END_FUNCTION (or analogous) envelope. To split the aggregate source back into separate blocks, copy each envelope (declaration through terminator) into its own STL source file or compile the aggregate file directly and let STEP 7 create the individual block objects.

5. The VOID Keyword: Compile Error Root Cause

The single most common compile error after an S5-to-S7 conversion is a parse failure caused by the return type placed immediately after the colon in the FC declaration. Two scenarios produce this error:

1. Missing VOID: Older converter outputs or hand-typed declarations omit the return type, leaving the parser expecting a type token that never arrives.
2. Stray/misplaced VOID: The S5-to-S7 converter often prefixes VOID in front of the block instead of after the colon, producing lines such as VOID FUNCTION FC 30. The grammar requires VOID to appear after the colon, not before FUNCTION.

Correct syntax accepted by the compiler:

FUNCTION FC 30 : VOID
BEGIN
...
END_FUNCTION

Incorrect syntax that fails to compile:

VOID FUNCTION FC 30       // wrong position
BEGIN
...
END_FUNCTION

Use Edit → Find/Replace with Regular expressions enabled to bulk-fix legacy sources:

• Search: ^VOID\s+FUNCTION\s+(FC|FB|OB)\s+(\d+)
• Replace: FUNCTION \1 \2 : VOID

For FBs that legitimately return a value, replace VOID with the correct type (INT, REAL, etc.). For multi-instance FBs, the FC equivalent remains VOID because the multi-instance model is FB-only.

6. Step-by-Step: Compile an STL Source into Blocks

1. In SIMATIC Manager, open the Sources folder of the target S7 program.
2. Double-click the STL source (e.g., PlantLibrary or S5_Migrated) to open the STL editor.
3. Verify the syntax of each block declaration. Confirm that every FUNCTION line carries a return type, every BEGIN is paired with a matching END_FUNCTION / END_ORGANIZATION_BLOCK / END_DATA_BLOCK / END_FUNCTION_BLOCK, and that block numbers are unique within their type.
4. Select File → Compile from the menu bar, or press Ctrl+B. STEP 7 parses the source and creates each declared block in Program Blocks. Existing blocks with the same number are overwritten after a confirmation prompt.
5. Watch the Compiler Output window. Errors are listed with the line number and a short description. Double-click an error to jump to the offending line.
6. Fix each error, save the source (Ctrl+S), and recompile until the output is clean.
7. After a successful compile, the newly generated blocks appear in the Program Blocks folder with the symbols from the source comments (when Symbolic Names is enabled).

7. Block Type Reference and Syntax Templates

The following templates cover the four block types you will encounter most often when migrating S5 programs. Paste them into a new STL source, then replace the placeholder numbers and code.

7.1 Function (FC) without Return Value

FUNCTION FC 100 : VOID
VAR_TEMP
    tAux    : INT;
END_VAR
BEGIN
NETWORK
TITLE = sample FC
      L   0;
      T   #tAux;
      BE;
END_FUNCTION

7.2 Function Block (FB) with Instance Data

FUNCTION_BLOCK FB 50
VAR
    iCounter  : INT;
    bEnable   : BOOL;
END_VAR
VAR_TEMP
    tLoop     : INT;
END_VAR
BEGIN
NETWORK
TITLE = sample FB
      A   #bEnable;
      JCN _SKIP;
      L   #iCounter;
      +   1;
      T   #iCounter;
_SKIP: NOP 0;
END_FUNCTION_BLOCK

7.3 Data Block (DB) with Address Assignment

DATA_BLOCK DB 200
  STRUCT
    iSetpoint   : INT;
    rActual     : REAL;
    bReady      : BOOL;
    sLabel      : STRING[20];
  END_STRUCT;
BEGIN
END_DATA_BLOCK

7.4 Organization Block (OB1 cyclic main)

ORGANIZATION_BLOCK OB 1
BEGIN
NETWORK
TITLE = cyclic main
      CALL FC 100;
      CALL FB 50 , DB 50;
      BE;
END_ORGANIZATION_BLOCK

8. Converting Block Types via Source Edit

STEP 7 does not provide a direct "convert FC to FB" wizard. The pragmatic path is source-level conversion:

1. Generate source from the existing FC (drag the FC onto a new STL source).
2. Edit the source: change FUNCTION FC x : VOID to FUNCTION_BLOCK FB y; replace END_FUNCTION with END_FUNCTION_BLOCK; convert any VAR_TEMP to VAR if you need retentive state across calls.
3. Compile the source. The new FB object appears in Program Blocks.
4. Generate an instance DB for the new FB: right-click Program Blocks → Insert New Object → Data Block; specify DB number and tie it to the FB via the Instance Of dropdown.

9. Verification Procedure

After a successful compile, perform these checks before downloading to the PLC:

1. Cross-reference: Open Options → Reference Data → Display. Verify that every called block exists, every referenced tag is declared, and there are no unresolved operands.
2. Symbol table consistency: Open the symbol table and confirm that every tag referenced in the new blocks has a defined symbol. Missing symbols produce downloads that succeed but operate on undefined addresses.
3. Block consistency check: In SIMATIC Manager, select Program Blocks and choose Edit → Check Block Consistency. Resolve any reported inconsistencies before download.
4. Download to PLCSIM: Use PLCSIM (S7-PLCSIM) to validate the compiled blocks in an offline simulation. Cycle power (STOP → RUN) and exercise the call hierarchy before connecting to real hardware.
5. Watch tables: Build a watch table that monitors the input, output, and intermediate tags of each newly compiled block. Trigger a single scan and verify that the block produces the expected results.
6. Compiler output review: Re-read the compiler output for warnings (yellow) as well as errors (red). The converter can emit warnings about implicit type conversions, missing BEGIN alignment, or deprecated mnemonics.

10. Troubleshooting Matrix

11. Best Practices for STL Source Maintenance

• One source file per logical unit. Keep migration sources separate from ongoing maintenance sources to avoid accidental overwrites.
• Header comments with block numbers. Insert a comment block at the top of each STL source listing every contained block number, version, and date. This aids diff/merge and review.
• External version control. STL sources are plain text — store them in Git, SVN, or TFS alongside the project. STEP 7 regenerates the binary blocks on every compile, but only the source text is diff-friendly.
• Consistent indentation. Use two-space indentation inside NETWORK blocks so that nested bit logic and arithmetic read cleanly.
• Explicit NETWORK separators. Every FC, FB, and OB network begins with NETWORK followed optionally by TITLE = .... This drives the LAD/FBD view if a colleague later switches representation.
• Avoid U( / O( nesting beyond 7 levels. The STL parser stack is limited; deep nesting produces obscure "Stack overflow" compile errors.
• Use symbols, not absolute addresses. Replace I 1.7 with "EM_Stop" in the symbol table; the converter already generates symbolic operand references that are far easier to maintain.

12. Working with TIA Portal (V20)

Engineers who migrated from SIMATIC Manager to TIA Portal can still manage STL and SCL blocks as external source files. The workflow is described in the official Saving blocks as external source files documentation. The same FUNCTION ... END_FUNCTION grammar applies. Compile is invoked by right-clicking the source in the project tree and selecting Compile → Software (rebuild all blocks). Errors land in the Compile tab of the Inspector window.

FAQ