TwinCAT Integration

ST LSP includes first-class support for TwinCAT 3 (Beckhoff) project structures, enabling full language intelligence across multi-file TwinCAT projects.

TwinCAT XML Files

TwinCAT stores PLC source code inside XML files. ST LSP transparently extracts the ST declarations and implementation from these files:

File extension	Contents
`.TcPOU`	PROGRAM, FUNCTION_BLOCK, or FUNCTION with Declaration + Implementation sections
`.TcGVL`	Global Variable List (`VAR_GLOBAL … END_VAR`)
`.TcDUT`	Data Unit Type (`TYPE … END_TYPE` for STRUCT, ENUM, ALIAS, UNION)
`.TcIO`	INTERFACE definition

Example .TcPOU structure:

<TcPlcObject>
  <POU Name="PumpController" Id="{...}">
    <Declaration><![CDATA[
FUNCTION_BLOCK PumpController
VAR_INPUT
    Enable : BOOL;
    Setpoint : REAL;
END_VAR
VAR
    pid : PID;
END_VAR
    ]]></Declaration>
    <Implementation>
      <ST><![CDATA[
IF Enable THEN
    pid(Setpoint := Setpoint);
END_IF
      ]]></ST>
    </Implementation>
  </POU>
</TcPlcObject>

ST LSP extracts the CDATA sections, joins Declaration and Implementation with a newline, and parses the result as plain ST. Diagnostic positions are mapped back to the correct line numbers in the original XML file.

Note: Plain .st files are also fully supported alongside .TcPOU files.

Project File Indexing

When a .tsproj or .plcproj file is present in the workspace, ST LSP automatically discovers and indexes all ST source files in the project, enabling cross-file navigation (Go-to-Definition, Find References, Rename) across the entire PLC project.

`.plcproj` (MSBuild format)

Standard TwinCAT PLC project files list source files as MSBuild <Compile> items:

<ItemGroup>
  <Compile Include="POUs\PumpController.TcPOU" />
  <Compile Include="GVLs\GlobalConstants.TcGVL" />
  <Compile Include="DUTs\E_PumpState.TcDUT" />
</ItemGroup>

`.tsproj` (TcSmProject format)

Top-level TwinCAT solution files that reference PLC sub-projects:

<TcSmProject>
  <Project>
    <Plc>
      <Project File="MyPLC\MyPLC.plcproj" />
    </Plc>
  </Project>
</TcSmProject>

Library references

ST LSP reads the library references from the .plcproj file to enable library-aware completion and library diagnostics:

<LibraryReference>
  <LibraryRef Name="Tc2_Standard" Version="*" Vendor="Beckhoff Automation GmbH" />
  <LibraryRef Name="Tc2_Math" Version="*" Vendor="Beckhoff Automation GmbH" />
</LibraryReference>

Only function blocks from referenced libraries are offered in completion lists.

Action Blocks

TwinCAT FUNCTION_BLOCKs can have named ACTION blocks — separate method-like bodies within a single .TcPOU file:

FUNCTION_BLOCK PumpController
VAR
    running : BOOL;
END_VAR

(* ACTION: Start *)
running := TRUE;

(* ACTION: Stop *)
running := FALSE;

ST LSP supports:

Go-to-Definition on PumpController.Start jumps to the ACTION body
Hover on action invocations shows the action name and owning FB
Document Symbols lists actions as children of the FB in the Outline

Pragma Documentation

TwinCAT uses {attribute '...'} pragmas to attach metadata to declarations. Hovering over a pragma shows its documentation:

Pragma	Purpose
`{attribute 'hide'}`	Hides the symbol from IntelliSense and object browsers
`{attribute 'obsolete' := 'message'}`	Marks the symbol as deprecated with a custom warning message
`{attribute 'no_copy'}`	Prevents the FB from being copied (for resource-managing FBs)
`{attribute 'monitoring' := 'call'}`	Controls how the symbol appears in the watch window
`{attribute 'qualified_only'}`	Requires enum values to be fully qualified (`E_Color.RED`, not `RED`)
`{attribute 'reflection'}`	Enables runtime reflection support
`{attribute 'call_after_init'}`	Invokes the FB automatically after PLC initialization
`{attribute 'TcNoFPU'}`	Instructs the compiler not to generate FPU instructions
`{attribute 'enable_dynamic_creation'}`	Allows `__NEW` dynamic instantiation
`{attribute 'OPC.UA.DA' := '1'}`	Exposes the symbol via OPC UA Data Access

Example:

{attribute 'obsolete' := 'Use NewPumpController instead'}
FUNCTION_BLOCK OldPumpController
(* Hover shows: Marks this symbol as deprecated.
   Callers will receive a compiler warning. *)

Standard Library Catalog

ST LSP includes a built-in catalog of Beckhoff standard library function blocks. The catalog provides:

Completion — FB names appear in suggestions when the library is referenced
Hover — inputs, outputs, and description shown on hover
Signature help — parameter list shown while typing a call

Tc2_Standard

The core IEC 61131-3 standard library, automatically catalogued:

Category	Function Blocks
Timers	`TON`, `TOF`, `TP`, `RTC`
Counters	`CTU`, `CTD`, `CTUD`
Edge detection	`R_TRIG`, `F_TRIG`
Bistables	`SR`, `RS`
String operations	`CONCAT`, `LEFT`, `RIGHT`, `MID`, `LEN`, `FIND`, …
Type conversion	`BOOL_TO_INT`, `INT_TO_REAL`, `REAL_TO_INT`, …
Math	`ABS`, `SQRT`, `SIN`, `COS`, `EXP`, `LOG`, …

Note: Additional Beckhoff libraries (Tc2_Math, Tc2_MC2, etc.) are recognized when referenced in the project file. The catalog is extensible via the stdlib.ts source file.