PLCSim
Simulate PLC programs in Structured Text (IEC 61131-3) directly in VS Code — no hardware needed.
Open a .st file, run the simulator, and interact with your program in real time. Free forever for learning and prototyping. Pro unlocks the tools you need for serious work.
Get started in 3 steps
1. Install PLCSim from the VS Code Marketplace.
2. Open the PLCSim panel from the Activity Bar (sidebar icon). Load one of the built-in examples or open your own .st file.
3. Run the simulator and interact in real time — toggle inputs, change values, watch outputs react. No compilation, no deploy, no hardware.
→ Full documentation at plcsim.appscat.cat
→ User Manual
Free — Full IEC 61131-3 Syntax & Simulator
Write and run complete Structured Text programs without restrictions.
Language (IEC 61131-3 Edition 3)
| Feature |
|
| Scalar types |
BOOL · SINT INT DINT LINT · USINT UINT UDINT ULINT · REAL LREAL · TIME DATE TIME_OF_DAY DATE_AND_TIME · STRING WSTRING · BYTE WORD DWORD LWORD |
| User-defined types |
STRUCT, ENUM (with optional default member), ARRAY (1D and 2D, including arrays of function block instances), typed literals (WORD#16#FF, DINT#-100) |
| References |
REFERENCE TO / REF_TO (IEC 61131-3 Edition 3 standard) — REF(), dereference ^, NULL, with clear runtime errors on NULL access |
| Variables & scope |
VAR, VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, VAR_TEMP, VAR_GLOBAL, PERSISTENT, RETAIN |
| Control flow |
IF / ELSIF / ELSE, FOR, WHILE, REPEAT, CASE, RETURN, EXIT, CONTINUE |
| Operators |
Arithmetic: + - * / MOD ** · Comparison: = <> < > <= >= · Logic: AND, OR, NOT, XOR · Bitwise (WORD/INT): AND, OR, XOR, NOT · Shift/Rotate: SHL, SHR, ROL, ROR |
| Comments |
Block (* ... *), line // ..., doc comments (** ... *) for auto-generated documentation |
| Timers |
TON, TOF, TP, TONR |
| Counters |
CTU, CTD, CTUD |
| Bistables & edges |
RS, SR, R_TRIG, F_TRIG |
| Standard functions |
30+ functions: String (LEN, CONCAT, REPLACE, LEFT, RIGHT, MID), Math (ABS, SQRT, SIN, COS, ROUND, FLOOR, CEIL, TRUNC), Type conversions (all scalar types), Time/Date (ADD_TIME, SUB_TOD, CONCAT_DATE_TOD, etc.), Array bounds (LOWER_BOUND, UPPER_BOUND) |
| Syntax highlighting |
Full .st language support |
| Examples |
20 ready-to-run programs included |
Simulator
| Feature |
|
| Interactive panel |
Toggle BOOL inputs, edit numeric values, watch outputs react in real time |
| Scan cycle control |
Configurable from 10 ms to 5000 ms |
| Trend charts |
Oscilloscope view for every variable — 30 s of history, auto-scrolling |
| Cold / Warm restart |
Explicit restart buttons in the Simulator panel — Cold clears RETAIN state, Warm preserves it, matching real PLC restart semantics |
Editor Intelligence (LSP)
| Feature |
|
| Real-time diagnostics |
Syntax and type errors underlined as you type — no need to run the simulator |
| Code completion |
IntelliSense for variables, function blocks, standard functions and keywords |
| Hover documentation |
Type information and signatures on hover |
| Go to Definition |
F12 to jump to any variable or function block declaration |
| Find All References |
See every usage of a symbol across your project |
| Rename Symbol |
F2 to safely rename a symbol and all its references in one step |
| Cross-file support |
All LSP features work across multiple .st files in the same workspace |
Pro — Debugging & Test
Tools to validate, debug and automate the verification of your ST logic.
| Feature |
|
| Native Debugger (DAP) |
Set breakpoints in .st files, step over statements (F10), step into function blocks (F11), inspect variables in the Variables panel and on hover — uses the VS Code Run & Debug panel natively |
| Watch Window |
Live variable list with filter, sort, pinned variables and change history |
| Single Scan / Step |
Pause the simulator and execute one scan cycle at a time |
| Snapshots |
Capture and restore simulator state — ideal for reproducing bugs |
| Unit Testing |
Write .plcsim.yaml test plans and run them from the VS Code Testing tab — set / assert / assert_range / wait_until on variables, assert_modbus to check protocol registers, and plans can target a Ladder .ld.json directly |
Pro — Ladder Diagram Editor
A visual editor for Ladder Diagram (LD) — the industry-standard graphical language for PLC logic — that compiles directly to the same Structured Text engine as the rest of PLCSim.
| Feature |
|
| Visual canvas |
Draw contacts, coils and parallel groups on an SVG canvas — undo/redo, live structural validation, auto-generated variable declarations |
| Unified tool + form pattern |
One toolbar per element family (Contact / Coil / Group / Block) — click a tool, click a placement point, fill in a small inline form. No hunting through a wall of buttons |
| Timer & counter blocks |
TON, TOF, CTU, CTD, CTUD as first-class Ladder elements — placed, configured and wired from the canvas, with automatic multi-instance aggregation so a stateful block is called exactly once per scan cycle even when driven from several rungs |
| Coil fork |
Multiple coils per branch, each driven by the accumulated logic since the start of the branch — the same wire-fork pattern used in TIA Portal, Studio 5000 and CODESYS to feed several outputs from shared input logic |
| Documentation generator |
Turn a Ladder Diagram into readable Markdown documentation, same tooling as the ST doc generator |
→ Also exposed to AI agents via the MCP server: plcsim_validate_ladder, plcsim_compile_ladder, plcsim_simulate_ladder (Free), plcsim_generate_ladder_docs (Pro)
Pro — Project Structure
Organise large programs as modular, maintainable projects.
| Feature |
|
| Custom Function Blocks |
FUNCTION_BLOCK definitions, nested instances |
| Multi-File Projects |
Split programs across types/, globals/ and blocks/ — loaded in dependency order |
Multi-File Project Layout
Create subdirectories in your workspace and PLCSim detects the structure automatically — no configuration needed:
workspace/
├── types/ ← TYPE … END_TYPE (structs, enums)
├── globals/ ← VAR_GLOBAL blocks
├── blocks/ ← FUNCTION_BLOCK definitions
└── Main.st ← main program logic (any name, any number of files)
Files are loaded in dependency order: types/ → globals/ → blocks/ → root .st files. Within each folder, alphabetical order.
Use PLCSim: New Project from the Command Palette to generate the structure automatically.
→ Full multi-file documentation
Pro — Export & Dialects
Take your validated logic to the real hardware.
| Feature |
|
| TIA Portal export |
Export programs as .scl files ready to import in Siemens TIA Portal |
| TwinCAT export (beta) |
Export programs as native TwinCAT files (.TcPOU / .TcDUT / .TcGVL) — import in TwinCAT XAE with Add > Existing Item |
| Siemens SCL dialect |
Native SCL syntax support |
| Beckhoff TwinCAT dialect |
TwinCAT ST syntax support |
| Schneider Unity dialect |
Unity Pro ST syntax support |
Pro — Industrial Communications
Connect the simulator to your real infrastructure.
| Feature |
|
| OPC-UA server |
Exposes all variables as OPC-UA nodes — connect any SCADA or HMI |
| Modbus TCP server |
Map variables to coils, discrete inputs, holding and input registers — and verify the mapping from a .plcsim.yaml test plan (assert_modbus), no external client needed |
| EtherNet/IP server |
Map variables to CIP tags |
| MQTT connector |
Publish variables to Node-RED, Home Assistant or any MQTT broker |
Pro — Object-Oriented Programming (Edition 3)
Modern PLC code with classes, inheritance, interfaces and polymorphic dispatch.
| Feature |
|
| Classes |
CLASS blocks with methods, properties, and internal state |
| Inheritance |
EXTENDS for single-parent inheritance; SUPER to call parent methods |
| Interfaces & polymorphism |
INTERFACE contracts with IMPLEMENTS; use an interface name as the type of a variable, parameter or array element — method calls dispatch to the actual type of the assigned instance |
| Properties |
Computed attributes with GET and SET accessors |
| Methods |
Callable procedures with parameters and return values |
| THIS |
Reference to current instance inside methods and properties |
| Instance collections |
ARRAY[..] OF <FB> and nested sub-instances (outer.inner.Method()) for the "N identical devices in a loop" pattern |
Pro — Documentation
Turn doc comments into shareable reference docs.
| Feature |
|
| ST Documentation Generator |
Generate Markdown documentation straight from (** ... *) doc comments — @description, @param, @returns, @author and @version tags for FUNCTION_BLOCK, FUNCTION and PROGRAM |
Free & Pro — AI Agents (MCP Server)
Give Claude Code, Claude Desktop, or any other MCP-compatible agent direct access to the PLCSim engine — validate, simulate, debug and export .st programs without VS Code open. The server ships bundled inside the extension — nothing to clone or build separately.
| Feature |
Tier |
|
plcsim_validate, plcsim_hover, plcsim_definition, plcsim_references, plcsim_rename, plcsim_simulate |
Free |
Same diagnostics, LSP and simulator as the editor |
plcsim_validate_ladder, plcsim_compile_ladder, plcsim_simulate_ladder |
Free |
Validate, compile and simulate a Ladder Diagram (JSON) without opening the visual editor |
plcsim_debug_run, plcsim_run_tests, plcsim_export, plcsim_generate_docs, plcsim_generate_ladder_docs |
Pro |
Debugging, test plans, export, documentation |
Point your MCP client at ~/.plcsim/mcp-server/index.mjs — created automatically the first time you open VS Code with PLCSim installed, and stable across future updates.
→ MCP Server setup guide
Free — AI Assistant (@plcsim)
An AI chat participant that writes IEC 61131-3 Structured Text code from a plain-language description — and never shows you code it hasn't verified against PLCSim's own compiler.
| Feature |
|
@plcsim chat participant |
Describe the logic you need in VS Code's Chat panel; get back a validated ST code block with a transparency footer ("compiled cleanly on first try" / "after N retries") |
| Built-in safety rules |
Fail-safe (NC) inputs, explicit IEC 60204-1 stop categories, bounded/deterministic loops, safe arithmetic, and the 22 PLCopen Safety function blocks named explicitly where they apply — real industrial conventions, not generic coding advice |
| Validation loop |
Every response is checked with the same LSP engine that powers editor diagnostics before you see it, with up to 3 automatic retries on error |
| Self-test |
For requests with a checkable input/output behavior, the assistant also writes a small test plan of its own and runs it against the generated code in PLCSim's simulator — if the code doesn't behave the way the assistant itself expected, that feeds back into the same retry loop. A consistency check, not a correctness guarantee |
| Safety warnings |
Static checks on the generated code, shown as non-blocking warnings: REAL equality (including the disguised x >= K AND x <= K), a decision without a default branch, over-deep nesting, and a naming heuristic for emergency-stop inputs used without NOT |
| Bring Your Own Key (BYOK) |
Works with GitHub Copilot Chat by default, or connect your own Anthropic (pay-per-token) or Google Gemini (free tier available) API key via PLCSim: Configure AI Provider — keys are stored in the OS keychain, never on disk |
→ AI Assistant setup guide
Early Adopter
Installed before October 1, 2027? You get full Pro access automatically — no license key needed, guaranteed forever.
Get Pro
→ plcsim.appscat.cat
Requirements
- VS Code 1.90 or later
- No additional software needed for Free features
- Mosquitto or any MQTT broker for MQTT connectivity (Pro)
- GitHub Copilot Chat, or your own Anthropic/Gemini API key, for the
@plcsim AI assistant
Extension Settings
| Setting |
Default |
Description |
plcsim.licenseKey |
— |
Your Pro license key |
plcsim.aiProvider |
copilot |
AI model source for @plcsim: copilot, anthropic or gemini |
plcsim.aiModel |
— (auto) |
Model ID for the active provider — leave empty for a sensible default |
License
See EULA
| |
