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`, `ARRAY` (1D and 2D), typed literals (`WORD#16#FF`, `DINT#-100`)
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	18 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

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

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
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 and holding registers
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, and interfaces.

Feature
Classes	`CLASS` blocks with methods, properties, and internal state
Inheritance	`EXTENDS` for single-parent inheritance; `SUPER` to call parent methods
Interfaces	`INTERFACE` contracts; `IMPLEMENTS` for conformance
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

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`

Early Adopter

Installed before July 1, 2026? You get full Pro access automatically — no license key needed, guaranteed forever.

Get Pro

→ plcsim.appscat.cat

Requirements

VS Code 1.85 or later
No additional software needed for Free features
Mosquitto or any MQTT broker for MQTT connectivity (Pro)

Extension Settings

Setting	Default	Description
`plcsim.licenseKey`	—	Your Pro license key

License

See EULA