PLCSim — Simulator

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PLCSim

Simulate PLC programs in Structured Text (IEC 61131-3) directly in VS Code — no hardware needed.

Write and run complete Structured Text programs directly in VS Code. No hardware, no compilation, no deployment. Free forever for learning and prototyping; Pro unlocks the tools you need for serious work.

Install free Buy Pro

Get started

VS Code 1.90 or later. No additional software required for Free features. MQTT broker required for MQTT connectivity (Pro).

Install PLCSim Search "PLCSim" in the VS Code Extensions panel and click Install.

Open the PLCSim panel Click the PLCSim icon in the Activity Bar. Load one of the built-in examples or open your own .st file.

Run the simulator Interact in real time — toggle inputs, change values and watch outputs react. No compilation, no deployment.

Full IEC 61131-3 Syntax & Simulator FREE

Language

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) — `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`
Logic & arithmetic	`AND`, `OR`, `NOT`, `XOR`, comparators, `+` `−` `` `/` `MOD` `*`
Bitwise operators	`AND`, `OR`, `XOR`, `NOT` (on WORD/INT)
Shift & rotate	`SHL`, `SHR`, `ROL`, `ROR`
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, Time/Date, Array bounds
Comments	Block `(* ... )`, line `// ...`, doc comments `(* ... *)` for auto-generated documentation
Syntax highlighting	Full `.st` language support
Examples	20 ready-to-run programs included

Simulator

Interactive panel	Toggle BOOL inputs, set INT/REAL values and watch outputs update instantly.
Scan cycle control	Configurable from 10 ms to 5000 ms with drift compensation
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

PLCSim — Simulator RUNNING (SCAN: 142)

NAME TYPE VALUE DIR

bsensorentry BOOL IN

bsensorexit BOOL IN

itarget INT −100+ IN

bmotor BOOL TRUE OUT

icounter INT 47 OUT

blotdone BOOL FALSE OUT

Trends — ILEN Logic Over Time

Unit Testing Framework PRO

Write YAML test plans and run them from the VS Code Testing tab.

# .plcsim.yaml test plan file
name: String Support Verification
steps:
  - set: sordercode
    value: "ORD-ABC-042"
  - wait: 100
  - assert: ilen
    expected: 11
  - assert: bready
    expected: true

Pro — Debugging & Test PRO

Native Debugger (DAP)	Set breakpoints in .st files, step over (F10), step into function blocks (F11), inspect variables in the Variables panel and on hover
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 YAML test plans and run them from the VS Code Testing tab.

AI Assistant (@plcsim) FREE

A 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.

@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 ("validated 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 — 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
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

Buy Pro

Editor Intelligence (LSP) FREE

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 — Project Structure PRO

Custom Function Blocks	FUNCTION_BLOCK definitions, nested instances
Multi-File Projects	Multi-file projects (types/, globals/, blocks/)

# PLCSim: New Project ← generates this layout
workspace/
├── types/   ← TYPE … END_TYPE (structs, enums)
├── globals/ ← VAR_GLOBAL blocks
├── blocks/  ← FUNCTION_BLOCK definitions
└── Main.st  ← main program logic

FB Instances PRO

▼ fbstation : station

bstart [input] false

ballrunning [output] true

banyfault [output] false

▼ fbmotor1 : motor

bstart [input] true

brunning [output] true

iruncount [output] 0

PLCSim — WATCH PANEL PRO RUNNING (SCAN: 0)

NAME TYPE VALUE DIR

★ sprefix STRING "ORD" OUT

★ sproduct STRING "ABC" OUT

★ sstatus STRING "Order: ORD-ABC-042" OUT

★ sordercode STRING "ORD-ABC-042" IN

★ smachinestate STRING "RUNNING" IN

★ ilen INT 11 OUT

★ idashpos INT 4 OUT

★ bready BOOL TRUE OUT

Ladder Diagram Editor PRO

A visual editor for Ladder Diagram (LD) that compiles directly to the same Structured Text engine as the rest of PLCSim.

Visual canvas	Draw contacts, coils and parallel groups on an SVG canvas — undo/redo, live structural validation and 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
Timer & counter blocks	TON, TOF, CTU, CTD and CTUD as first-class Ladder elements — placed and wired from the canvas, with automatic multi-instance aggregation
Coil fork	Multiple coils per branch, each driven by the accumulated logic since the start of the branch — the same pattern used in TIA Portal, Studio 5000 and CODESYS
Documentation generator	Turn a Ladder Diagram into readable Markdown documentation, same tooling as the ST doc generator

Pro — Export & Dialects PRO

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	TwinCAT ST syntax support
Schneider Unity	Unity Pro ST syntax support

Pro — Industrial Communications PRO

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 MQTT brokers

// protocols.config.json
{
  "protocols": [
    { "modbusTCP":  { "enabled": true, "port": 5020 } },
    { "ethernetIP": { "enabled": true, "port": 44818 } },
    { "opcua":     { "enabled": true, "port": 4840,
                   "nodeIdFormat": "generic" } },
    { "mqtt":      { "enabled": true,
                   "brokerUrl": "mqtt://localhost:1883" } }
  ]
}

Pro — Object-Oriented Programming PRO

Modern IEC 61131-3 Edition 3 with classes, inheritance, interfaces and polymorphic dispatch.

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 — 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 PRO

ST Documentation Generator

Generate Markdown documentation from (** ... *) doc comments — @description, @param, @returns, @author and @version tags for FUNCTION_BLOCK, FUNCTION and PROGRAM

AI Agents (MCP Server) FREE PRO

Connect Claude Code, Claude Desktop or any MCP client directly to the PLCSim simulation engine, without VS Code open.

Free tools	plcsim_validate, plcsim_hover, plcsim_definition, plcsim_references, plcsim_rename, plcsim_simulate, plcsim_validate_ladder, plcsim_compile_ladder, plcsim_simulate_ladder — same diagnostics, LSP, simulator and Ladder engine as the editor
Pro tools	plcsim_debug_run, plcsim_run_tests, plcsim_export, plcsim_generate_docs, plcsim_generate_ladder_docs — debugging, tests, export and documentation (ST and Ladder)

Server at ~/.plcsim/mcp-server/index.mjs — created automatically, stable across updates.