Engineering · 03PLC programming, in a tab

RungCraft

A PLC ladder and structured-text editor with a real scan-cycle simulator, in a browser tab.

Any modern browser LAD · ST No install Open core (MIT) Free during launch

Request the build The open core lives here

Version

0.3.0 Released

Runs

Any modern browser · macOS / Windows launcher

Projects

IndexedDB · .ladproj.json

Licence

Open core (MIT) Public repository

01Ladder

Ladder editing that behaves like ladder logic

Contacts and coils in every form the work needs: normally open, normally closed, edge, set/reset and negated. Series and parallel branches, multi-coil rungs, drag re-ordering, and inline operand editing with autocomplete.

A variable can be created where it is used, rather than in a dialog three screens away. Keyboard-first shortcuts, undo and redo throughout.

A drawn ladder-logic rung: power rails on both sides, two contacts in series with a sealing branch, a coil, a timer block with PT and ET values, and a structured-text block below. — One rung with a sealing branch, a timer block, and structured text in the same program. This is a notation drawing made for this page, not a screenshot of the editor.

02Structured text

ST that shares the same program

Syntax highlighting and live diagnostics, the full statement set, timers and counters, arrays and UDT fields, Chinese identifiers, and T#1s time literals.

FC and FB calls work across LAD and ST in both directions, so a block can be written in whichever notation is clearer and called from the other.

Live diagnostics as you type, inside the block
Arrays and UDT fields with nesting
Mixed blocks LAD and ST block by block, in one project

IF "Motor_Start" AND NOT "Motor_Stop" THEN
    "Conveyor_Run" := TRUE;
    "Run_Timer".IN := TRUE;
END_IF;

"Run_Timer"(PT := T#5s);

IF "Run_Timer".Q THEN
    "Conveyor_Run" := FALSE;
END_IF;

Structured text in the editor's notation, including Chinese identifiers and time literals.

03Program structure

The parts a real project needs

Not a ladder sketchpad: data blocks, function blocks with interfaces, instance DBs, and a schema that survives a version change.

Blocks and interfaces

DB, FC and FB with typed interfaces, instance DBs, multi-instance and recursion checks.

Datatypes

PLC datatypes (UDT) with arrays and nesting, cross-referenced across the project.

Projects that migrate

IndexedDB storage, .ladproj.json import/export, and a Zod-versioned schema with migrations.

04Simulation

A scan cycle, not a green light

The simulator runs in a Web Worker with its own scan cycle: run, pause, step, stop and reset, with scan time reported and a speed multiplier for slow logic.

Boolean energy flow is live on the rungs and operand values update in place. Forcing and instance monitoring let you hold an input and watch the program respond, rather than guessing.

Engine: Web Worker scan cycle
Controls: Run · pause · step · stop · reset
Watch: Energy flow, operand values, instance state
Override: Forcing, scan-time and speed multiplier

05HMI and recipes

A screen and a recipe table, bound to tags

Enough operator interface to demonstrate a sequence or test a control scheme, without a separate SCADA project.

Screens

Buttons, switches, lamps
Numeric display and numeric input
Text and bar graphs
Every element bound to a PLC tag

Recipes

Recipe tables bound to tags
Download to the running program
Capture current values back into a recipe
CSV and JSON import/export

06Built-in demos

Five projects to open first

Each demo exercises a different part of the editor, so the feature set is testable in the first ten minutes.

Motor start-stop Sealing circuit with start, stop and a run lamp — plus an HMI screen driving the same tags.

Traffic light Timer and counter instructions sequencing three lamps.

Conveyor counting Edge detection and counting on a digital input.

Traffic light, in ST The same sequence written as structured text, for comparison.

Tank control Function block, instance DB, UDT and analog value — with an HMI screen and a recipe table.

07Specification

Build and licence

The core engine is open source (MIT) and buildable from the public repository, which keeps it usable for teaching and evaluation. Current builds and later add-on features are commercial — and everything is free during the launch period.

Request the build The open core lives here

Free during launch Download · coming soonDownload

Download links open when the binary bucket is live. Until then, ask and we will send the installer directly.

Capabilities · version 0.3.0
Item	Value
Notations	LAD · ST, mixed in one project
Storage	IndexedDB · .ladproj.json
Simulation	Web Worker scan cycle, forcing
HMI	Screens and recipes bound to tags
Demos	5 built in
Distribution	Browser · macOS / Windows launcher
Core licence	Open core (MIT)
Price now	Free now