Four languages, one program model

A nautilus program file is .st, .ld, .fbd, or .sfc. Pick the language per task and mix them freely in one controller: the demo project runs its control loop in FBD, its interlocks in ladder, and its plant simulation in ST, all against one tag store.

Text first

IEC 61131-3 defines ladder, function block, and sequential function chart graphically. nautilus gives each one a text form that carries the standard’s semantics with no vendor dialect: rung text for ladder, a netlist for FBD, and the standard’s own textual keywords for SFC. The text file is the program. The VS Code extension projects it into a full diagram editor where every gesture is a structural edit to the text underneath, so a wiring change, a new rung, or a new step shows up in git diff as a few readable lines.

One compile chain

Each graphical language transpiles toward Structured Text, one hop for FBD and SFC and two for ladder, and ST lowers to the typed IR the runtime executes:

 .st  ─────────────────────────►  IR
 .fbd ── transpile ──► ST ──────►  IR
 .ld  ── transpile ──► FBD ──► ST ─►  IR
 .sfc ── transpile ──► ST ──────►  IR

Because the hops preserve line maps, a diagnostic anywhere in the chain lands on your source: an .ld error points at the rung, an .sfc error at the step or transition. And because everything ends in one IR, the whole toolchain is shared. Built-in functions and function blocks, user FUNCTION_BLOCKs from .st, .ld, or .fbd library files, arrays and structs, nautilus check, acceptance tests, live values, online edits, and visual diffs behave the same in every language.

Which language for which job

Logic	Language	Why
Interlocks, permissives, latches, annunciators	Ladder	Reads as the relay logic it replaces; power flow is the review
Loops, signal conditioning, continuous control	Function Block Diagram	Data flow between blocks is the natural shape
Math, arrays, structs, physics, anything algorithmic	Structured Text	The full language; the other three lower to it
Batch sequences, state machines, startup and shutdown	Sequential Function Chart	Steps and transitions are the spec of the process

nautilus new --language ld (or fbd, sfc) scaffolds a minimal project in that language. Tasks are declared in nautilus.yaml, one program file each, at their own scan rate. See Function blocks, libraries, and tasks for structuring logic across files and tasks, and the language reference for evaluation semantics and every built-in.