Function Block Diagram Online: Draw and Simulate FBD Free in Your Browser

Searching for a way to build a function block diagram online usually means one of three jobs: you need to learn FBD without installing a vendor suite, you need to test a block network before it goes near a controller, or you need a clean diagram for a report or assignment. All three used to require Windows, a multi-gigabyte IDE and sometimes a license; none of them do anymore. This page covers what a browser-based FBD editor can genuinely do in 2026, where the limits are, and a two-minute first circuit so you can judge for yourself.

What "Function Block Diagram Online" Gets You

In the PLC-Ladder editor, FBD works in the browser with no install or signup wall: drag the standard IEC 61131-3 blocks onto networks — AND/OR/NOT, TON/TOF/TP, CTU/CTD, R_TRIG, SR/RS, comparators, math (every block and its pins, catalogued) — wire outputs to inputs, and press Run. The simulation executes a real scan cycle, so the diagram animates: you watch signals flow left to right, timers accumulate, latches hold. Three properties matter more than the convenience:

It's compiled, not just drawn. The editor refuses an INT wired into a BOOL pin, an undeclared timer instance, an output driven twice — the compiler checks that make a diagram trustworthy rather than merely tidy.

It's the same logic in three views. Flip the running FBD to ladder or structured text and back; edits carry across, because the languages are one logic. For learners this is the feature: see (Start OR Motor) AND StopOK as gates, rungs and text until the equivalence is reflex.

It's shareable up the stack. A running network can be exposed over OPC UA, so an HMI or UaExpert can drive the simulated blocks — which turns "online toy" into "pre-commissioning tool."

Your First Online FBD in Two Minutes

The seal-in with a run-delay — the canonical starter:

1. New program, FBD view. Drag an OR block; wire inputs Start and Motor into it.
2. Drag an AND block; wire the OR output and StopOK into it; wire the AND output to Motor. (The feedback wire is the memory — the tutorial explains why.)
3. Drag a TON, instance RunDelay, PT := T#5s; wire Motor → IN, Q → Pump.
4. Run. Toggle Start on and off — Motor latches; watch ET climb to five seconds and Pump come on; drop StopOK and everything releases.

That's a compiled, animated, correct FBD network, built faster than a vendor IDE finishes loading. From there, the eight worked circuits each take a few minutes to recreate.

Honest Limits of Browser FBD

A browser editor won't download to your PLC (vendor IDEs keep that job), won't emulate vendor dialects (Siemens' & labels, Rockwell's FBD routine quirks — concepts transfer, spellings differ), and won't simulate your machine, only your logic — a 3D plant sim or the real commissioning pass still owns physical reality. If you need a specific vendor's environment offline, the free simulator comparison maps the alternatives honestly. For learning FBD, testing block logic, teaching a class, or producing verified diagrams for documentation, the browser is now simply the fastest tool for the job — and the only one that works identically on the lab PC, the Chromebook and your phone.