Ladder Logic Simulator — Toy Rung Editors vs Real Trainers

Search "ladder logic simulator" and the results split into two products wearing the same name. One lets you draw rungs and watch virtual power flow through them, free, in about thirty seconds. The other runs your rungs on a faithful scan cycle, throws faults at them, and refuses to pass you until the logic survives. The first is a toy. The second is a trainer. Both have a legitimate use, and this page tells you which one your situation calls for, plus the minimum instruction set any simulator needs before it can carry you past week one.

Run your first rung free →

The short answer

• A rung editor that animates power flow is fine for five minutes of curiosity. Learning the trade requires a simulator that grades your work and reproduces real scan-cycle behaviour.
• The instruction floor for serious practice: contacts and coils, the three IEC timers, up/down counters, comparison instructions and a move. Anything less and you stall inside two weeks.
• Ladder is still the right first PLC language for the SA job market: it's what's on the panel HMI when the line stops, and it's what interviews test on a whiteboard.
• Our simulator's free tier gives you the unlimited sandbox plus six graded lessons, in the browser, nothing installed.
• After free, it's $12–$29 a month (roughly R220–R540). SA classroom courses run R4 100 to R17 595 for a fixed week.

A simulator that grades you versus one that just runs

Here's the test that separates the two categories: write a start-stop rung with the stop contact the wrong way around, examine-if-closed against a normally-open stop button. In a toy, it runs. Power flows, the coil energises when you click start, everything animates pleasingly. You walk away believing you can write a motor circuit. In a trainer, the grader breaks the stop wire in one of its test scenarios, your motor keeps running with a severed stop circuit, and the exercise fails with a message telling you which scenario killed it and why that matters on a real machine.

That wrongly-typed stop contact is not a contrived example. It's probably the most common beginner error in ladder, it's invisible in normal operation, and it's exactly the class of mistake that animated power flow can never catch, because the program does "work". It just fails dangerous instead of failing safe.

So when you evaluate any ladder logic simulator, look past the rung drawing (everyone has rung drawing) and ask four harder questions.

Does it run a real scan? Top to bottom, left to right, inputs frozen into an image at the start of the cycle, outputs written at the end, last write wins. These rules are why ladder behaves the way it does: why a seal-in uses last scan's output state, why rung order can change behaviour, why an output written in two places is a bug. The semantics come from IEC 61131-3 (iec.ch); a tool that evaluates rungs instantly and independently, the way a circuit diagram would, is teaching you electrical intuition and PLC superstition at the same time.

Can it inject faults? Broken sensor wire, stuck contact, miswired output. Fault-finding is the half of the job that pays, and you can't practise it in a tool where everything always works.

Does it grade? Our curriculum runs each exercise against twelve automated scenarios. The point isn't the score; it's that a machine with no interest in your feelings tells you your rung fails when stop and start are pressed in the same scan. Our opinion, plainly: a simulator that can't fail you can't teach you. Everything below "graded" is reference material with animations.

Can you watch and manipulate live state? Bit states visible on every contact and coil while the program runs, timer accumulators counting where you can see them, and the ability to force an input to chase a branch. Debugging is mostly looking; the tool has to give you something to look at.

The free rung editors fail most of these. That's not an insult — they're built as visualisers, not trainers, and as visualisers some are genuinely neat. The mismatch only hurts when someone tries to ride one from curiosity all the way to employability.

The instruction floor: what a serious ladder simulator must cover

Vocabulary check. If a simulator's palette is missing any of the following, you'll hit its ceiling before you hit your own.

Contacts and coils, all the flavours. Examine-if-closed, examine-if-open, output coil, set/reset pair. This is the alphabet, and crucially the simulator must handle the NO/NC instruction against NO/NC field-device distinction honestly, because that's where the fail-safe stop logic above lives. The contacts and coils reference covers the full family and the wiring conventions behind it.

The three IEC timers. TON for on-delay, TOF for off-delay, TP for fixed pulse. Real sequences are built out of timers more than any other instruction, and timer misuse (re-triggering a TP, resetting a TON mid-time) is a rich seam of beginner bugs that only shows up under a live scan. Details and patterns in the TON/TOF/TP reference.

Counters, up and down. CTU, CTD, and the combined CTUD with its load and reset behaviour. Counting parts, counting bottles into a crate, counting pump starts for duty rotation. The counter reference walks the family.

Comparison instructions. Equal, greater-than, less-than and the in-range patterns built from them, because the moment an analog value enters your program, rung conditions stop being purely boolean. Covered in the comparison instruction reference.

Move. Copying a value into a timer preset, staging a recipe parameter, latching a reading for an HMI. Unglamorous and everywhere. The move and copy reference explains the variants.

That floor is roughly the instruction set behind the classic interview exercises and behind most of what a junior actually edits in the field. Math, shift registers and string handling matter later; a tool can lack those and still carry you for months. A tool missing timers or comparison can't carry you past a fortnight.

A ladder-first path through the exercises

If you learn best with a sequence rather than a feature list, this is the ladder-only spine of our curriculum, each step one new idea on top of the last.

Start with the start-stop seal-in. One rung, three contacts, one coil, and the single most important pattern in industrial control. You'll be asked to produce this from memory at some point in your career; get it into your hands early.

Then the jog versus latch motor exercise, which forces you to articulate the difference between momentary control and latched control, the distinction the seal-in exists to create. Most learners can build a latch before they can explain when you'd deliberately not want one. The grader checks both.

Third, the star-delta starter. Timers meet interlocks: two contactors that must never be on together, a transition timed in fractions of a second, and a fault scenario that overlaps them if your interlock is decorative. This is the first exercise that feels like real panel work, because it is real panel work.

Fourth, the bottle counting line. Counters driving decisions: count bottles into a crate, stop the infeed at the preset, handle the crate swap without losing product. Counting sounds trivial until product keeps moving while your count is being reset.

By the end of that arc you've used every instruction in the floor list above, under grading, with faults. Background theory for the whole run lives in ladder logic basics if you want the reading before the doing. And if you're weighing this against the broader product (curriculum scope, wiring track, sensor library, what the paid tiers add), the simulator hub page is the complete picture; this page stays on ladder.

When a free rung editor is actually enough

Against our own interest: there are real situations where a free browser rung editor is the right tool and a paid trainer is overkill.

You're curious whether ladder looks like something you could stand. Ten minutes in any free editor answers that. (Though our free sandbox answers it too, with a real scan engine underneath, so the comparison isn't quite either/or.)

You want to sketch one interlock idea and watch it, the rung equivalent of a napkin drawing. An editor with power-flow animation is honestly pleasant for this.

You're teaching a non-technical colleague what "the PLC" does in one sitting, and animated power flow communicates faster than any explanation of scan cycles would.

Where the free editor stops being enough is the moment your question changes from "what does ladder look like" to "can I do this work". That question needs scan fidelity, faults and a grader, and the general background on ladder's history and conventions — worth a read at Wikipedia's ladder logic entry — won't substitute for being failed twelve ways by an exercise until your rung is actually right.

What it costs

Option	Price	In rand (approx.)
Simulator — Free tier	$0	R0 — unlimited sandbox + first six graded lessons
Simulator — Basic	$12/month	~R220/month — full curriculum and wiring track
Simulator — Pro	$29/month	~R540/month — adds sensor school, cert packs, portfolio export
Simulator — Teams	$199/seat/year (min 5 seats)	~R3 700/seat/year — training centres and employers
SA classroom course	R4 100 – R17 595	one to five days, fixed dates

The free rung editors cost nothing forever, and for the napkin-sketch use cases above, nothing is the right price. The comparison that matters is Basic at about R220 a month against a R12 000+ classroom week, for the months it takes to build real fluency. Tier details on the pricing page; the full SA market survey is in PLC course prices in South Africa.

Common questions

Is simulated ladder the same as TIA Portal ladder?

The behaviour is the same; the dialect isn't. Scan order, timer semantics, seal-in behaviour and the rest follow IEC 61131-3 in both, so logic you can write in our simulator behaves identically to S7 ladder. What changes in TIA Portal is the surroundings: absolute addressing and data blocks, Siemens instruction naming, and the IDE itself, which is its own multi-week familiarisation. Arrive with the logic already fluent and that familiarisation is the whole job, which is a far smaller job.

Can a free online rung editor teach me ladder logic?

It can teach you to read ladder and to predict power flow, which is genuinely the first rung of the skill. It can't teach you scan-dependent behaviour, fault response or correctness under edge cases, because it has no scan, no faults and no grader. Treat free editors as the picture-book stage: real, useful, and finished within your first week.

Do I need electrical theory before learning ladder logic?

No, and this surprises people. Ladder was designed in the 1970s so plant electricians could read programs the way they read relay circuit diagrams, so electricians start with a real head start in reading fluency. But the conventions (why stop circuits use NC field devices, what a seal-in is doing) are all teachable inside the simulator from zero. Plenty of our learners come from IT or from no technical background at all; they take an extra week on wiring conventions and then track the same curve.

Should I learn ladder or structured text first?

Ladder first, for one practical reason: when a line stops in an SA plant, the program someone pulls up at the panel is overwhelmingly ladder, and interview whiteboards follow plant reality. Structured text is worth learning second; it's better for math, loops and data handling, and our structured text guide covers when each language wins. Learning ST first and ladder never is a viable path in some industries overseas. It's a poor bet here.

How long until I can write rungs from a blank canvas?

Reading rungs comes in days. Writing the start-stop seal-in from nothing, without referring to an example, takes most learners a focused weekend. The four-exercise arc above (seal-in, jog/latch, star-delta, bottle counter) is two to four weeks at a few evenings a week, and at the end of it you're writing graded, fault-surviving logic rather than copying patterns. Slower than a brochure promise. Honest, though.

Open the sandbox — write a rung tonight →

What we don't claim

We're not SAQA-registered and not MerSETA-accredited, and completing our ladder curriculum earns a completion record — plus, on the Pro tier, a portfolio export — not an NQF-listed qualification. We don't claim simulated ladder practice covers panel wiring, terminations or live commissioning; the simulator builds the logic-and-diagnosis half of the work and the physical half still needs real equipment. We don't claim the free rung editors out there are useless, because for sketching and curiosity they're fine; our claim is narrower, that graded scan-faithful practice is what builds employable skill, and we've laid out the checklist above so you can hold any tool to it, ours included.