Real machine physics
Every scenario simulates real equipment. Tanks fill, motors spin, valves modulate — driven by your actual ladder logic.
IEC 61131-3 compliant
Ladder Diagram Simulator — Free & IEC-Compliant
Draw contact/coil rungs and execute them against virtual inputs and outputs. IEC 61131-3 notation throughout — NO contacts, NC contacts, TON, CTU, R_TRIG — all in your browser with no install.
How it feels
Built to build intuition.
A browser tab that behaves like a real PLC bench — without the hardware budget.
Live I/O experimentation
Toggle inputs by hand to see how the PLC responds. No wiring, no hardware — just click and learn.
Certificate-backed interview prep
Pass an interview track and earn a downloadable PDF certificate. Pro users get solution walk-throughs with expert commentary on every scenario.
A ladder diagram simulating a motor start-stop seal-in. No install. Watch contacts pass power, timer accumulators tick, and outputs energise in real time.
Ladder diagram 101
What is a ladder diagram simulator?
A ladder diagram simulator is software that lets you draw contact/coil rungs and execute them against virtual inputs and outputs, so you can verify the program without real hardware. You define your variables, wire up the rungs, press Run, and watch the IO image update scan by scan — exactly as it would on a physical PLC.
Paper and textbook exercises teach you ladder diagram syntax. A simulator teaches you to read runtime behaviour — why a seal-in rung stays energised after the START button is released, how a TON timer accumulates elapsed time on every scan, and where race conditions can appear when two rungs write to the same coil in different orders. That understanding is what separates someone who has studied a diagram from someone who can actually commission one.
Traditional ladder diagram tools — CoDeSys, Siemens TIA Portal, Allen-Bradley Studio 5000, Festo FluidSIM — are desktop installs tied to specific operating systems, hardware dongles, or expensive licenses. A browser-based simulator removes every one of those constraints. If you can open a URL, you can run a ladder diagram.
Instruction set
What ladder diagram notation is covered?
Every instruction below is implemented to the IEC 61131-3 specification. Contact and coil symbols use IEC notation; AB and Siemens aliases are mapped to the same execution primitives under the hood.
Contacts
-| |- normally open (NO)-|/|- normally closed (NC)-|P|- rising edge (R_TRIG)-|N|- falling edge (F_TRIG)
Coils
-( )- standard output coil-(S)- set (latch)-(R)- reset (unlatch)-(/)- negated output
Timers
TON — on-delayTOF — off-delayTP — pulse timer
Counters
CTU — count upCTD — count downCTUD — up/down
Compare
EQ, NE (equality)GT, GE, LT, LE (relational)
Math
ADD, SUB, MUL, DIV, MOD
Flow Control
JMP / LBL (program flow)Nested rungs (parallel branches)
Advanced
PID function blockComparison blocks on analog tags
Dialect support
Three dialects of ladder diagrams supported
Ladder diagram conventions vary by PLC vendor. A Siemens technician writes contacts as --| |-- and refers to the program as a "ladder diagram" per IEC 61131-3. An Allen-Bradley technician calls the same thing "ladder logic" and names contacts XIC and XIO. The underlying scan-cycle model is identical; only the notation and instruction mnemonics differ.
This simulator lets you switch dialects on any scenario without rewriting your program. The execution engine is the same — you are learning the same concepts regardless of which syntax you use.
IEC 61131-3Default
The international standard. Contact symbols -| |-, coil symbols -( )-, timer and counter names TON / CTU. Preferred by Siemens, Beckhoff, CODESYS, and European academic curricula.
Allen-BradleySupported
Rockwell Automation / Studio 5000 style. Contacts named XIC and XIO, coils named OTE / OTL / OTU. Dominant in North American manufacturing. The dialect most US PLC job listings specify.
Siemens TIA PortalSupported
Siemens-specific syntax for LAD (Ladder) programs. Contact and coil symbols follow IEC conventions but use Siemens data types, addressing, and function block signatures.
Practice scenarios
Ladder diagram scenarios to practise with
Each scenario loads a machine simulation and a set of auto-graded objectives. Write your ladder diagram, press Run, and the test harness tells you which objectives pass and which fail. No manual marking.
Motor Start-Stop
The canonical three-wire seal-in circuit. A START push-button latches the motor contactor through its own auxiliary contact. An E-stop and thermal overload on normally-closed contacts break the rung. The foundational ladder diagram every technician must know.
BeginnerSafetyCoils
Traffic Light
Sequential timing with TON timers. Green, amber, red phases driven entirely by timer .Q outputs and reset logic. Add a pedestrian walk-request input and watch the IEC 61131-3 rung structure handle event priority.
BeginnerTimersSequencing
Conveyor Sort
Sensor-based diverter logic. A photoelectric sensor detects item presence; a compare instruction checks a tag count; a solenoid diverter fires on the correct items. Introduces conditional branching inside a ladder diagram.
IntermediateCompareSensors
Tank Fill
Analog level comparison using IEC compare instructions (GE, LE) against a simulated 4–20 mA level transmitter. Level high and level low switch points drive fill and drain valves. A realistic SCADA-style ladder diagram in the browser.
IntermediateAnalogCompare
Elevator
State-machine control across multiple floors. Floor-call and direction bits drive a sequencer through IDLE, MOVING, and DOOR states. One of the more complex ladder diagrams in the set — good preparation for multi-step machine control interviews.
AdvancedState MachineSequencing
E-Stop Reset
IEC 61131-3 safety circuit topology. Latching fault bits, monitored reset push-button, and a guard that prevents auto-restart after an E-stop. The ladder diagram structure here follows the reasoning examiners expect in functional safety discussions.
IntermediateSafetyIEC 62061
Comparison
Comparing ladder diagram simulators
Most ladder diagram tools are either expensive, Windows-only, or both. Here is an honest comparison of the options available.
| Tool | Cost | Platform | Standard | Scenarios |
|---|---|---|---|---|
| FluidSIM | ~€200 | Desktop (Windows) | Festo-specific | — |
| LogixPro | $200+ | Desktop (Windows) | Allen-Bradley only | ~10 |
| CoDeSys | Free to €€€ | Desktop (Win/Mac) | IEC 61131-3 | None built-in |
| PLC-Fiddle | Free | Browser | Limited | Limited |
| PLC Simulator (this) | Free / Pro | Browser (any OS) | IEC 61131-3 + AB + Siemens | 40 auto-graded |
FAQ
Ladder diagram simulator — frequently asked.
Yes. The simulator is free for two scenarios with no credit card required. A Pro subscription unlocks all 40 scenarios, 18 lessons, 12 quizzes, and 6 interview tracks — everything you need to go from first rung to job-ready.
Open the ladder diagram simulator.
No install. No license. Start with a traffic-light sequence and work your way up to safety circuits and state machines. The nine-scenario Beginner Track is completely free.