PLC Ladder Logic Simulator

PLC Ladder Logic Simulator — Learn Programmable Logic Controller Programming Online

Why PLCs Are Still Everywhere After 50 Years

Modicon shipped the first PLC in 1968. The control engineering profession has reinvented itself a dozen times since — we now have embedded microcontrollers cheaper than a button, full Linux SBCs in industrial enclosures, and cloud-connected smart sensors with built-in analytics. None of these have displaced PLCs from the factory floor. The reasons are practical, not nostalgic.

PLCs survive because the trade-offs that matter on a factory floor have not changed. They are deterministic: the scan cycle is fixed and known, so timing is repeatable. They are rugged: industrial enclosures rated to IP65 or higher, conformal-coated boards, robust against electrical noise and temperature swings. They are maintainable: a maintenance engineer can read ladder logic without being a software developer, because the diagram looks exactly like the relay-logic circuits the same engineer trained on. And they are certifiable: safety-rated PLCs (SIL2, SIL3) have a regulatory pedigree that no general-purpose microcontroller offers.

The Seal-In Circuit — The First Pattern Every PLC Programmer Learns

Every PLC course starts with the same example: a motor that should start when the Start button is pressed and keep running until Stop is pressed. The naive ladder — start button contact in series with motor coil — only runs the motor while the button is held. Release the button, motor stops. Useless.

The fix is the seal-in (latching) circuit. In parallel with the Start button contact, add a normally-open contact from the motor output coil. When Start is pressed, the motor coil energises; that energises the output bit, which closes the parallel contact, which keeps the rung TRUE even after the Start button is released. The Stop button (normally-closed contact, in series) breaks the rung and de-energises the coil, ending the latch.

This single pattern — coil contact in parallel with start, NC stop in series — underlies every motor starter, conveyor-belt run command, lighting control, and pump start logic in industry. Once you internalise it, the rest of PLC programming is variations on the theme.

Timer Use Cases That Show Up in Every Real Program

• Bottle-filling line. Sensor detects bottle, TON timer holds the fill valve open for 2 seconds. After preset, valve closes, bottle moves on. Adjusting the preset adjusts the fill volume.
• Conveyor belt anti-jam. If conveyor runs for more than 30 seconds without a part-detected sensor pulse, TON triggers a fault output and stops the line. Manual reset required.
• Air-compressor off-delay. Pressure switch triggers compressor start. TOF timer keeps the cooling fan running for 60 seconds after the compressor stops, so heat dissipates before the fan shuts off.
• Flashing fault indicator. Pair of TON timers driving an oscillator: when fault active, light flashes at 1 Hz. The same pattern in software that a 555 timer chip would do in hardware.

References for PLC Programming

• IEC 61131-3:2013 — Programmable controllers — Part 3: Programming languages. The defining standard for ladder logic, structured text, and the other four IEC languages.
• Petruzella, F. D. — Programmable Logic Controllers, 5th ed. The widely-used vocational textbook.
• Bolton, W. — Programmable Logic Controllers, 6th ed. The mechanical-engineer-friendly introduction.
• Siemens, Rockwell, Mitsubishi PLC programming manuals — freely available online. Each vendor has its own dialect of IEC 61131-3, but the core is portable.

A Programmable Logic Controller (PLC) is the backbone of modern industrial automation. From manufacturing assembly lines to water treatment plants, PLCs execute control logic that keeps processes running safely and efficiently. Ladder logic, defined in the IEC 61131-3 standard, remains the most widely used PLC programming language worldwide. This free simulator lets you build ladder diagrams with 22 instruction types, simulate scan cycles in real time, and master PLC programming through hands-on practice.

This free PLC ladder logic simulator provides a complete virtual PLC environment directly in your browser. Using standard IEC 61131-3 instruction symbols, you can place contacts, coils, timers, counters, compare blocks, and math instructions onto ladder rungs and observe real-time scan cycle execution. Toggle digital inputs, watch output LEDs respond, and learn how industrial automation programs work through interactive experimentation. Whether you are preparing for a Siemens, Allen-Bradley, or Mitsubishi PLC certification, the fundamental ladder logic concepts are universal and transferable across all PLC brands.

Understanding the PLC Scan Cycle

Every PLC operates in a continuous scan cycle consisting of three phases. First, the Input Scan reads all physical inputs (pushbuttons, sensors, limit switches) and stores their states in an input image table in memory. Second, the Program Scan executes the ladder logic program from the first rung to the last, evaluating each contact condition and updating coil states in an output image table. Third, the Output Scan transfers the output image table values to the physical output modules (motor contactors, solenoid valves, indicator lights). A typical scan cycle takes 1 to 10 milliseconds depending on program size and CPU speed. This cyclic operation ensures deterministic, repeatable behaviour essential for industrial process control.

Contacts and Coils — The Building Blocks

In ladder logic, contacts represent input conditions and coils represent output actions. A Normally Open (NO) contact passes power when its associated address is TRUE (energised). A Normally Closed (NC) contact passes power when its address is FALSE (de-energised) — this is essential for emergency stop circuits where a broken wire must stop the machine. Contacts arranged in series create AND logic (all must be true), while contacts on parallel branches create OR logic (any can be true). The seal-in circuit, where an output coil’s own NO contact is placed in parallel with the start button, is the most fundamental PLC pattern — equivalent to a relay self-holding circuit.

Timers and Counters in PLC Programming

Timers introduce time-based control into ladder programs. The TON (On-Delay Timer) is the most common: it starts timing when its input goes TRUE and energises its output bit after the preset time elapses. If the input drops before the preset, the timer resets to zero. The TOF (Off-Delay Timer) passes the input signal immediately and keeps the output energised for the preset duration after the input turns OFF — useful for cooling fans that must run after a motor stops. The TP (Pulse Timer) generates a fixed-width output pulse on each rising edge of its input, regardless of input duration. Counters track events: CTU (Count Up) increments on each rising edge of its count input and sets its output when the accumulated value reaches the preset. CTD (Count Down) decrements from a preset value and sets its output when reaching zero.

IEC 61131-3 Addressing and Data Types

The IEC 61131-3 standard defines a consistent addressing scheme used across PLC manufacturers. I (or %I) prefixes denote physical inputs, Q (or %Q) denotes physical outputs, and M (or %M) denotes internal memory bits (markers). Byte-level addressing uses the format I0.0 where the first number is the byte address and the second is the bit within that byte (0–7). Word registers such as MW0, MW2 store 16-bit integer values for timer presets, counter values, and arithmetic operations. Understanding this addressing scheme is essential for writing structured, maintainable PLC programs across Siemens S7, Allen-Bradley CompactLogix, Mitsubishi FX, and other platforms.

Common PLC Ladder Logic Instruction Reference

Who Uses This PLC Ladder Logic Simulator?

This PLC training simulator is designed for Technical and Vocational Education and Training (TVET) students studying industrial automation, electrical engineering, and mechatronics. Vocational instructors use it as a classroom teaching aid to demonstrate ladder logic concepts before students work on physical PLC trainers. Maintenance electricians use it to practice troubleshooting and understand existing plant programs. Automation engineers prototype control sequences before downloading to actual PLCs. The four learning modes (Simulate, Explore, Practice, Quiz) provide a complete educational progression from theory to hands-on application, making it an ideal free PLC programming trainer for learners at any level.

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