This free online simulator lets you build and run PLC ladder logic — the most widely used IEC 61131-3 programming language — directly in the browser. Wire up rungs with normally-open and normally-closed contacts, output coils, timers and counters, then toggle inputs in real time and watch the controller solve each rung on every scan. It's a hands-on learning tool for automation technicians, controls engineers, and students learning how PLC programs actually execute.
The tool models the core elements of a real ladder program:
• Rungs — horizontal lines of logic evaluated top to bottom. • Contacts — normally-open (NO) and normally-closed (NC) inputs that pass or block power flow. • Coils — outputs energized when their rung's logic is true. • Timers and counters — TON/TOF/TP timing and up/down counting blocks. • Inputs and outputs — toggle field inputs live and watch outputs respond. • Scan execution — see power flow resolve rung by rung each cycle.
Ladder logic is drawn like a relay circuit: two vertical 'power rails' with horizontal rungs between them. Each rung is a chain of contacts (conditions) ending in a coil (an action). A normally-open contact passes power only when its input is TRUE; a normally-closed contact passes power only when its input is FALSE. If a complete path of TRUE contacts exists from the left rail to the coil, the coil energizes. Contacts in series form AND logic; contacts in parallel form OR logic. Timers and counters add delays and counting so the program can sequence operations over time.
1. Build or load a rung with contacts and a coil. 2. Toggle the input bits to TRUE/FALSE to simulate field conditions. 3. Watch each rung solve — energized paths and coils highlight as the scan cycle runs — to confirm your logic behaves as intended.
Ladder logic is a graphical PLC programming language that represents control logic as rungs resembling a relay ladder diagram. Each rung links input conditions (contacts) to an output (coil). It is one of the five languages defined by IEC 61131-3 and the most common way to program industrial PLCs because it maps cleanly onto electrical control thinking.
A normally-open (NO) contact passes power when its associated input is TRUE (energized) and blocks it when FALSE. A normally-closed (NC) contact is the opposite: it passes power when the input is FALSE and blocks it when TRUE. NO is used for "act when present," NC for "act when absent" — for example, a NC contact on a stop button keeps a motor running until the button is pressed.
A coil is the output element at the right end of a rung. When a complete path of true contacts connects the left rail to the coil, the coil energizes — turning on an output, setting an internal bit, or driving a timer/counter. Coil types include standard output, latch (set), and unlatch (reset).
The scan cycle is the repeating loop a PLC runs: read all inputs, execute the program logic rung by rung, then update all outputs (plus housekeeping like comms and diagnostics). Because the PLC solves the whole program every few milliseconds, outputs reflect the latest input states almost continuously. Scan time is how long one full cycle takes.
IEC 61131-3 is the international standard for PLC programming languages. It defines five: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL, now deprecated), and Sequential Function Chart (SFC). Ladder logic is the most widely used for discrete control.