Ladder logic is a graphical PLC programming language drawn like an electrical ladder diagram: two vertical power rails joined by horizontal rungs of contacts (conditions) and coils (outputs). Every scan, the PLC evaluates each rung left to right, top to bottom, and energizes a coil when a path of true contacts reaches it.
Learn the symbols, read worked rungs step by step, then study eight recurring ladder patterns with circuit diagrams, physical-machine context and links into 170 source-catalogued practice records. Build a first contact-and-coil program in your browser without an account.
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Try this in the browserConnect each rung to the machine
A rung is useful only when you can explain the equipment state it creates. These examples connect the abstract contact-and-coil shapes to motors, sensors, sequence timing, permissives and process stages.
Why patterns matter
A motor start/stop circuit is not invented from scratch each time: the seal-in rung is a recognizable control pattern. An interlock blocking two contactors from energizing simultaneously is another. A timer cascade advancing a timed sequence step by step is another. Pattern recognition helps you form a hypothesis about unfamiliar code, but you still verify tag meaning, scan order, hardware state and the machine specification.
These patterns cover common building blocks in discrete machine control. Learning them is not about memorizing rungs: it is about understanding what problem each pattern solves, what assumptions it makes and how you would prove the resulting behavior.
Each pattern on this page includes a structural diagram (pattern shape only — no complete program, no tag addresses), a plain-English explanation of what it does and when it is used, and links to the live scenarios on this platform that exercise that exact pattern. The scenarios are the place to build the pattern yourself. This page is the conceptual foundation.
Direct answers
Ladder logic is a graphical PLC programming language. Each rung evaluates conditions on the left and drives instructions or outputs toward the right during the controller scan. It is the most common first language in PLC programming.
A ladder diagram uses two vertical rails with horizontal rungs. Electrical relay diagrams describe wired control; PLC ladder diagrams use similar notation to express software instructions evaluated by a controller.
Relay logic changes behavior by rewiring components. PLC ladder logic changes software running on a programmable logic controller while field devices remain mapped to I/O. PLCs add timers, counters, data and diagnostics, but the electrical safety circuit still has its own requirements.
Read a ladder diagram
Most PLC ladder diagrams use a small set of symbols. Allen-Bradley and IEC 61131-3 tools draw them slightly differently and use different instruction names, so both are listed. Each symbol refers to a bit or value in memory, not to a physical wire. The full reference, with vendor variations, is on the ladder logic symbols page, and the mnemonics are decoded on the PLC codes reference.
| Symbol | Name | Allen-Bradley | IEC 61131-3 | What it does |
|---|---|---|---|---|
| --[ ]-- | Normally open contact | XIC | NO contact | True when its bit is 1, for example while an input is on. |
| --[/]-- | Normally closed contact | XIO | NC contact | True when its bit is 0. It inverts the bit in logic; it says nothing about how the field device is wired. |
| --( )-- | Output coil | OTE | Coil | Writes the rung result to its bit every scan: 1 when the rung is true, 0 when it is false. |
| --(L)-- / --(S)-- | Latch (set) coil | OTL | Set coil (S) | Sets its bit to 1 when the rung is true and leaves it on after the rung goes false. |
| --(U)-- / --(R)-- | Unlatch (reset) coil | OTU | Reset coil (R) | Clears its bit to 0 when the rung is true. Pair every latch with a reset path. |
| --[P]-- | Rising-edge (one-shot) | ONS | P contact or R_TRIG | True for exactly one scan when the condition changes from false to true. |
| [ TON ] | On-delay timer | TON | TON block | Starts timing when enabled and sets its done output after the preset time. |
| [ CTU ] | Count-up counter | CTU | CTU block | Adds one on each false-to-true transition and sets its done output at the preset. |
| [ GRT ] / [ > ] | Compare block | GRT, LES, EQU | GT, LT, EQ | Passes power when the comparison of two values is true, such as Level > 80. |
Worked examples
Every rung is a Boolean expression. Trace power from the left rail: series contacts are AND, parallel branches are OR, and a normally closed contact is NOT. These four small rungs cover the logic that the patterns further down are built from.
Press = Left_PB AND Right_PB
|--[ Left_PB ]--[ Right_PB ]--------( Press )--|Power reaches the coil only if every contact in the path is true. This is the logic behind two-hand start; a real two-hand control also needs safety-rated devices and a simultaneity check.
| Left_PB | Right_PB | Press |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 1 | 1 |
Alarm = Door1_Open OR Door2_Open
|--+--[ Door1_Open ]--+--------( Alarm )--|
| | |
| +--[ Door2_Open ]--+A parallel branch gives power a second path. Either open door is enough to sound the alarm.
| Door1_Open | Door2_Open | Alarm |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 1 | 1 |
Fill_Valve = NOT High_Level
|--[/ High_Level ]------------------( Fill_Valve )--|The valve stays open while the high-level switch is off and closes as soon as the tank reaches the high level.
| High_Level | Fill_Valve |
|---|---|
| 0 | 1 |
| 1 | 0 |
Motor = (Start_PB OR Motor) AND Stop_OK
|--+--[ Start_PB ]--+--[ Stop_OK ]--( Motor )--|
| | |
| +--[ Motor ]-----+Stop_OK is a normally closed Stop pushbutton, so it is 1 until someone presses Stop (or the wire breaks). The coil feeds its own contact, which holds the rung true after Start is released.
| Scan | Start_PB | Stop_OK | Motor result | Why |
|---|---|---|---|---|
| 1 | 1 | 1 | 1 | Start path is true, so the coil energizes. |
| 2 | 0 | 1 | 1 | Start released, but the Motor branch holds the rung true. |
| 3 | 0 | 0 | 0 | Stop pressed: the series contact opens the only path. |
| 4 | 0 | 1 | 0 | Stop released, but nothing reseals until Start is pressed again. |
Watch these rules happen live in the PLC scan cycle visualizer, or read the full guide to reading ladder logic.
Build a contact-and-coil rung, flip the input and watch the coil energize. No account or install.
The most common ladder circuit in industry
A momentary Start pushbutton energizes an output coil. A parallel contact — fed by the same output bit — holds the coil energized after the Start button is released. A normally-closed Stop contact in series with the parallel branch breaks the circuit on demand. The output coil "seals itself in" through its own contact.
Virtually every motor circuit. Conveyor drives. Pump contactors. Any load that must stay on after a momentary start command and turn off on a momentary stop command. The pattern also appears in alarm latches, fault holds, and mode-enable circuits where any version of seal-in logic is required.
Pattern structure (concept only)
Preventing two outputs from being simultaneously true
Two outputs (A and B) must never both be energized at the same time. A normally-closed contact of output A is placed in series with the rung driving output B, and vice versa. If A is true, its NC contact in the B rung opens, blocking B. This is a software interlock; in real panels it is always paired with a hardwired electrical interlock (wired NC contacts across the physical contactors).
Forward/reverse motor drives where reversing a spinning motor is mechanically destructive. Star and delta contactors in a star-delta starter — energizing both simultaneously would short two motor terminals. Dual-coil solenoid valves where both coils energized is a fault state. Any dual-output exclusion requirement.
Pattern structure (concept only)
Triggering logic on the rising or falling edge of a signal
A one-shot (OSR/R_TRIG) instruction produces a single-scan true output on the rising edge of its input signal — regardless of how long the input stays true. A standard normally-open contact stays true for as long as the input is true; the one-shot is true for exactly one scan. The equivalent falling-edge instruction (OSF/F_TRIG) triggers on the 1→0 (true-to-false) transition.
Counting items on a conveyor — a photoelectric sensor sees each item for multiple scans; the one-shot ensures each item counts as exactly one pulse. Incrementing a counter only once per button press even if the button is held. Triggering a timed delay on the first scan that a condition becomes true. Detecting the leading edge of a process alarm before it is acknowledged.
Pattern structure (concept only)
Chaining TON timers to step through a timed sequence
Multiple TON (on-delay) timers are chained so that the done bit of timer N enables the coil of timer N+1 and simultaneously de-energizes the output for step N. Each step occupies a time window defined by its TON preset. When the final timer's done bit fires, it resets the chain and the sequence repeats from step 1. The done bit of each timer in the chain also drives its corresponding output.
Traffic light sequencing — the canonical example. Machine warm-up sequences with timed dwell phases. Wash cycle timers in a CIP or dishwash sequence. Alarm acknowledgement timeout windows. Any application where a series of outputs must each be active for a fixed duration in a repeating cycle.
Pattern structure (concept only)
Counting events and triggering logic at a threshold
A CTU (count-up) instruction increments its accumulator by one on each rising edge of its count input. When the accumulator reaches the preset value, the done bit goes true. A CTD (count-down) decrements from the preset to zero. The accumulator can be read at any time for display or comparison logic. A reset coil sets the accumulator back to zero.
Counting boxes on a conveyor and triggering a diverter or stop signal at the batch size. Counting pump starts for maintenance hour scheduling. Counting parts through a machine cycle to verify correct operation. Tracking occupancy (entry CTU − exit CTU = current count). Any scenario where cumulative events drive a threshold action.
Pattern structure (concept only)
Latching and unlatching a bit with separate coil instructions
A SET (latch) coil instruction sets its bit true when its rung goes true and holds it true even when the rung goes false — the bit remains latched until a RESET (unlatch) instruction on a separate rung sets it false. Unlike a seal-in circuit, the SET/RESET latch is controlled by two completely independent rungs. In IEC syntax this is the S and R coil pair; in Allen-Bradley it is OTL (Output Latch) and OTU (Output Unlatch).
Fault latch circuits where an alarm state must persist until an operator resets it — even across a power cycle if stored in retentive memory. Mode-select logic where pressing a mode button sets a mode bit and pressing another mode button resets it. Any scenario where set and reset conditions are physically or logically separate enough that combining them in a seal-in rung would be confusing.
Pattern structure (concept only)
Requiring all safety conditions to be clear before allowing a start
A permissive is a condition that must be satisfied before a machine action is allowed. Multiple permissives are wired in series in the logic — each as a normally-closed contact for a fault bit, or a normally-open contact for a ready bit. Every permissive in the chain must be true simultaneously for the permissive rung output to be true. The permissive rung output then gates the start command. A single failed permissive blocks the entire chain.
Boiler startup: purge complete AND gas pressure OK AND no flame present must all be true before pilot ignition is permitted. Conveyor start: guard door closed AND e-stop reset AND upstream conveyor running must all be true before the drive can start. Any machine with multiple independent safety or readiness conditions that must all be verified before motion is permitted.
Pattern structure (concept only)
Organizing complex machine behavior into named states with defined transitions
A state machine assigns an integer step counter (or a set of mutually exclusive state bits) to represent which phase of the machine cycle is currently active. Each rung in the program is conditioned on the current step value. Transition logic advances the step counter when the exit conditions for the current step are met. Only one step is active at a time, and only the rungs for that step execute.
Garage door controllers where the door can be Opening, Open, Closing, Closed, or Faulted. Batch mixer sequences where the machine moves through Fill, Mix, Heat, Hold, Drain, and Clean phases. Elevator controllers where the cab can be Idle, Moving Up, Moving Down, Door Opening, Door Open, or Door Closing. Any machine with a defined sequence of named operating modes where the active mode determines what the PLC should be doing.
Pattern structure (concept only)
Why runnable wins
A screenshot of a ladder diagram explains the structure but not the behavior. When you run the same circuit in a live simulation and watch the seal-in contact hold the motor on after you release the Start button — or watch the interlock block the reverse contactor when the forward contactor is energized — the pattern shifts from something you recognize to something you understand. That difference is what the grader measures.
Each pattern on this page links directly to a scenario in the catalog. Open it in a new tab, write the rung, run it. The grader tests that your circuit behaves correctly under the test inputs — not just that it compiles.
You can implement a seal-in as a parallel contact or as a SET coil. The grader does not care about the method — it tests whether the output stays true after the start input goes false and goes false when the stop input fires. Correct behavior is the standard.
The advanced scenarios combine multiple patterns from this page. The elevator uses seal-in, interlock, state machine, and safety permissive chains together. Recognizing the individual patterns inside the larger program is the skill industrial employers test in interviews.
Quick reference
Triggering logic on the rising or falling edge of a signal
Organizing complex machine behavior into named states with defined transitions
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Technical reference and worked-example guide
Direct answer
The reader can explain each example as an input-state-output contract, identify its owned memory and test it at normal, boundary and fault conditions.
Written for pLC beginners and technicians studying start-stop, seal-in, interlock, timer, counter, sequence and alarm patterns before adapting them to a machine.
Process requirement, input states, output owner, memory, scan order, priority, timing, count, permissive, feedback, alarm, reset and restart policy.
Operator or field condition through rung continuity and stored state to command, independent feedback, alarm and acceptance evidence.
One start-stop, timer, counter and sequence example executed from a known initial state and explained scan by scan.
Simultaneous start and stop, held input, exact preset, repeated edge, feedback failure, reset, mode change, power return and stale state.
A requirement, contact-truth, state, scan-order, timing, count, output-owner, feedback or reset defect.
The selected pattern rewritten for the exact target and regression-tested against machine-specific requirements.
Learn input-to-output truth, stop-priority start-stop, seal-in state, interlocks, TON timing, CTU counting, step sequences, alarms and feedback mismatch in that order.
Declare the initial state and expected outputs, then exercise normal operation, simultaneous inputs, exact time or count boundaries, reset, restart and at least one failed-feedback case.
PLC code and example path
Read the instruction behavior, inspect a complete pattern, run its edge cases and then adapt it as a project.