IEC 61131-3 Ladder Logic
The international standard that unifies PLC programming across vendors. Uses VAR blocks and symbolic addressing. Widely supported by Codesys, Siemens TIA Portal (LD), and most modern PLCs.
The same Button to Light, Seal-In (Motor Start/Stop), AND Gate, OR Gate, and TON On-Delay programs written in 9 PLC dialects. Click any cell to copy — no account required.
Want hands-on practice in any of these dialects? Pro tier unlocks all 9 — simulate real machines, earn a certificate.
Try a lesson| Program | IEC 61131-3 | Allen-Bradley | Siemens STL |
|---|---|---|---|
Button to LightA momentary push-button directly energises an output coil. The simplest possible rung.boolean | | | |
Seal-In (Motor Start/Stop)Classic 3-wire motor control: START latches the motor output via its own feedback contact; STOP (NC) breaks the seal.boolean | | | |
AND GateOutput energises only when both inputs are simultaneously active — the fundamental series rung.boolean | | | |
OR GateOutput energises when either input is active — the fundamental parallel rung.boolean | | | |
TON On-DelayOutput turns on after the start input has been active. IEC/AB show a real TON timer block; other dialects show an equivalent latch pattern.timer | | | |
The international standard that unifies PLC programming across vendors. Uses VAR blocks and symbolic addressing. Widely supported by Codesys, Siemens TIA Portal (LD), and most modern PLCs.
Rockwell Automation's instruction set uses XIC, XIO, OTE mnemonics and file-based addresses (I:0/0). Powers the MicroLogix, SLC-500, and ControlLogix families.
Step 7 / TIA Portal's low-level text language. Boolean stack instructions:A (AND), O (OR), = (coil). Often called "Siemens ladder logic" by practitioners.
Uses LD, AND, OR, OUT with native X/Y/M device addressing. Dominant in Japanese automotive and semiconductor manufacturing.
Similar mnemonics to IEC IL but with two-token negation:LD NOT, AND NOT, OR NOT. Channel-based addressing (e.g. 0.00).
Popular in cost-sensitive OEM markets. Uses octal-group addressing (X0–X7 = first 8 inputs). ANI /ORI for normally-closed contacts instead of separate NC tokens.
A tested learning subset using R, MR, and DM devices with KV-style contact, coil, timer, counter, and arithmetic mnemonics.
Industrial automation sites often run multiple PLC brands — an Allen-Bradley ControlLogix on the packaging line, a Siemens S7 on the press, Mitsubishi on the conveyor. Technicians who can read and translate between dialects are far more flexible and valuable. This free reference shows the same logic in each dialect so you can spot the patterns quickly: seal-in in AB looks like XIC START OR XIC MOTOR AND XIO STOP OTE MOTOR; in Siemens it is A START / O MOTOR / AN STOP / = MOTOR. Same concept, different syntax.
All programs on this page are live — you can open them in the Open in editor links or copy the code and paste it into the simulator's editor to run it against real machine scenarios.
Competency and practice field guide
Direct answer
PLC dialect learning pathway becomes useful when it connects source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior with machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed i/o and physical result, then proves the same start-stop, timer, counter and edge cases explained and reproduced in two selected dialects under normal, boundary, fault and recovery conditions. The objective is a repeatable engineering or learning result, not merely activity inside a page or tool.
This guide is written for pLC learners moving between IEC concepts and Allen-Bradley, Siemens, Mitsubishi, Omron or Delta conventions without memorizing isolated mnemonics. The intended result is specific: the learner can express a neutral behavior contract, identify platform-owned state and verify the translated behavior at exact boundaries.

System map / 02
Treat these as connected checkpoints. Each checkpoint has an expected state, an observable state and a boundary to the next part of the system. That structure prevents a software indication from being mistaken for physical proof.
source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior. For structured cross-vendor PLC dialect learning, record the initial condition, actor, requested change, observable result and stopping condition before selecting a tool or implementation.
machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed I/O and physical result. Separate request, internal state, output or service, physical or user-visible result and independent feedback so each boundary can be inspected.
the same start-stop, timer, counter and edge cases explained and reproduced in two selected dialects. Run more than one cycle from a known state and retain the values, timings or artifacts that demonstrate repeatability.
first scan, retained data, skipped calls, task-period change, time conversion, overflow, online edit, download and power return. Choose minimum, maximum, simultaneous, delayed or restart conditions that reveal assumptions hidden by the happy path.
a terminology, address, instance, parameter, scan, timing, retention, data-type or workflow mismatch. Preserve the first symptom, divide the system at a measurable boundary and change one condition only after predicting the result.
each transferred case compiled and regression-tested in the current official target environment. Restore normal state, remove temporary changes, repeat affected checks and document which claims remain limited to the learning environment.
Procedure / 03
Run the steps in order the first time. Later, the same structure becomes a diagnostic loop: define the expected condition, observe the boundary, interpret the difference and choose one proving action.
Convert source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior into initial conditions, one stimulus and observable pass criteria.
Evidence: Another person can repeat the case without guessing the intended result.
Avoid: Using page completion or an animation as the acceptance criterion.
Document machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed i/o and physical result and name who owns each state or decision.
Evidence: Every request and result has a source, destination and useful inspection point.
Avoid: Using the same value as command, status and independent feedback.
Apply the same start-stop, timer, counter and edge cases explained and reproduced in two selected dialects from a clean start and record the expected evidence.
Evidence: Repeated runs produce the same bounded result.
Avoid: Changing several parameters before a baseline exists.
Test first scan, retained data, skipped calls, task-period change, time conversion, overflow, online edit, download and power return without changing the acceptance contract.
Evidence: Limits, timing and restart behavior reach defined states.
Avoid: Testing only one ideal sequence.
Introduce or analyse a terminology, address, instance, parameter, scan, timing, retention, data-type or workflow mismatch and locate the first disagreement.
Evidence: The proving action distinguishes the leading hypotheses.
Avoid: Resetting, forcing or replacing before evidence is retained.
Complete each transferred case compiled and regression-tested in the current official target environment and repeat the affected regression cases.
Evidence: A learner completes the surface by explaining the result, passing a changed case and identifying what still requires supervised target-equipment practice.
Avoid: Treating an acknowledged message or one successful rerun as handover.
Diagnostic matrix / 04
The table is a reasoning aid, not a parts-replacement chart. Preserve the initial symptom, inspect the named boundary and use the interpretation to choose the next controlled test. Site safety procedures and equipment manuals remain authoritative.
| Observed symptom | Inspect | Interpretation | Next proving action |
|---|---|---|---|
| The expected result is unclear | Requirement, initial state, actor, stimulus, units and pass condition | The learner, instructor and assessor may be solving different versions of the task. | Rewrite one observable acceptance case before continuing. |
| Internal state changes but the outcome does not | Request, final owner, output or service boundary and independent feedback | A software or interface indication proves intent at one layer, not the complete outcome. | Trace the first boundary after the changing state. |
| Normal case passes but an edge case fails | Limits, timing, simultaneous events, reset and restart assumptions | The implementation contains a hidden assumption exposed by the changed condition. | Add the failed boundary as a permanent regression case. |
| The failure disappears after reset | Original symptom, histories, diagnostics, timestamps and active cause | Reset changed evidence or state without proving the initiating cause. | Reproduce under a controlled condition and preserve pre/post-event data. |
| Simulator and target disagree | Model boundary, software version, task timing, I/O behavior, data types and configuration | A learning model and the intended target do not share one of the recorded assumptions. | Reduce the case and verify against current target documentation. |
| The result cannot be explained | Prediction, observation, proving action, alternative hypotheses and limitations | Activity occurred but the evidence is not yet transferable or reviewable. | Have the learner defend the signal path and repeat a changed case. |
Product evidence / 05
The browser platform can retain programs, scenario results, attempts and observable machine state so practice is attached to evidence rather than seat time alone.
Dialect lessons do not convert native projects or emulate vendor firmware; current official tools, documentation and target tests govern implementation.
Commissioning notebook / 06
Use these as written briefs rather than click-through instructions. For every case, state the expected condition before acting, retain the first useful observation and explain why the final result proves the requirement. A different program or component choice can still be correct when it produces the same bounded behavior and evidence.
Case 01
predict → observe → prove
Engineering context. source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior. For structured cross-vendor PLC dialect learning, record the initial condition, actor, requested change, observable result and stopping condition before selecting a tool or implementation. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.
Controlled setup. Use the “Write the acceptance case” stage of the workflow: convert source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior into initial conditions, one stimulus and observable pass criteria. The acceptance record should show this result: another person can repeat the case without guessing the intended result. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.
Fault challenge. Introduce or analyse “The expected result is unclear” as one bounded deviation. Inspect requirement, initial state, actor, stimulus, units and pass condition The working interpretation is that the learner, instructor and assessor may be solving different versions of the task. The next proving action is to rewrite one observable acceptance case before continuing. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.
Review and recovery. The most common trap here is using page completion or an animation as the acceptance criterion. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.
Explain it aloud: Which PLC dialect should I learn first? A defensible short answer is: Learn one platform well enough to explain I/O, scan, state, timers and diagnostics, while writing vendor-neutral behavior contracts that can be transferred later.
Case 02
predict → observe → prove
Engineering context. machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed I/O and physical result. Separate request, internal state, output or service, physical or user-visible result and independent feedback so each boundary can be inspected. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.
Controlled setup. Use the “Build the map” stage of the workflow: document machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed i/o and physical result and name who owns each state or decision. The acceptance record should show this result: every request and result has a source, destination and useful inspection point. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.
Fault challenge. Introduce or analyse “Internal state changes but the outcome does not” as one bounded deviation. Inspect request, final owner, output or service boundary and independent feedback The working interpretation is that a software or interface indication proves intent at one layer, not the complete outcome. The next proving action is to trace the first boundary after the changing state. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.
Review and recovery. The most common trap here is using the same value as command, status and independent feedback. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.
Explain it aloud: Are PLC dialects interchangeable? A defensible short answer is: No. Common control intent transfers, but names, parameters, instance storage, task execution, data types, reset and restart behavior can differ.
Case 03
predict → observe → prove
Engineering context. the same start-stop, timer, counter and edge cases explained and reproduced in two selected dialects. Run more than one cycle from a known state and retain the values, timings or artifacts that demonstrate repeatability. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.
Controlled setup. Use the “Run the baseline” stage of the workflow: apply the same start-stop, timer, counter and edge cases explained and reproduced in two selected dialects from a clean start and record the expected evidence. The acceptance record should show this result: repeated runs produce the same bounded result. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.
Fault challenge. Introduce or analyse “Normal case passes but an edge case fails” as one bounded deviation. Inspect limits, timing, simultaneous events, reset and restart assumptions The working interpretation is that the implementation contains a hidden assumption exposed by the changed condition. The next proving action is to add the failed boundary as a permanent regression case. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.
Review and recovery. The most common trap here is changing several parameters before a baseline exists. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.
Explain it aloud: What should I learn first about structured cross-vendor PLC dialect learning? A defensible short answer is: Start with the operating contract and evidence path: source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior, followed by machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed i/o and physical result. Add advanced features only after the baseline is predictable.
Case 04
predict → observe → prove
Engineering context. first scan, retained data, skipped calls, task-period change, time conversion, overflow, online edit, download and power return. Choose minimum, maximum, simultaneous, delayed or restart conditions that reveal assumptions hidden by the happy path. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.
Controlled setup. Use the “Challenge assumptions” stage of the workflow: test first scan, retained data, skipped calls, task-period change, time conversion, overflow, online edit, download and power return without changing the acceptance contract. The acceptance record should show this result: limits, timing and restart behavior reach defined states. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.
Fault challenge. Introduce or analyse “The failure disappears after reset” as one bounded deviation. Inspect original symptom, histories, diagnostics, timestamps and active cause The working interpretation is that reset changed evidence or state without proving the initiating cause. The next proving action is to reproduce under a controlled condition and preserve pre/post-event data. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.
Review and recovery. The most common trap here is testing only one ideal sequence. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.
Explain it aloud: How do I practise structured cross-vendor PLC dialect learning effectively? A defensible short answer is: Use short cases with known initial conditions, a written prediction, one action and an observable result. Then alter a boundary or fault and explain why the evidence changed.
Case 05
predict → observe → prove
Engineering context. a terminology, address, instance, parameter, scan, timing, retention, data-type or workflow mismatch. Preserve the first symptom, divide the system at a measurable boundary and change one condition only after predicting the result. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.
Controlled setup. Use the “Isolate one failure” stage of the workflow: introduce or analyse a terminology, address, instance, parameter, scan, timing, retention, data-type or workflow mismatch and locate the first disagreement. The acceptance record should show this result: the proving action distinguishes the leading hypotheses. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.
Fault challenge. Introduce or analyse “Simulator and target disagree” as one bounded deviation. Inspect model boundary, software version, task timing, I/O behavior, data types and configuration The working interpretation is that a learning model and the intended target do not share one of the recorded assumptions. The next proving action is to reduce the case and verify against current target documentation. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.
Review and recovery. The most common trap here is resetting, forcing or replacing before evidence is retained. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.
Explain it aloud: What counts as proof of competence? A defensible short answer is: A repeatable artifact or system result plus an explanation of the signal path is stronger than time spent, screenshots or a copied answer. Physical competence requires separate supervised evidence.
Case 06
predict → observe → prove
Engineering context. each transferred case compiled and regression-tested in the current official target environment. Restore normal state, remove temporary changes, repeat affected checks and document which claims remain limited to the learning environment. Begin with a written normal condition and identify which request, state, physical result or communication value will provide independent confirmation. Do not begin by changing the configuration; the initial state is part of the evidence and should remain reproducible.
Controlled setup. Use the “Close the evidence loop” stage of the workflow: complete each transferred case compiled and regression-tested in the current official target environment and repeat the affected regression cases. The acceptance record should show this result: a learner completes the surface by explaining the result, passing a changed case and identifying what still requires supervised target-equipment practice. Record initial conditions, the exact stimulus and the observation point so another learner can repeat the case without relying on your memory.
Fault challenge. Introduce or analyse “The result cannot be explained” as one bounded deviation. Inspect prediction, observation, proving action, alternative hypotheses and limitations The working interpretation is that activity occurred but the evidence is not yet transferable or reviewable. The next proving action is to have the learner defend the signal path and repeat a changed case. Change only one condition before observing the result, and preserve timestamps or measurements where timing matters.
Review and recovery. The most common trap here is treating an acknowledged message or one successful rerun as handover. After restoring the cause, repeat the normal case and at least one stop, timeout, disconnect or restart boundary relevant to this topic. Remove temporary forces and bypasses, return the model to a known state and retain the evidence that both operation and recovery are deliberate.
Explain it aloud: Why test faults and restart behavior? A defensible short answer is: Because a terminology, address, instance, parameter, scan, timing, retention, data-type or workflow mismatch or first scan, retained data, skipped calls, task-period change, time conversion, overflow, online edit, download and power return can expose assumptions that never appear during ideal startup and steady operation.
Answer surface / 07
These concise answers define the operating, training and product boundaries most often missed in broad summaries. The full workflow and diagnostic table above provide the evidence behind them.
Learn one platform well enough to explain I/O, scan, state, timers and diagnostics, while writing vendor-neutral behavior contracts that can be transferred later.
No. Common control intent transfers, but names, parameters, instance storage, task execution, data types, reset and restart behavior can differ.
Start with the operating contract and evidence path: source and target controller, engineering version, program organization, task or scan model, tag or device model, instructions, instances, data types, time and restart behavior, followed by machine requirement through neutral state-and-transition contract, source implementation, target implementation, observed i/o and physical result. Add advanced features only after the baseline is predictable.
Use short cases with known initial conditions, a written prediction, one action and an observable result. Then alter a boundary or fault and explain why the evidence changed.
A repeatable artifact or system result plus an explanation of the signal path is stronger than time spent, screenshots or a copied answer. Physical competence requires separate supervised evidence.
Because a terminology, address, instance, parameter, scan, timing, retention, data-type or workflow mismatch or first scan, retained data, skipped calls, task-period change, time conversion, overflow, online edit, download and power return can expose assumptions that never appear during ideal startup and steady operation.
No. It can build concepts and diagnostic reasoning. Exact firmware, I/O electrical behavior, networking, safety and commissioning require current official tools, documentation and target equipment.
Keep the requirement, initial state, program or configuration, observed values, fault hypothesis, proving action, recovery result and a concise limitations statement.
Continue the signal path / 08
PLC dialect transfer path
Compare address families and mnemonics around the same machine task, then validate the final project in the target vendor tool.