Learn what contacts, coils, latches, one-shots, counters, program-flow and data instructions do during execution. Compare representative Allen-Bradley, Siemens, IEC 61131-3 and Mitsubishi conventions, then practice the underlying logic in a browser-based training simulator.
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Try this in the browserFrom field signal to instruction result
A mnemonic only becomes useful when you can connect it to the field device, controller memory, task execution and physical result. These diagrams make that complete path visible.
Direct answers
PLC code is the executable control logic stored in a programmable logic controller. It may be written as ladder diagram, function block diagram, structured text, sequential function chart or another vendor-supported form. An instruction is one operation inside that program; the full code also includes tags, routines, tasks, data types and controller configuration.
This page is a catalog of common PLC operations. Capitalized Instruction List can also mean the legacy IEC 61131-3 textual language that writes logic as mnemonic statements. If that is what you need, use the PLC Instruction List simulator and guide.
Foundations
PLC platforms share control concepts inherited from relay logic and computing, but they do not expose one universal instruction set. Allen-Bradley's Studio 5000 uses mnemonics such as XIC, OTE and JSR. Siemens and IEC-oriented environments use graphical elements and function blocks with platform-specific details. Mitsubishi programming tools use mnemonics such as LD, OUT, SET, RST and PLS. Available instructions, parameters and runtime behavior depend on controller family and software version.
The transferable skill is predicting how Boolean state, transitions, retained values and task execution affect a machine. Comparable contacts and rising-edge detectors solve similar control problems, but they are not automatically interchangeable: storage, prescan, enable, scope and update rules can differ. Use this guide to build a mental model, then verify exact behavior in the target vendor's documentation.
Each instruction below gets its own page: what it does during the scan, a ladder diagram showing it in context, a timing diagram where behavior is stateful, the vendor naming table, two worked examples and the mistakes that actually bite people in the field. Start with contact-and-coil behavior in the no-account browser exercise; after signup, the scenario catalog covers broader instruction combinations in practical machine contexts.
The reference
Looking for timers? TON, TOF and RTO have a dedicated deep-dive on the PLC timers page, counters get the full treatment on PLC counters, and every symbol is drawn out on the ladder logic symbols reference.
Build the first contact-and-coil rung without an account, toggle a field input and observe the output. Then save your result and continue into guided scenarios that combine timers, counters, latches, data and program structure.
Instruction-set field guide
Direct answer
PLC instructions are operations evaluated by the controller runtime. Learn each instruction from its input contract, output or state change, scan behavior, reset behavior, boundary cases and vendor terminology—not from the symbol alone.
This guide is written for beginners building a transferable instruction vocabulary and experienced technicians comparing IEC-style, Allen-Bradley-style and Siemens-style naming. The intended result is specific: the learner can choose an instruction for an observable requirement, predict its behavior over scans and write a test that distinguishes correct state from a plausible-looking rung.
System map / 02
Contact-style instructions read a Boolean expression and contribute rung truth; they do not directly represent whether a physical contact carries energy.
Coils and assignment instructions write state. Multiple writers create priority and maintenance risks that must be intentional.
Set/reset, retentive timers and counters preserve information until a defined reset or initialization event occurs.
Timers accumulate according to enable, reset, time base and task execution; a displayed preset alone does not define behavior.
Counters depend on edge detection, accumulated range, done logic and reset policy rather than an abstract quantity.
Moves, comparisons, arithmetic and conversions require type, range, precision, overflow and validity contracts.
Procedure / 03
Write the observable condition, state change and reset or stop behavior.
Evidence: The instruction choice is justified by behavior.
Avoid: Choosing a mnemonic first.
List input types, output type, internal state and platform assumptions.
Evidence: Unsupported or ambiguous operands are found before coding.
Avoid: Assuming all integer and time types behave alike.
Walk false, true, held, falling and reset conditions across repeated execution.
Evidence: A small state table explains the instruction completely.
Avoid: Reasoning from one static screenshot.
Use one clear instruction pattern with named tags and one owner for final state.
Evidence: The rung or statement maps directly to the requirement.
Avoid: Hiding state changes behind clever branches.
Exercise zero, preset, maximum credible value, overflow and restart where applicable.
Evidence: Boundary behavior is recorded and deterministic.
Avoid: Testing one nominal input.
Compare the exact vendor manual and run on the intended controller toolchain.
Evidence: Platform-specific status and fault behavior is confirmed.
Avoid: Treating transferable semantics as vendor parity.
Diagnostic matrix / 04
| Observed symptom | Inspect | Interpretation | Next proving action |
|---|---|---|---|
| Contact appears inverted | Tag value, instruction type, physical normal state and naming | A normally closed instruction is logical negation, which may differ from the device’s electrical description. | Write the truth table. |
| Latched bit will not clear | Every set/reset writer, reset priority, mode and retentive initialization | Another writer or untested state may reassert the bit. | Cross-reference all writes. |
| Timer resets unexpectedly | Enable continuity, non-retentive behavior, reset rung and task state | A brief false scan can clear accumulated time. | Trend enable and accumulator. |
| Counter skips or doubles | Input edge, one-shot, scan rate, filtering and numeric range | The counted event contract does not match the sampled level. | Create one deterministic event pulse. |
| Comparison never true | Data type, units, scaling, precision and signedness | Operands may represent different engineering facts despite similar numbers. | Inspect typed values before changing thresholds. |
| Conversion changes value | Source range, destination range, rounding, truncation and overflow policy | The destination cannot preserve every source value. | Test endpoints and fractional cases. |
Product evidence / 05
The public reference links supported contacts, coils, timers, counters, comparisons, data movement, conversion and control instructions to parser/runtime fixtures and runnable scenarios.
Answer surface / 07
Contacts, coils, set/reset, timers, counters, comparisons and data movement form a common introductory set. Names and exact semantics vary by platform.
In Allen-Bradley terminology, XIC is true when the examined bit is true and XIO is true when it is false. They examine data rather than physical continuity.
TON is also the common IEC on-delay function-block name, but invocation, instance data and timing details differ by environment.
Use retained state only when the requirement defines how it is set, reset, initialized and recovered after faults or restart.
It converts a rising or falling condition into one execution event, useful when a held input must increment or trigger only once.
Some runtimes allow it, but the later effective write can obscure ownership. Prefer one final owner or explicit arbitration.
Create a state table, reset to known state, apply transitions over scans and assert output plus retained internal state at boundaries.
Concepts often transfer, but mnemonics, data structures, status bits, time bases and fault behavior require exact platform documentation.
Continue the signal path / 08
PLC code and example path
Read the instruction behavior, inspect a complete pattern, run its edge cases and then adapt it as a project.