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Courses/Industrial Control & PLCs/PLC Fundamentals

lesson · 10min · Lesson 23 of 30

Ladder logic basics: contacts and coils

Course syllabusCourse overview
01Motor Starters
  1. ReadDOL starter: main and control circuit
  2. ReadStar-delta starter: wiring and timer
  3. ReadSoft starters: operation and parameters
  4. ReadVariable speed drives: inverter drives
  5. quizMotor starter quiz
02Contactors & Overloads
  1. ReadContactor construction and ratings
  2. ReadAuxiliary contacts and interlocking
  3. ReadThermal overload relays: setting the dial
  4. ReadElectronic overloads and motor protection relays
  5. exerciseContactor circuit problems
03Control Circuit Diagrams
  1. ReadIEC 60617 symbols: reading control diagrams
  2. ReadLadder diagrams: European and American styles
  3. ReadDrawing a full DOL control circuit
  4. ReadForward-reverse motor control
  5. exerciseDiagram reading exercises
04Safety Systems
  1. ReadEmergency stop requirements: IEC 60204
  2. ReadSafety relays and safety PLCs
  3. ReadLight curtains and safety interlocks
  4. ReadSafe isolation: EAWR and multi-energy control
  5. quizSafety systems quiz
05PLC Fundamentals
  1. ReadWhat is a PLC and where is it used?
  2. ReadPLC architecture: CPU, I/O modules, power
  3. ReadLadder logic basics: contacts and coils
  4. ReadTimers and counters in ladder logic
  5. exerciseWriting a simple motor control program
06Panel Layout & Installation
  1. ReadPanel design: layout and component spacing
  2. ReadCable management inside panels
  3. ReadLabelling, ferrules, and documentation
  4. ReadPanel testing and commissioning checklist
  5. quizFinal assessment
Lesson · 10min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Ladder logic basics: contacts and coils

Industrial Control · Lesson 23 · Advanced

AdvancedReview: professional review pending

Purpose

Read and write ladder contacts/coils using Boolean semantics, scan order and single-writer discipline.

Before you beginControl diagrams and starter circuits · Machine safety and protection fundamentals

Learning objectives

  • Trace data and energy paths
  • Predict deterministic state changes
  • Design maintainable panel implementation
  • Verify before handover

A ladder contact instruction examines a Boolean value; it is not necessarily a physical contact. A coil writes a value or command according to rung truth and platform execution rules.

PLC SCAN AND LADDER STATEPLC SCAN AND LADDER STATESUPPLYSTARTERmotor · loadshort circuit · overload · command · interlock · isolation

Core theory

Series examine instructions implement AND; parallel branches implement OR; a normally-closed/examine-if-off instruction tests the inverse state. Always name the underlying tag and physical interpretation.

PLC scans can make rung order, task order, latches and multiple writers significant. Assign each actuator command one controlled owner and use intermediate state logic.

Input bounce, output latency, communications updates, initialization, retentive memory and online edits must be considered. Safety logic needs its certified environment and validation.

Terms, symbols, and units
TermMeaningSymbolUnit
Examine instructionProgram element testing a Boolean stateNot applicableNot applicable
CoilInstruction assigning or setting a Boolean resultNot applicableNot applicable
Single writerOne controlled location responsible for a command valueNot applicableNot applicable
Worked exampleRUN = Start AND NOT StopFault, with Start=true and StopFault=false.

Assumptions: The supplied PLC behavior and equipment data are authoritative for the example; Safety functions are separately specified and validated.

  1. Invert: NOT StopFault = true.
  2. AND: true AND true = true.
  3. Output: RUN becomes true for this scan, subject to surrounding state and task rules.

RUN evaluates true in the supplied Boolean state.

Reasonableness check: The result follows the declared scan, wiring and assembly boundaries rather than assuming every platform behaves identically.

Common mistakes
  • Treating field input state and program contact instruction as the same thing
  • Writing one output from several uncontrolled locations
  • Using spare physical space as proof of thermal or EMC capacity

Where this appears in practice

PLC and panel engineering connects field signals, deterministic software, power/control distribution, safety functions, documentation and commissioning evidence.

SafetyPLC force, online edit, manual output or simulation can move machinery. Establish authorization, exclusion zones, safety-state controls and rollback before change; isolate and prove dead before panel wiring work.
Local code checkConfirm the exact PLC/runtime semantics and manufacturer manuals, current IEC/BS EN 61131-3, machine risk assessment, IEC/BS EN 60204-1, applicable IEC/BS EN 61439 assembly part, EMC/cybersecurity requirements, short-circuit rating and documented design/routine verification. Standard PLC logic is not a safety function unless the complete implementation is designed and validated accordingly.

Knowledge check

Does a normally-closed ladder instruction prove a physical NC contact is wired?

No. It tests an inverse Boolean; trace the tag and field circuit.

Answer: No. It tests an inverse Boolean; trace the tag and field circuit.

Practical exercise

Convert five rungs to Boolean expressions and identify multiple-writer, seal-in and restart defects.

Summary

  • PLC logic operates on a sampled process image
  • Panel layout is an electrical/thermal/EMC design
  • Commissioning must prove hardware, software and safety behavior

Sources and review

  • IEC 61131-3:2025: Programmable-controller languages: IEC; 2025; International
  • IEC 60204-1:2016+AMD1:2021: Electrical equipment of machines: IEC; 6.1; International
  • IEC 61439-1:2020: Low-voltage assemblies: general rules: IEC; 2020 with current corrigenda; International

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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