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Educational use: ElectraCore supports learning and preliminary checks. It does not replace a competent electrician or engineer. Verify results, equipment data, and current local regulations before installation or live work.
Courses/Industrial Control & PLCs/PLC Fundamentals

lesson · 9min · Lesson 22 of 30

PLC architecture: CPU, I/O modules, power

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 · 9min
POWER TRIANGLEφP: real (W)Q (VAr)S (VA)PF = P / S= cos φS² = P² + Q²
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PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

PLC architecture: CPU, I/O modules, power

Industrial Control · Lesson 22 · Advanced

AdvancedReview: professional review pending

Purpose

Trace PLC power, CPU, memory, backplane/network and I/O boundaries including electrical interface types.

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

The CPU executes configured tasks, but field behavior depends on power supplies, racks/backplanes, input thresholds, output technology, commons, isolation groups and external devices.

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

Core theory

Map supply voltage/current, inrush, grounding/bonding, module power budget, expansion-bus limits and behavior through brownout/power restoration.

For inputs, distinguish sourcing/sinking, AC/DC, voltage thresholds, filtering and common references. For outputs, distinguish relay, transistor and triac switching, leakage, protection and load suppression.

Document channel/group isolation, field supply fusing, terminal assignments, shield/reference treatment, hot-swap permissions, diagnostics and safe replacement procedure.

Terms, symbols, and units
TermMeaningSymbolUnit
Process imageMemory representation of sampled inputs and commanded outputsNot applicableNot applicable
SourcingProviding current toward a load/inputNot applicableNot applicable
BackplaneInternal power/data interconnection among controller modulesNot applicableNot applicable
Worked exampleA rack supply provides 2.0 A; CPU uses 0.65 A and modules use 0.30, 0.42 and 0.38 A.

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

  1. Sum: Load=0.65+0.30+0.42+0.38=1.75 A.
  2. Margin: Nominal remaining capacity=2.00−1.75=0.25 A.
  3. Verify: Check startup, environmental derating, expansion and manufacturer design rules.

Arithmetic margin is 0.25 A, but suitability still needs dynamic and derating evidence.

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

Can a transistor output switch any AC or DC load within its current number?

No. Output technology, polarity, voltage, leakage, inrush and suppression all matter.

Answer: No. Output technology, polarity, voltage, leakage, inrush and suppression all matter.

Practical exercise

Create an I/O and power budget including commons, fuses, isolation groups, load inrush and fail states.

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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