Plc and ladder logic fundamentals

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Module 1.2 — Inputs, Outputs, and the CPU
Module 1 · Lesson 2 · PLC & Ladder Logic Fundamentals

Inputs, Outputs, and the CPU — The Three-Part Anatomy

Watch a signal travel from a real field switch, through the CPU's decision-making, out to a real field device — the entire PLC anatomy in one animation.
Live · Signal Flow From Field Input to Field Output
SWITCH field input INPUT MODULE CPU runs the ladder logic program OUTPUT MODULE LAMP field output
Toggle the switch and watch the signal travel through all three stages of PLC anatomy.

Three Physical Parts, One Continuous Signal Path

Inputs — physical field devices (switches, sensors, pushbuttons) that send real-world signals INTO the PLC, converted to digital form by the input module. CPU — the "brain" that reads the current input states, runs your ladder logic program against them, and decides what the outputs should do. Outputs — the output module converts the CPU's decisions back into real electrical signals that drive field devices like lamps, motor starters, or valves.

Field-Practical Framing When troubleshooting, always think in this same three-stage order: is the physical field device actually working (input side)? Is the CPU's logic doing what you expect (program side)? Is the output module actually energizing the field device correctly (output side)? Isolating which of these three stages has failed is the single most useful troubleshooting habit in this entire course.

Check Your Understanding

What is the CPU's core job in a PLC?
Answer: To read current input states, run the ladder logic program against them, and decide what the outputs should do.
Why is thinking in "input → CPU → output" stages useful for troubleshooting?
Answer: It helps isolate exactly which stage has failed — a field device problem, a logic problem, or an output module problem — rather than guessing randomly.

QUICK RECAP

  • Inputs bring real-world field signals into the PLC via the input module
  • The CPU runs your ladder logic program against current input states
  • Outputs convert CPU decisions back into real signals driving field devices
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PLC & Ladder Logic Fundamentals — Module 1, Lesson 2
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