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PLC Systems

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A Programmable Logic Controller (PLC) is a ruggedized industrial computer designed to control machines and processes. It reads inputs, executes logic in a deterministic scan, and updates outputs — repeated thousands of times per second.

Overview

PLCs replaced banks of relays in the late 1960s and are now the standard control element in factories, water plants, oil & gas, and packaging. They are designed for decades of unattended operation in harsh environments.

Architecture

  • CPU — runs the program; battery- or capacitor-backed memory.
  • Power supply — typically 24 VDC.
  • I/O modules — digital in/out, analog in/out, specialty (motion, weigh, comms).
  • Backplane / chassis or DIN-rail mount.
  • Communication — Ethernet, serial, fieldbus.
  • Programming terminal — laptop with vendor software.

Scan Cycle

  1. Read inputs into the input image table.
  2. Execute program logic top-to-bottom.
  3. Write outputs from the output image table.
  4. Housekeeping — communications, diagnostics.

Typical scan times: 1–20 ms. Avoid I/O reads mid-scan; use the image table so logic sees a consistent snapshot.

IEC 61131-3 Languages

  • LD (Ladder Diagram) — graphical relay logic.
  • FBD (Function Block Diagram) — graphical, signal-flow.
  • ST (Structured Text) — Pascal-like text.
  • SFC (Sequential Function Chart) — state-machine sequencing.
  • IL (Instruction List) — legacy assembler-style (deprecated in 3rd ed).

I/O & Tags

  • Digital inputs: dry contacts, sourcing/sinking 24 V.
  • Digital outputs: relay, transistor (sinking/sourcing), TRIAC.
  • Analog: 4–20 mA, 0–10 V, RTD, thermocouple.
  • Tag-based addressing > absolute (e.g. Conveyor_1.Run).
  • UDTs (user-defined types) keep tag structures consistent.

Major Platforms

  • Rockwell / Allen-Bradley — ControlLogix, CompactLogix, Micro800; Studio 5000.
  • Siemens — S7-1200, S7-1500; TIA Portal.
  • Schneider — Modicon M580, M340; EcoStruxure Control Expert.
  • Mitsubishi — MELSEC iQ-R, iQ-F.
  • Omron — NX, NJ; Sysmac Studio.
  • Beckhoff — TwinCAT 3 (Windows-based, soft PLC).
  • Codesys-based — Wago, Eaton, ifm, many OEMs.
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