What are ladder diagrams and sequence listing?

PLC (Programmable Logic Controllers)
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What are ladder diagrams and sequence listing

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Ladder Diagrams and Sequence Listing Explained with Reference to Provided Image

Ladder Diagrams

A graphical programming language called ladder diagrams or ladder logic is used to create software for Programmable Logic Controllers (PLCs). Commonly employed in industrial automation, they look like electrical relay logic diagrams.

Key Features:

  • Main Characteristics: Every “rung” stands for a logical operation or control sequence.
  • Usually, the left rail indicates the power source (like L+), and the right rail indicates the return (like L- or neutral).
  • Each rung’s symbols stand for inputs—like sensors or switches—and outputs—like coils or actuators.
  • The logic runs from top to bottom and left to right.

In Provided Image:

You have three rungs, each representing a transition condition for a sequence step:

  • Rung 1: LOAD LS input (I:1.2) when NOT electrified (because NC - Normally Closed).
  • Rung 2: Both TROLLEY PS (I:1.4) must be ON and LOAD BS (I:1.5) must be OFF.
  • Rung 3: OFF UNLOAD LS (I:1.3) must also be NC type.

Every rung lets the sequence move on by setting a memory coil (such M:1.4, M:1.5, M:1.6) showing the transition condition has been satisfied.

Sequence Listing

A sequence listing is a methodical, step-by-step record of how an automated system should operate. It specifies and breaks down the procedure into logical steps and it define:

  • Conditions for moving between phases.
  • Actions performed at every stage.
  • Needed actuator or sensor statuses.

Example From Provided Image:

  • Step 1: Trolley travels ahead until the Loading Limit Switch (LOAD LS) is triggered, NC being open. Condition M:1.4 is enabled…
  • Step 2: Trolley sits. It moves on to the next stage only when:
    • The Pallet Sensor (TROLLEY PS) detects a pallet (wired NO - therefore contact shuts when pallet is present).
    • The Loading Area Busy Sensor (LOAD BS) does not register the crane (NO stays open). This establishes M:1.5.
  • Step 3:Trolley runs backward. M:1.6 is set when the Unloading Limit Switch (UNLOAD LS) triggers (NC opens).

Ladder Diagram = Visual implementation of control logic. Sequence Listing = Text-based, step-by-step functional description. Together, they allow structured and predictable automation programming.