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PLC

Ladder Logic

Contact and coil patterns: interlocks, latching, and edge detection.

PLC

Overview

Ladder logic expresses control as power flow through contacts and coils — the lingua franca of discrete automation, readable by electricians and auditable rung by rung.

Engineering purpose

Interlocks, sequences, and motor control where visual traceability during live troubleshooting matters more than computational elegance.

How it works

Each rung evaluates left-to-right per scan; seal-in branches latch outputs past momentary commands; edge instructions convert level signals to one-shot events; cross-references track every address.

  • Seal-in circuits hold outputs after momentary commands; every stop condition must break the seal.
  • Use rising/falling edge instructions for one-shot actions to avoid re-triggering every scan.
  • Interlocks must be structured so no single rung can bypass a safety condition.

Common faults

Double coil assignments producing flicker; seal-ins that a stop condition fails to break; edge instructions duplicated so events fire twice; interlock bypass hidden in a parallel branch.

Diagnostic checks

  1. 1Trace rung power flow online to find the blocking contact.
  2. 2Search cross-references for double coil assignments.
  3. 3Audit every safety-relevant rung for parallel paths that could bypass the interlock condition.

Safety notes

A ladder interlock is an availability function, not a safety function — protective stops must remain hardwired or in certified safety logic regardless of program structure.

Commissioning notes

Walk each sequence step with the machine in manual, watching rung power flow online; archive a cross-reference listing with the as-commissioned program.

Related concepts

Contacts and coils, seal-in/latch, edge detection, cross-reference, interlocking, scan order.

ladder · rung · contact · coil · latch · seal · interlock · لدر · نردبانی · کنتاکت · کویل · اینترلاک

Related engineering cases

  • S7-1200 BF after CPU swapSiemensPLC
  • S7-1500 cycle watchdog from SCL loopSiemensPLC
  • FX5 output flicker from double coilMitsubishiPLC

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When Hermes Brain uses this article

Cited when questions mention rungs, coils, latching behavior, outputs that stick or flicker, or interlock logic in any ladder-programmed PLC.

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