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Lesson 61 of 78 · PLC Architecture & Sequences

PLC Scan, I/O & Control Architecture

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A programmable logic controller is a deterministic industrial computer designed around physical I/O, machine sequencing, communications, diagnostics, and robust lifecycle support. IEC 61131-3:2025 specifies structured text, ladder diagram, function block diagram, and sequential function chart elements 1.

A PLC repeatedly reads inputs, executes logic, writes outputs, performs diagnostics and communication, then repeats, with physical process timing alongside
PLC logic is evaluated in a repeating scan while the physical process continues to move between scans. Credit: StudyCorner original diagram · CC BY 4.0 · Source

Scan behavior matters

Conceptually the PLC reads inputs into an image, executes logic in order, writes output states, services communications and diagnostics, then repeats. Immediate I/O, tasks, interrupts, motion systems, and distributed devices complicate this model, but the scan remains a useful timing foundation.

If a photoeye pulse lasts 4 ms and the worst-case task plus I/O update path is 12 ms, the event may be missed. Use hardware capture, a faster task, pulse stretching, or a sensor protocol designed for the requirement. “The PLC is fast” is not a timing analysis.

I/O list as contract

For every point record tag, device, location, type, electrical range, normal and safe/fault state, update requirement, scaling, filter, alarm, terminal, drawing reference, and test method. Distinguish command from proof: Conveyor_RunCmd is not Conveyor_RunningFb.

Architecture layers

Field devices sense and act. PLC and motion controllers execute deterministic control. HMI/SCADA supervises and records. MES coordinates operations. ERP plans business resources. ISA-95 gives a technology-neutral vocabulary for these boundaries 2. Do not let a slow business-system outage directly destabilize essential local control.

OT constraints

NIST notes that operational technology interacts with the physical world and has distinct safety, reliability, and performance requirements 3. Manage accounts, change control, remote access, backups, network segmentation, time, and asset inventory without treating a PLC as an ordinary office endpoint.

Architecture artifact

Draw the control architecture for a two-station cell. Add update times, ownership, network boundaries, local behavior during link loss, data retention, and which functions must remain available offline.

Source trail

References

  1. 1
    IEC 61131-3:2025 - Programmable Controllers, Part 3: Programming Languages. International Electrotechnical Commission. 2025. verifiedCurrent syntax and semantics for structured text, ladder diagram, function block diagram, and sequential function chart organization. Cited at: language suite.
  2. 2
    ISA-95 - Enterprise-Control System Integration. International Society of Automation. verifiedTechnology-neutral models for equipment hierarchy, manufacturing operations, and the interfaces among plant control and business systems. Cited at: levels and interfaces.
  3. 3
    NIST SP 800-82 Rev. 3 - Guide to Operational Technology Security. National Institute of Standards and Technology. 2023. verifiedSecurity guidance for OT systems, including PLC, DCS, SCADA, physical-process interactions, reliability, performance, and safety constraints. Cited at: OT scope.

Check your understanding

  1. What is the usual conceptual PLC scan order?
  2. Why document signal fail state?