PLC Project Architecture: From Signals, HMI, and Diagnostics. A Practical Guide

Writing working PLC code is only half the job. True mastery begins where code becomes architecture: clear, scalable, easy to debug and maintain. Chaotically scattered variables, "magic" addresses, fragmented logic, and no connection to the HMI turn a project into a nightmare for whoever has to support it a year later — or even a month later.

This practical guide is dedicated to designing a PLC project as a system, not as a random collection of programs. It systematically covers all key aspects of industrial automation project architecture using Structured Text (ST) according to IEC 61131-3.

The book covers in detail:

Project architecture — hierarchy of Program Organization Units (POU), task partitioning, cyclic and interrupt-driven programs, global data organisation (VAR_GLOBAL), modular structure.

Signal organisation — naming standardisation (tags), structuring of input/output signals, signal types (discrete, analog, packetised), validation and scaling.

HMI/SCADA interface — dedicated interface data block, structured HMI variables, operator commands, status, mode, alarm and warning display.

Diagnostics — signal-level diagnostics (open circuit, short circuit, out of range), equipment diagnostics (module status, communication), program diagnostics (timeouts, freezes, error counters).

Alarm management — alarm philosophy, priorities, acknowledgement, alarm history, HMI integration.

Logging and trends — data collection, history storage, trend display, diagnostic logs.

Testing and commissioning — structure for FAT and SAT, simulation modes, debug modes.

The book is built on practical examples and includes ready-made architecture templates that can be adapted for specific projects. All examples are provided in Structured Text (ST) with detailed comments.

The manual is intended for automation engineers, PLC programmers, control system architects, students, and anyone who wants to create not just working code, but professionally organised projects that are easy to maintain, scale, and hand over to other specialists.