PLC, HMI, SCADA, DCS: who does what in the automation chain.

Industrial automation is organized in layers, like a pyramid: at the bottom the field (sensors, actuators, motors), then the PLC that controls them, above it HMI and SCADA for visualization and supervision, higher up the MES that organizes production, and at the top the ERP that runs the business. Each layer talks to the one above and the one below — and the floors are almost never skipped.

The rule that brings order to any quote: each layer has its own time horizon. When you evaluate a system, ask which horizon it acts on — it's the fastest way to tell whether you're being offered the right tool on the right floor.

The PLC (Programmable Logic Controller) is the industrial computer that executes the machine's logic: it reads sensors, decides, and commands actuators — open the valve, stop the conveyor, start the motor. It does this in cycles of a few milliseconds, thousands of times a minute, for years, in environments where an ordinary PC wouldn't survive. If the PLC says the press is stopped, the press is stopped. Typical brands: Siemens S7-1500, Rockwell/Allen-Bradley, Beckhoff, Omron, Schneider.

Two things every buyer should know. First: the PLC also handles functional safety — the logic that protects people and machines (guards, e-stops, zones) under standards such as ISO 13849, on dedicated safety hardware or CPUs. Second: a PLC lives 15-20 years, far longer than any software on the upper floors. That's why integration matters so much: the systems you buy over the next few years will have to talk to the PLCs you already own.

The HMI (Human-Machine Interface) is what the operator sees and touches. Typical examples of a human-machine interface: the touch screen on the press showing temperature and the running recipe, the packaging-line panel with start/stop and piece counters, the machining-center screen with alarm diagnostics. The horizon is the single machine or station, here and now: status, parameters, commands.

The real choice today is between a dedicated panel and a web-based HMI. The traditional panel — a physical terminal with proprietary software, engineered with tools like Siemens WinCC — is rugged but rigid: every change goes through a technician, every workstation is a license and a piece of hardware to maintain. A web-based HMI runs in a browser on any screen — industrial panel, shop-floor tablet, office PC — and is updated in one place. It's the approach behind ARIA HMI, and the direction the market is moving in.

SCADA (Supervisory Control And Data Acquisition) moves up one floor: it doesn't watch one machine, it watches the line or the whole plant. It collects data from all the PLCs, lays it out on plant-wide synoptics, centralizes alarms and — the most important difference from an HMI — keeps history: trends, patterns, shift-to-shift comparison, reconstructing what happened at 3 a.m. The HMI shows one machine's now; SCADA keeps the plant's memory. Names you'll come across: WinCC, Ignition, Movicon, Zenon, and the new generation of web-based SCADA such as ARIA SCADA.

SCADA supervises and commands the process, but it doesn't organize production: work orders, lot traceability, OEE, and scheduling are the MES's job, the floor above. The boundary between the two deserves a guide of its own — you'll find it at the bottom of this page. For now, the rule is enough: SCADA answers «what is the plant doing», the MES answers «what should production do».

The DCS (Distributed Control System) was born for continuous-process plants — chemicals, petrochemicals, energy, paper, water treatment — where the plant never stops and every regulation loop (a flow loop, a reactor temperature) must be controlled continuously and redundantly. The key word is «distributed»: control doesn't sit in one PLC per machine, but in controllers distributed along the plant, engineered, supplied, and supervised as a single integrated system.

The difference from SCADA is architectural, not cosmetic. A SCADA setup is composable: you pick the PLCs, you pick the supervision layer, you make them talk — flexible, suited to manufacturing where machines come from different builders. A DCS is integrated by design: control and supervision are born together, from the same vendor (Siemens PCS 7, ABB, Emerson, Honeywell), with redundancy and process engineering built in. In discrete manufacturing a DCS is almost always oversized; in a chemical plant a composable SCADA is almost always undersized.

At the field level, between PLCs, sensors, and drives, the fieldbuses do the talking: Profinet and Profibus in the Siemens world, EtherNet/IP in the Rockwell world, Modbus a bit everywhere, EtherCAT where tight synchronization is needed. These are deterministic networks: the packet arrives when it must, every time. It's the realm of guaranteed timing, and you don't touch it without someone who knows it.

Toward supervision and IT, the standard that changed the rules is OPC UA: a common language through which SCADA, MES, and business systems read data from PLCs of different brands, with standard security and a standard data model. Alongside it live the native protocols — S7 for Siemens — and MQTT where data has to travel light toward cloud and analytics. This is where an integrator's quality shows: making a heterogeneous machine park talk — new and legacy — without rewriting the PLCs.

When an automation project disappoints, it's rarely because a product «doesn't work». Almost always, one layer was asked to do another layer's job. These are the mistakes we run into most often in the projects we inherit — all avoidable with the map from the previous chapters.