What is a PLC? — A Field Engineer's Guide
A Programmable Logic Controller (PLC) is a ruggedised industrial computer built to run deterministic control logic on plant — boilers, pumps, motors, valves, robots, conveyors. In a building context, a PLC sits behind a BMS where the control task is too fast, too critical or too high-IO-density for a DDC controller.
PLC vs DDC vs SCADA
A DDC (Direct Digital Controller) like a Trend IQ4 is optimised for HVAC sequence-of-operation — slow loops, BACnet, hundreds of points. A PLC is optimised for high-speed deterministic logic — fast loops, EtherNet/IP or Profinet, thousands of points. SCADA (Supervisory Control and Data Acquisition) is the visualisation, alarming and data-historian layer that sits above PLCs.
In a hospital you'll typically see a BMS DDC running AHUs and a PLC running the medical-gas alarm system — different criticality, different tools.
The Major PLC Manufacturers
Siemens (TIA Portal — S7-1200, S7-1500), Rockwell / Allen-Bradley (Studio 5000 — CompactLogix, ControlLogix, MicroLogix, SLC-500), Mitsubishi Electric (GX Works — FX, Q-Series, iQ-R), Schneider Electric (EcoStruxure Control Expert — Modicon M340, M580), ABB (Automation Builder — AC500), Omron, Beckhoff (TwinCAT) and B&R (Automation Studio).
IEC 61131-3 Programming Languages
Five standardised languages: Ladder Diagram (LD) for discrete logic; Structured Text (ST) for maths and loops; Function Block Diagram (FBD) for reusable libraries; Sequential Function Chart (SFC) for state machines; Instruction List (IL) for legacy code. Modern projects typically use LD + ST + FBD.
Safety PLCs
Safety-rated PLCs (Siemens Safety Advanced F-CPU, Rockwell GuardLogix, Pilz PNOZmulti) are designed for safety functions — emergency stop, light curtain, two-hand control. They carry SIL or PL certification and are validated to BS EN ISO 13849-1 / IEC 62061.
External References & Standards
FAQs
Is a PLC the same as a BMS controller?
No. A PLC is built for deterministic high-speed industrial logic; a BMS controller (DDC) is built for HVAC sequence-of-operation. They overlap, but a BMS controller is not safety-rated and a PLC doesn't natively speak BACnet.
Which PLC is most common in buildings?
Siemens TIA Portal is dominant in M&E plant (chillers, generators, BMS-adjacent skids). Allen-Bradley dominates manufacturing and pharma; Mitsubishi is common in lifts and packaged plant.
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