Encyclopedia
OT components
What each device is, what it does, where it sits in the Purdue model, what it talks to, and what happens if it is compromised.
Level 1
Programmable Logic Controller (PLC)
A ruggedised industrial computer that executes deterministic control logic against field I/O.
Level 1
Programmable Automation Controller (PAC)
A controller with PLC determinism plus richer computing, networking and data handling capability.
Level 1
Remote Terminal Unit (RTU)
A field controller at a geographically remote site that collects telemetry and executes control commands from a master station.
Level 2
Human-Machine Interface (HMI)
The operator's window into the process — panel-mounted or PC-based graphics showing state and providing control actions.
Level 2 – Level 1
Distributed Control System (DCS)
An integrated control platform for continuous processes, combining controllers, operator stations, engineering tools and a proprietary control network.
Level 2 – Level 3
SCADA Server
The supervisory application that polls field devices, maintains the real-time database and drives operator displays and alarms.
Level 2 – Level 3
Engineering Workstation (EWS)
The computer holding engineering software and controller projects — the legitimate path to change control logic.
Level 3 (with Level 3.5 replica)
Process Historian
A time-series database storing process values, events and alarms for analysis, reporting and compliance.
Level 1 (safety)
Safety Instrumented System (SIS)
An independent protection layer that takes the process to a safe state when defined conditions are exceeded.
Level 1 (safety)
Safety PLC
A safety-rated controller executing certified safety functions, often alongside standard control tasks in machine environments.
Level 1
Intelligent Electronic Device (IED)
A microprocessor-based substation device performing protection, control, metering or monitoring functions.
Level 1
Protective Relay
A device that detects electrical faults and trips circuit breakers to protect equipment and people.
Level 1 – Level 0
Variable Frequency Drive (VFD)
A power electronics device controlling motor speed and torque.
Level 1 – Level 0
Motor Controller / MCC
Switchgear and intelligent starters that energise and protect motors.
Level 1
Robot Controller
The controller executing motion programs for an industrial robot.
Level 2 – Level 0
Industrial Ethernet Switch
A hardened managed or unmanaged switch providing plant-floor connectivity.
Level 3.5 / zone boundaries
Industrial Firewall
A firewall, often with industrial protocol awareness, enforcing conduit rules between zones.
Level 3.5
Remote Access Gateway
The controlled service through which remote engineers and vendors reach OT systems.
Level 2 – Level 3
Data Acquisition Server
A server that polls field devices and normalises data for supervisory and historian consumption.
Level 2 – Level 3
OPC Server
A translation service exposing device data through OPC interfaces to client applications.
Level 2 – Level 1
Serial Terminal Server
A device converting serial links to TCP so legacy equipment becomes network reachable.
Level 3.5 and above
IIoT Edge Gateway
A device collecting OT data and forwarding it to cloud or enterprise analytics, sometimes with edge compute.
Level 0
Sensor / Transmitter
A device measuring a physical quantity and reporting it to the control system.
Level 0
Actuator / Final Element
The device that acts on the process — valves, dampers, pumps, motors.
Level 3
Plant Asset Management Server
A system managing field-device configuration, calibration and diagnostics at scale.
Level 3
Domain Controller in OT
Directory services dedicated to the OT environment, providing authentication for OT hosts.
Level 3.5
Jump Server / Bastion
A hardened intermediate host that all interactive access into OT must traverse.
Level 3
OT Backup Server
The system holding recoverable copies of OT configurations, images, projects and logic.
Level 3 – Level 2
Time Server
The authoritative time source (often GPS-disciplined NTP/PTP) for OT systems.
Level 1
Generic Controller
Any device executing automated control logic where a more specific classification is not yet known.
Level 0
Field Device
Collective term for instrumentation and final elements installed on the process.