OT Technologies & Field Devices

A PLC (programmable logic controller) is a solid-state, reprogrammable device that executes control logic against local inputs and outputs, typically within a single machine or process area, on a fixed scan cycle — read inputs, execute logic, write outputs, repeat, as covered in Lesson 2. PLCs are the workhorse of discrete manufacturing and are increasingly common in process industries too, usually speaking Modbus TCP, EtherNet/IP, or PROFINET to whatever else shares their network segment.

A RTU (remote terminal unit) is built for a different problem: monitoring and controlling equipment at a site that's geographically remote from the control center — a pipeline valve station, a water tower, a pump station forty miles from the nearest office. RTUs typically run on DNP3 or Modbus, often over cellular, licensed radio, or leased telephone circuits, and report back to a SCADA master at intervals rather than continuously, exactly as Lesson 1 described.

An IED (intelligent electronic device) is most often found in electrical power systems — a protective relay or smart meter monitoring one specific piece of equipment, such as a feeder breaker or a transformer. What sets an IED apart is speed and independence: when it detects a fault condition (an overcurrent, a short), it can trip a breaker within a few electrical cycles — well under a second — without waiting for a command from anywhere else. IEDs typically speak IEC 61850 or DNP3 on a substation LAN.

A sensor measures some property of the physical world — temperature, pressure, level, flow, position — and converts it into a signal a controller can read. Many legacy sensors use a 4–20 milliamp analog current loop, where 4 mA represents the low end of the measurement range and 20 mA represents the high end; a controller's analog input card reads that current and converts it back into an engineering value. Newer sensors may report digitally over protocols like HART or directly over an industrial Ethernet protocol, but the underlying job is identical: turn a physical quantity into something a PLC, RTU, or IED can act on.

An actuator is the mirror image: it converts a controller's electrical output signal back into physical action — opening or closing a valve, starting or stopping a motor, moving a damper. Where a sensor is an input device, an actuator is an output device, and the two together form the two ends of every control loop: measure something, decide something, do something.

A HMI (human-machine interface) is the screen and software an operator uses to watch a process in real time and issue manual commands — open this valve, acknowledge this alarm, start this pump. Despite running specialized SCADA or HMI software, an HMI is almost always, underneath, a full general-purpose operating system — usually some version of Windows — which means it carries that operating system's entire attack surface in addition to whatever the HMI software itself exposes.

An engineering workstation is a different tool for a different job: a computer, often running vendor-specific programming software, used by engineers and technicians to write, upload, and modify the control logic running on PLCs, RTUs, and IEDs. Where an HMI lets an operator watch and nudge a running process, an engineering workstation can rewrite what the process does — which makes it one of the most sensitive machines in the entire environment, because it typically has direct, trusted write-access to every controller it manages.

A historian is a time-series database that continuously logs process values — temperatures, flows, levels, alarms — for trending, troubleshooting, and regulatory reporting. A historian doesn't control anything; its job is memory, not action. But because historians are useful to people well beyond the control room — engineers building reports, managers reviewing production numbers — they're also one of the more common bridges between the OT network and business-facing systems.

A variable frequency drive (VFD), often just called a drive, is a device that controls a motor's speed by varying the frequency and voltage of the power supplied to it, rather than running the motor at one fixed speed. Drives are a common type of actuator in their own right — a PLC sends a speed command, and the drive translates that into the electrical waveform an induction motor actually needs to spin at that speed. When a drive fails, the motor it controls typically either stops (a safe default in most designs) or, in rarer and more concerning failure modes, runs at an uncommanded speed.

A safety instrumented system (SIS) is fundamentally different from every device covered so far, and Lesson 2 already told you why: it sits above the CIA triad and outside the basic process control system entirely. A SIS has its own sensors, its own logic solver, and its own final elements (like a dedicated shutdown valve), and it's designed and certified so that its ability to detect a hazardous condition and force the process into a safe state never depends on whether the PLC, HMI, or any network between them is working correctly.

Putting the loop together

Every device covered in this lesson fits into one picture: field devices sense and act, controllers decide, and supervisory systems let people watch, program, and remember — with the safety instrumented system running alongside as a deliberately separate, independent path.

Select a device below to see its role, its typical network exposure, and — using the Simulate a Failure button — what happens if that specific device stops working correctly. Nothing here touches a real device, network, or protocol; it's a model built to compare consequences across device types.

Five questions covering the lesson's key distinctions. Answer each, then select Grade My Quiz to see your score and an explanation for every item.

• Sensors Reference (course site) — a companion reference page with additional detail on common OT sensor types and signal formats.
• Actuators Reference (course site) — a companion reference page covering common OT actuator types, from solenoid valves to variable frequency drives.