What are Field Components?
Field components are all physical components installed on or close to the production process, such as sensors, actuators, I/O modules, cabling and field enclosures, which together form the link between the real process and the control logic in a PLC or DCS. They are also called field equipment and form the lowest layer of an automation system; in the Purdue Model they sit largely at level 0. Where a field device is specifically the instrument that measures or acts on the process, the term field components also covers everything that carries that instrument’s signal to the control system. They communicate with the control system through hard wiring, through Remote IO or through fieldbuses.
🧠 What do field components do?
Field components have four basic tasks:
- Measure - sensors and transmitters convert physical process variables such as temperature, pressure, level, flow and position into an electrical signal.
- Act - actuators such as motors, control valves, solenoid valves and relays convert a control signal into movement, force or a switching action.
- Detect - proximity sensors, limit switches and photoelectric sensors report whether a product or machine part is present or has reached an end position.
- Transmit - I/O modules, fieldbus couplers and cabling carry signals to the control system and on to SCADA and the HMI.
Only together do these components form a control loop: the sensor measures, the controller calculates and the actuator corrects the process.
🧱 What types of field components are there?
| Type | Example component | Function |
|---|---|---|
| Sensor and Transmitter | Temperature sensor (PT100), pressure transmitter, level meter | Measure physical quantities and deliver a standard signal |
| Switching sensors | Limit switch, proximity sensor, photoelectric sensor, encoder | Detect presence, position or speed |
| Actuator | Electric motor, solenoid valve, control valve with positioner | Carry out actions: move, open, close, dose |
| Motor control | motor starter, variable frequency drive, soft starter, drive | Start, stop and control motors |
| I/O | Digital and analogue input cards, Remote IO, IO-Link master | Translate field signals into data for the PLC |
| Field infrastructure | Cables, junction boxes, marshalling cabinets, 24 V DC power supplies | Connect, distribute and power the field components |
| Ex isolation | Zener barrier, galvanic isolator | Limit the energy to equipment in an Ex zone |
| Signalling | Indicator lights, buzzers, local control stations | Visual or audible feedback for operators |
Limit switches and emergency stops are sometimes confused, but they have different roles: a limit switch reports a position to the control system, while an emergency stop is a safety command device that usually acts through a safety relay or safety PLC.
🔄 How does a signal travel from the field to the control system?
In a classic installation a measurement signal follows a fixed route: from the instrument through a field cable to a junction box in the plant, then through a multicore cable to a marshalling cabinet in the equipment room, possibly through an Ex isolator, and finally to an input card of the control system. A control signal travels the same way in the opposite direction. With Remote IO, fieldbuses and Ethernet-APL, the I/O moves into the field and a single network cable replaces dozens of cores.
| Signal type | Characteristic | Typical values | Use |
|---|---|---|---|
| Digital 24 V DC | On/off according to IEC 61131-2 | Type 1 high from 15 V, Type 3 from 11 V, about 2 mA | Switches, proximity sensors, valves |
| 4-20 mA | Analogue with a live zero | Valid 3.8-20.5 mA, fault at ≤ 3.6 mA or ≥ 21 mA (NAMUR NE 43) | Classic process instrumentation |
| HART | Digital over the 4-20 mA loop | 1,200 bit/s | Configuration and diagnostics alongside the measured value |
| PROFIBUS PA / Foundation Fieldbus H1 | Digital fieldbus | 31.25 kbit/s, several devices per cable | Process industry, including Ex zones |
| IO-Link | Digital point-to-point | Max. 20 m cable (IEC 61131-9) | The last metre to the sensor and actuator |
| Ethernet-APL | Ethernet over two wires | 10 Mbit/s, trunk 1,000 m, spur 200 m | New process plants; spurs can be intrinsically safe |
IEC 61131-2 defines three types of digital input: Type 1 for mechanical contacts, Type 2 for sensors with a higher current draw and Type 3 for modern low-power electronic sensors. Type 3 is the usual choice for new input cards, because it handles both mechanical contacts and most electronic sensors. With a two-wire proximity sensor, always check that its residual voltage and leakage current match the input type of the card, otherwise the PLC sees a signal that is not there.
🏭 Where are field components used?
- Machine building - drives, servo motors, position detection and safety switches
- Process industry - pressure, temperature, level and flow measurements in tanks and pipes, with control valves as actuators
- Logistics and warehousing - object detection, barcode readers and conveyor control
- Food industry - level and temperature monitoring with hygienic, washdown-rated designs
- Building management - climate control in building automation and access control
- Water and energy - pumping stations, flow meters and substations, often controlled through an RTU
✅ What requirements must field components meet?
Field components operate in a harsh environment and must work reliably for years. The main requirements:
- Enclosure - the IP Code according to IEC 60529 indicates the protection against dust and water. IP65 withstands water jets, IP67 temporary immersion.
- Explosion protection - in an Ex zone the ATEX Directive 2014/34/EU applies to equipment. The installation follows IEC 60079-14, whose sixth edition was published in August 2024 and now explicitly includes the initial inspection before use. Intrinsically safe circuits are separated with a zener barrier, which needs a dedicated intrinsically safe earth, or with a galvanic isolator that has no such earthing requirement. In new installations galvanic isolators have largely replaced zener barriers.
- Electrical installation - low-voltage installations in the Netherlands follow NEN 1010, machines follow IEC 60204-1, and work on the installation is governed by NEN 3140.
- EMC - analogue signals run through shielded twisted-pair cables, separated from the motor cables of variable frequency drives.
- Functional safety - sensors and valves in an SIS are part of a safety instrumented function with a SIL according to IEC 61511. They must be demonstrably suitable for it (SIL capability according to IEC 61508 or proven in use) and are proof tested periodically.
- Calibration - measuring instruments drift over time and are calibrated at fixed intervals.
🧭 How do you approach field components in a project?
- Start with the P&ID - every instrument gets a tag number, from which the instrument list and the I/O list follow.
- Choose signal types and standards - decide per application between hard wiring, IO-Link, fieldbus or Ethernet-APL, and limit the number of brands and protocols.
- Design the field infrastructure - junction boxes, cable routes, marshalling and the control cabinet, with spare capacity for future expansion.
- Install according to the standards - IP rating, Ex requirements and EMC measures; record the Ex documentation.
- Test in stages - a FAT at the supplier, then a loop check of every signal from instrument to screen and a SAT on site.
- Manage the lifecycle - register every component in the asset inventory and plan replacement in lifecycle management, because field components often stay in service for fifteen years or more.
🔐 Why are field components relevant to OT security?
Classic field signals such as 4-20 mA and PROFIBUS PA have no authentication. For a long time this was acceptable because the field was physically isolated. With smart instruments, IO-Link masters with a web interface and Ethernet-APL, however, the field gets IP addresses and therefore a network attack surface. IEC 62443-4-2 therefore sets requirements for components, with a separate category for embedded devices. New field components with a data connection fall under the Cyber Resilience Act: reporting of actively exploited vulnerabilities applies from 11 September 2026, the full requirements from 11 December 2027. Architectures such as NAMUR Open Architecture route diagnostic data from the field through a separate channel to condition monitoring, without touching the process control.
❓ Frequently asked questions
What is the difference between field components and field devices?
A field device is an instrument that itself measures or acts on the process, such as a transmitter, control valve or motor. Field components is the broader term: besides field devices it also covers I/O modules, cabling, junction boxes, Ex isolators and power supplies. Everything in the field that is needed to connect the process to the control system is a field component.
At which Purdue level are field components?
Sensors and actuators sit at level 0 of the Purdue Model, the level of the physical process. The PLCs and DCS controllers that read and control them sit at level 1, and I/O cards in the control cabinet are usually counted at that level too. Remote IO and smart field devices with an Ethernet connection make that boundary less sharp in practice.
Do field components always communicate via a fieldbus?
Field components by no means always communicate via a fieldbus. In many plants they are still hard-wired with 24 V DC signals or 4-20 mA loops to I/O cards in a control cabinet. Fieldbuses, IO-Link and Ethernet-APL are used increasingly, however, because they save cabling and also deliver diagnostic information.
What is a marshalling cabinet?
A marshalling cabinet is an enclosure where the field cables come in and are cross-wired on terminals to the cables that run to the I/O cards of the control system. This lets the wiring in the field be designed independently of the layout of the I/O cards. In Ex installations the galvanic isolators are often mounted there as well.
How long do field components last?
Field components often stay in service for fifteen years or more, far longer than IT equipment. Mechanical parts such as valves and limit switches wear sooner and need periodic maintenance, while measuring instruments are calibrated at fixed intervals. Record the lifecycle and the availability of spare parts for every field component.
Do field components need to be secured?
Field components need to be secured because a changed measuring range or a wrong valve position has a direct effect on the process and on safety. Classic field protocols have no authentication, so protection comes mainly from write protection, network segmentation and restricted access to engineering tools. New smart field components placed on the EU market from 11 December 2027 must also comply with the Cyber Resilience Act.
📌 In summary
Field components are the physical “eyes, ears and hands” of an automation system: the sensors, actuators, I/O and field infrastructure at level 0 that connect the real process to the control system. They work with signals ranging from 24 V DC and 4-20 mA to IO-Link, fieldbuses and Ethernet-APL, must withstand dust, moisture, vibration and sometimes an explosive atmosphere, and often stay in service for fifteen years or more. They provide the interaction between the digital control system and the real world of machines, products and processes. A good design starts with the P&ID and only ends with a tested and documented signal all the way to the operator’s screen.