What is IO-Link?
IO-Link is an open, internationally standardised point-to-point interface (IEC 61131-9) through which sensors and actuators communicate digitally with an IO-Link master over an ordinary, unshielded 3- or 5-wire cable, carrying process data, parameters and diagnostics. It is the first globally standardised IO technology for the last metre to the sensor and actuator. IO-Link does not replace a fieldbus but works underneath it: the master passes the data to a PLC via ProfiNET, IP, EtherCAT or Modbus TCP. A simple switch or transmitter thus becomes a smart device that identifies itself, can be configured remotely and reports its own health.
🧠 What problem does IO-Link solve?
A classic proximity sensor delivers a single bit: on or off. An analogue transmitter delivers a single value, for instance over 4-20 mA. Everything else the device knows, such as its temperature, a dirty lens, operating hours or an internal fault, remains invisible. Settings are made with push buttons or a potentiometer on site, and when a device is replaced a technician has to enter every setting again.
IO-Link addresses this with three principles:
- Digital over the existing connection - the same M12, M8 or M5 connector and the same 24 V signal levels from IEC 61131-2, but now with data communication.
- One device per port - each master port serves exactly one device, so there are no bus addresses, terminating resistors or bus load calculations.
- Standardised description - every device has an IODD file, so every engineering tool presents it in the same way.
The result is fewer variants in stock, faster commissioning and diagnostics down to sensor level. According to PROFIBUS & PROFINET International (PI), the umbrella of the IO-Link Community, 9.7 million IO-Link nodes were added in 2024, bringing the total to around 61 million.
🔧 How does IO-Link work technically?
An IO-Link system consists of a master (often with 4 or 8 ports, as an IP67 module in the field or as a card in the control cabinet), devices (sensors, actuators, valve terminals, hubs) and an engineering tool that reads the IODD.
| Characteristic | Value |
|---|---|
| Standard | IEC 61131-9 (SDCI, first edition 2013) |
| Topology | Point-to-point, one device per master port |
| Cable | Unshielded, 3- or 5-wire, maximum 20 m, ≥ 0.34 mm² |
| Connectors | M5, M8 or M12 (A-coded) |
| Power | 24 V DC via pins 1 and 3, at least 200 mA |
| Communication | Pin 4 (C/Q), 24 V level |
| Speeds | COM1 4.8 kbaud, COM2 38.4 kbaud, COM3 230.4 kbaud |
| Process data | 0 to 32 bytes per direction per cycle |
| Cycle time | Minimum around 0.4 ms at COM3; stored in the IODD |
| Error handling | A frame is retried twice before the master reports a failure |
A port has four modes: IO-Link, DI (digital input), DQ (digital output) and deactivated. An IO-Link device starts in SIO mode (standard I/O) and then behaves like an ordinary switch. Only after a wake-up pulse from the master does it switch to digital communication. This is why an IO-Link sensor also works on a standard input card.
For ports, the specification distinguishes Port Class A, where pins 2 and 5 are defined by the manufacturer (usually an extra switching channel), and Port Class B, where pins 2 and 5 provide an additional, galvanically isolated power supply for devices with higher consumption, such as valve terminals and small drives.
📦 What data does IO-Link exchange?
IO-Link has four types of data:
- Process data - cyclic, for example a distance, a pressure or a switching state.
- Value status (PortQualifier) - cyclic, indicates whether the process data is valid.
- Device data - acyclic on request: parameters, identification (manufacturer, type, serial number) and diagnostics.
- Events - acyclic from the device: warnings and fault messages, such as a short circuit or overheating.
The IODD (IO Device Description) is a zip archive containing a main XML file, language files, images and the manufacturer’s declaration. That declaration has been mandatory for devices placed on the market since 1 July 2011. The IODDfinder of the IO-Link Community holds more than 11,000 device descriptions. Profiles such as the Smart Sensor Profile (version 1.2 from January 2024) and the Common Profile ensure that comparable sensors from different brands use the same data structure.
An important feature since specification V1.1 is data storage: the master keeps a copy of the parameters. With the Backup/Restore setting, the master automatically writes all settings back to the new device after a replacement. This works only when both master and device support V1.1; a V1.0 device does run on a V1.1 master, but without this backup.
🔄 How does IO-Link compare with fieldbuses and other technologies?
| Technology | Topology | Typical role | Difference from IO-Link |
|---|---|---|---|
| IO-Link | Point-to-point, max. 20 m | Last metre to sensor and actuator | Reference |
| AS-Interface | Bus, two-wire flat cable | Many simple binary signals | Addressed bus, little data per slave |
| HART | Digital over 4-20 mA | Process instrumentation | Analogue signal remains primary, low data rate |
| ProfiNET, IP, EtherCAT | Ethernet network | Communication between PLC and masters | Carries IO-Link data to the controller |
| Single Pair Ethernet | Ethernet over one wire pair | Ethernet into the field device | IP down to the sensor, but costlier per node |
| IO-Link Wireless | Radio, point-to-point | Moving parts, tool changers | Same data model, wireless physical layer |
For the upward connection there are specifications for PROFINET, EtherNet/IP, EtherCAT, AS-Interface, CANopen and SERCOS, among others. For the IT side there is the OPC UA for IO-Link Companion Specification (2018) and the JSON Integration for IO-Link, which lets a master offer data via a REST API or MQTT. Sensor data can then go straight to an edge device, MES or cloud platform, in parallel with control. This makes IO-Link a building block of Industry 4.0 and Smart Industry.
🛡️ What are IO-Link Safety and IO-Link Wireless?
IO-Link Safety adds a safety layer to master and device (FS-Master and FS-Device) based on the black channel principle: the ordinary IO-Link channel transports the safety data, and the safety layer detects errors. Implementations can reach SIL 3 according to IEC 61508 and comply with ISO 13849. The technology is standardised as IEC 61139-2. A port on an FS-Master can also run in OSSDe mode, so you can connect existing safety devices with classic OSSD outputs, such as a light curtain, to the same master. Towards the controller the safety data usually travels via PROFIsafe or another safety protocol.
IO-Link Wireless replaces only the physical layer with radio in the 2.4 GHz band, based on IEEE 802.15.1, using frequency hopping and blacklisting of disturbed channels. Towards the controller, the wireless master behaves like an ordinary master.
🔐 How do you secure an IO-Link installation?
The IO-Link protocol itself has no authentication or encryption. On a point-to-point link of at most 20 metres that is acceptable, because an attacker needs physical access to the cable. The real risk lies in the master: it has an Ethernet port, often a web interface and sometimes an IoT port with REST or MQTT. Anyone who reaches the master can change sensor parameters, such as a switching distance or a measuring range. Vendors have released security updates in the past for vulnerabilities in the web interfaces of IO-Link masters.
Practical measures:
- Segment - place masters in their own zone according to IEC 62443 and network segmentation; the IoT port does not belong on the office network.
- Restrict the IoT port - disable REST, MQTT and the web interface if you do not use them, or set them to read-only.
- Keep an inventory - record masters and their firmware versions in your asset inventory.
- Protect parameters - use the device’s parameter lock where possible and keep IODD and parameter files under version control.
- Mind the regulations - the Cyber Resilience Act sets requirements for products with digital elements, with reporting obligations from 11 September 2026 and full application from 11 December 2027. The Machinery Regulation applies from 20 January 2027 and requires protection against corruption of safety functions.
🏭 How do you approach an IO-Link project?
- Choose the master - fieldbus or protocol to the PLC, number of ports, Port Class A or B, IP rating and whether you need an IoT port.
- Select devices with an IODD - check in the IODDfinder whether the IODD is available and which profile the device supports.
- Plan the cabling - at most 20 m per port; use 5-wire cable for Port Class B. A hub can bundle classic binary signals onto one port.
- Configure in the engineering tool - import the IODD, set parameters and cycle time, and set data storage to Backup/Restore.
- Use the diagnostics - link events and sensor values to condition monitoring, predictive maintenance or your CMMS.
- Document - record types, parameters and firmware versions for asset management and replacement.
An example from the packaging industry: on a format change, the PLC sends new switching points to dozens of sensors via IO-Link, instead of a technician adjusting them one by one. A dirty optical sensor also reports itself before it fails. Other typical applications are grippers and valve terminals on assembly lines, smart pressure and temperature sensors in the process industry and sensors on conveyors in logistics systems.
❓ Frequently asked questions
Is IO-Link a fieldbus?
No, IO-Link is not a fieldbus but a point-to-point interface between one master port and one device. The IO-Link master itself is a participant on a fieldbus or industrial Ethernet network, such as PROFINET or EtherNet/IP. IO-Link therefore complements fieldbuses for the last metre to the sensor.
What cable does IO-Link need?
IO-Link uses a standard unshielded 3- or 5-wire sensor cable with M5, M8 or M12 connectors. The maximum cable length between master and device is 20 metres. Port Class B with its additional power supply requires a 5-wire cable.
What is an IODD?
An IODD (IO Device Description) is the standardised description file of an IO-Link device, a zip archive with an XML file, language files and images. It describes identification, parameters, process data and diagnostics. Engineering tools read the IODD, so every IO-Link device can be configured in the same way.
Can an IO-Link sensor work on a standard digital input?
Yes, an IO-Link sensor starts in SIO mode and then behaves like an ordinary switching sensor. On a standard digital input card it therefore works as a classic switch. Only on an IO-Link master does it switch to digital communication with parameters and diagnostics.
How fast is IO-Link?
IO-Link has three speeds: 4.8, 38.4 and 230.4 kbaud. Each cycle carries at most 32 bytes of process data per direction, with a minimum cycle time of around 0.4 ms at 230.4 kbaud. That makes IO-Link fast enough for most sensors and actuators, but it is not intended for high-speed motion control.
Is IO-Link secure against cyberattacks?
The IO-Link protocol has no authentication or encryption of its own, but it is limited to a cable of at most 20 metres. The biggest risk lies on the Ethernet side of the IO-Link master, such as its web interface and IoT port. Segmentation, disabling unused services and up-to-date firmware reduce that risk.
📌 In summary
IO-Link is the standardised digital point-to-point connection (IEC 61131-9) that makes ordinary sensors and actuators smart: process data, parameters and diagnostics over a standard cable of up to 20 metres. With the IODD, automatic parameter backup and links to PROFINET, OPC UA and MQTT, IO-Link connects the field level with OT and IT systems; secure the master above all, because that is where the link to the network lies.