IO-Link

IO-Link is a standardized, digital point-to-point communication system for connecting sensors and actuators to automation systems. It sits below PROFINET, PROFIBUS, or Modbus: the IO-Link segment connects field devices to an IO-Link master, which then forwards the data via the higher-level fieldbus or industrial Ethernet network to the PLC.

IO-Link is not a fieldbus at the device level – it is a point-to-point connection between a master port and a device, using standard sensor cables, typically three-wire.

How IO-Link Is Structured

An IO-Link system consists of three elements. The IO-Link device is the sensor or actuator – for example a proximity switch, pressure sensor, valve manifold, RFID reader, or photoelectric sensor. The IO-Link master is the hub with multiple ports, each port a dedicated connection to one device. The PLC or SCADA system receives data from the master via PROFINET, EtherNet/IP, or another higher-level network.

Device descriptions are provided through an IODD file, which informs engineering tools and systems about the parameters and data points a device offers.

What Data IO-Link Delivers

IO-Link goes far beyond a simple switching signal. Cyclically, it transmits process data such as measured values, states, and switching points. Acyclically, it transmits parameter data such as threshold values, filters, device settings, and recipe parameters. In addition, it delivers diagnostic events – contamination, short circuit, temperature warnings, undervoltage – as well as device identification including serial numbers.

This combination of process value, diagnostics, and parameterization is the real advantage over conventional sensor technology.

Why IO-Link Is Used

IO-Link bridges the gap from simple binary sensing to parameterizable, diagnostics-capable peripherals – without complex fieldbus infrastructure at the device level. Faster commissioning through automatic parameterization instead of manual adjustment, fewer unplanned downtimes through early diagnostic events instead of sudden failures, stress-free device replacement because parameters can be reloaded from the master, and a richer data foundation for condition monitoring and quality assurance.

Typical Use Cases on the Shop Floor

Condition monitoring is the most common application: wear and contamination indicators from sensors make maintenance plannable rather than reactive. For rapid format changes, recipe parameters can be automatically pushed to devices instead of set manually. For process monitoring, pressure, temperature, and flow are delivered digitally and with less noise than analog signals. And RFID readers over IO-Link enable identification and status capture as a building block for traceability concepts.

IO-Link and MES: The Realistic Data Path

MES systems almost never communicate with IO-Link directly. The typical path is: IO-Link device → IO-Link master → PLC/SCADA/edge → OT-IT interface → MES or BI. Value is only created when diagnostic events and device states are modeled and evaluated in the MES – for example as downtime cause codes or early indicators for preventive maintenance. Using IO-Link only for process values while ignoring diagnostics wastes the greatest lever for OEE improvement.

Common Mistakes

Using only the process value while ignoring diagnostics eliminates IO-Link's main benefit. No standardization of tags and units makes later analytics costly. Missing parameter management means format changes and device replacements become manual again. And unclear expectations around update rates lead to incorrect planning of cyclic versus acyclic data.

FAQ

What is the difference between IO-Link and PROFINET? PROFINET is the higher-level industrial Ethernet network for communication between PLCs, drives, and field devices at the network level. IO-Link is the point-to-point connection at the lowest level – between a master port and an individual sensor or actuator. Both complement each other: IO-Link delivers the device data, PROFINET transports it further.

Is IO-Link suitable for brownfield systems? Yes. IO-Link uses standard sensor cables and can therefore be retrofitted into existing systems without replacing complete wiring. IO-Link-capable devices are often pin-compatible with existing digital sensors.

How are IO-Link diagnostic data made usable in a MES? Diagnostic data flows through PLC and edge into the OT/IT integration layer. In the MES, it is modeled as events – for example as a downtime cause, maintenance trigger, or quality event. The prerequisite is that signal meaning (e.g., "sensor contaminated") is preserved when crossing from OT to IT.

What is an IODD file? IODD stands for IO Device Description. It is a standardized XML file that describes which parameters, data points, and diagnostic information an IO-Link device provides. Engineering tools and integration software read the IODD to automatically configure and evaluate devices.