IO-Link Smart Sensors

IO-Link is a point-to-point communication standard (IEC 61131-9) that runs over the same three-wire unshielded cable already used for standard sensors. The cable carries 24 V power, the conventional SIO (standard IO) bit, and the IO-Link serial channel on a single conductor — no extra wiring, no fieldbus topology, no star cabling required.

The IO-Link master is a module that sits on a fieldbus (PROFINET, EtherNet/IP, EtherCAT) and provides four to eight ports, each of which connects to one IO-Link device. Every port is independently configurable: you can run port 1 as a conventional digital input while ports 2 and 3 run full IO-Link at COM3 speed.

Three baud rates are defined: COM1 at 4.8 kbps for simple devices, COM2 at 38.4 kbps for most sensors, and COM3 at 230.4 kbps for high-speed process data. A modern inductive or photoelectric sensor typically negotiates COM2 automatically during the port activation handshake.

The data channel is split into three parts. Process data carries the primary measurement — the bit or word value the PLC reads every scan. Service data is on-demand read/write access to device parameters such as switching point, output logic, and filter time. The device IODD (IO Device Description) file describes every readable and writable parameter in a machine-readable XML format that engineering tools use to build configuration UIs automatically.

Diagnostics are what make IO-Link compelling for maintenance teams. A device can push an event asynchronously — lens dirty, target too close, supply voltage low — without waiting for the PLC to poll it. The master forwards the event to the controller within one IO-Link cycle. This turns a silent trip into an actionable alarm with a plain-English description.