IO-Link: smart sensors on a point-to-point link
IO-Link (IEC 61131-9) connects one sensor or actuator to a master port over a standard 3-wire cable: process data, parameters, IODD files and device replacement.
Idea
IO-Link is not a fieldbus but a point-to-point serial link between an IO-Link master port and one device (sensor, actuator, valve terminal, small I/O hub). The master sits on PROFINET, EtherNet/IP, EtherCAT or a PLC backplane and passes data upward.
Physical layer
- Standard unshielded 3- or 5-wire sensor cable, M12/M8 connectors, up to 20 m.
- Pin 4 (C/Q) carries IO-Link communication or works as a normal switching signal (SIO mode) — an IO-Link sensor can replace a plain one.
- Port class A: pin 2 free or extra DI/DO; class B: additional galvanically separated actuator supply on pins 2/5.
- Transmission rates: COM1 4.8, COM2 38.4, COM3 230.4 kbit/s — chosen by the device.
Data
| Type | Use |
|---|---|
| Process data | Cyclic values (e.g. distance + switching state), a few bytes |
| Service data (ISDU) | Parameters addressed by index/sub-index, read/write on demand |
| Events | Diagnostics such as overtemperature, short circuit |
Every device is described by an IODD (IO Device Description, XML) with its parameters, process data layout and texts.
Why engineers like it
- Parameters (switch points, filters, units) are set from the PLC or a tool instead of buttons on the sensor.
- Data storage: the master keeps a copy of the parameters; a replaced device of the same type receives them automatically.
- Diagnostics reach the PLC instead of only an LED.
Typical issues
Wrong process data mapping (byte order inside the IO-Link process data), using a class A port where class B power is needed, and validation settings (vendor/device ID) that reject a replacement device of another revision.
Connections
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Sources
- IO-Link — technology — IO-Link Community
Updated 2026-10-02