How IO-Link connects one smart sensor per port and carries its readings, identity, and diagnostics digitally.
IO-Link is a point-to-point connection, standardized as IEC 61131-9. Each sensor or actuator (the device) has its own cable to its own port on an IO-Link master. There is no bus, no address to set, and no termination: one port, one device.
The cable is an ordinary unshielded three- or five-core sensor cable, up to 20 m long. The same pin that carries the digital conversation can also act as a plain 24 V switching signal, so an IO-Link sensor still works as a simple switch on an ordinary input.
Two modes. A port runs in one of two modes:
| Speed | Rate |
|---|---|
| COM1 | 4.8 kbaud |
| COM2 | 38.4 kbaud |
| COM3 | 230.4 kbaud |
A device uses one fixed speed. A master built to version 1.1 of the specification supports all three, so it can talk to any device.
What travels on the link. Three kinds of data share the connection:
How a master identifies a device. When communication starts, the master reads the device’s Vendor ID and Device ID. Together they identify the exact product. A master can be set to accept only an expected device on a port and reject anything else, which catches a wrong sensor plugged into a port.
IODD files. Each device model has an IODD (IO Device Description): an XML file from the manufacturer that describes its process data layout, parameters, units, and events. A master or configuration tool uses it to turn raw bytes into named, scaled values. IODDs are published in the IO-Link Community’s IODDfinder.
Data storage. Under IO-Link version 1.1, the master can keep a copy of a device’s parameters. When a failed sensor is swapped for an identical one, the master writes the saved parameters into the replacement automatically.
IO-Link uses an M12 A-coded connector. Pin 1 (L+) supplies 24 V to the sensor, pin 3 is the return, pin 4 (C/Q) carries IO-Link communication, and pin 2 (I/Q) is commonly a spare digital input.
| Pin | Name | Class A port | Class B port |
|---|---|---|---|
| 1 | L+ | 24 V supply | 24 V supply |
| 2 | I/Q or 2L+ | Digital input or output, as the master’s manufacturer defines | Second 24 V supply, for actuators |
| 3 | L− | 0 V | 0 V |
| 4 | C/Q | IO-Link communication, or SIO switching signal | IO-Link communication, or SIO switching signal |
| 5 | 2M | Not used, or manufacturer-defined | 0 V of the second supply |
Do not plug a device that uses pin 2 or pin 5 into a port of the other class without checking both datasheets: on a Class B port pins 2 and 5 carry a supply, which a Class A device may treat as a signal.
The sensor sends its reading as a digital value with known units, so there is no analog span to set and nothing to drift.
The value arrives exactly as the sensor measured it, in the units its IODD describes. Cable length and electrical noise do not shift it: a message either arrives intact, or it is detected as bad and discarded. The sensor also reports its own faults as events, so a controller can tell “the level is low” from “the sensor has failed”.