IO-Link is the point-to-point digital communication interface between a master and a sensor or actuator, standardised as IEC 61131-9. It replaces the analogue or binary signal of the last stretch of cable with a three-wire serial link that carries configuration parameters, device identity and diagnostics on top of the process value.
Where IO-Link shows up on the plant floor
IO-Link lives in the last metre of the installation: pressure, temperature, level, distance and flow sensors, RFID readers, signal towers, grippers and vacuum cups, compressed-air flow meters. The master is typically an IP67-rated field block mounted on the machine itself and connected upwards over PROFINET or EtherNet/IP. Its adoption has gone hand in hand with packaging and food and beverage lines, where two arguments outweigh the rest: replacing a sensor without reparameterising it by hand, because the master stores its configuration and writes it back to the replacement, and getting early warning when a sensor fouls or drifts out of alignment, rather than discovering it through a quality reject.
IO-Link versus a fieldbus
IO-Link is not a fieldbus, and that is the most confused distinction. There is no shared topology, no node addresses and no multiple devices per line: each master port serves exactly one device, over standard unshielded three-wire sensor cable of up to 20 metres with the usual M12 connector. The bus sits above, between master and controller. Against CANopen or AS-Interface, which are genuine multi-drop buses, IO-Link makes a different offer: digitalise the last stretch without changing the electrical installation or the cable type. One detail that settles arguments on the floor: an IO-Link device wired to a conventional digital input works in standard mode as an ordinary switching sensor, though everything digital is lost in that mode.
Getting data out of an IO-Link installation
IO-Link handles two data planes, and understanding the difference decides everything else. The first is process data: cyclic, up to 32 bytes per direction, placed by the master into its fieldbus image and seen by the controller like any other input. The second is parameter data and events: acyclic, travelling on an on-demand service channel, and unless someone explicitly programmed it, never reaching the PLC at all.
The practical consequence is that connecting to the controller returns only what the machine builder chose to map, which is almost always the primary value and little else. Everything that justifies IO-Link in the first place (secondary measurements, operating hours, internal sensor temperature, contamination warnings, serial number and device reference) sits on the acyclic channel and needs another path.
That path exists and it is the interesting angle for a data layer: many masters expose their own network interface, usually an OPC-UA server or a web service returning JSON, independent of the fieldbus cycle. Reading there touches no PLC program, consumes no controller scan time and does not disturb communication with the machine controller. There is also an OPC-UA companion specification for IO-Link, published jointly by the IO-Link community and the OPC Foundation, standardising how master and devices are represented in the address space.
Semantics come from the IODD file each manufacturer publishes for its device: names, types, units, ranges and the meaning of every status bit. With it, an extraction layer labels and scales values instead of maintaining a byte map by hand, which is precisely the work that eats integration projects. On timing, each device declares its own minimum cycle and it depends on the communication rate: roughly 0.4 ms at COM3 (230.4 kbit/s), around 2.3 ms at COM2 (38.4 kbit/s) and up to some 18 ms at COM1 (4.8 kbit/s). The real limit for a data layer is not there but in the polling period of the master interface. And on stoppages: reading through the master interface needs none, whereas mapping new process data into the PLC image, or swapping an analogue sensor for an IO-Link one, does mean an intervention and a window at that station.
Related terms
IO-Link sits below the fieldbus, feeding masters connected over PROFINET or EtherNet/IP, and it is one of the most direct ways to give substance to industrial IoT without deploying a parallel sensing layer. Its data is usually published over OPC-UA towards the analytics layer. At Captia Connect it maps to sensor connectivity: integrating both new signals and those already present in plant instrumentation.