industrial//instrumentation//IO-Link
IO-Link is a point-to-point digital link between one sensor or actuator and a port of an IO-Link master, standardized as IEC 61131-9, and it is used to bring process values, parameters and diagnostics from simple field devices into a control system over the same three-wire cable a plain switching sensor uses. It is the last metre of industrial communication, below any fieldbus: the master gathers eight or so devices and hands their data upward over PROFINET, EtherNet/IP or another network.
IO-Link is a point-to-point digital link between one sensor or actuator and a port of an IO-Link master, standardized as IEC 61131-9, and it is used to bring process values, parameters and diagnostics from simple field devices into a control system over the same three-wire cable a plain switching sensor uses. It is the last metre of industrial communication, below any fieldbus: the master gathers eight or so devices and hands their data upward over PROFINET, EtherNet/IP or another network.
A photoelectric sensor on a conveyor shows the difference. Wired as a classic switching input, it gives the PLC one bit: object present or not. Wired to an IO-Link port, the same sensor also reports its received light level (a dirty lens shows up as a falling margin weeks before it starts missing boxes), its operating hours and its temperature, and the PLC can rewrite its switching threshold when the product changes, with no technician at the machine.
IO-Link moves the meaning into the device without moving the wiring.
The value arrives with status and diagnostics, as from a smart sensor, yet the cable is a standard unshielded three- to five-wire lead of at most 20 m.
Each port works in one of two modes. In SIO mode (standard input-output) it is an ordinary digital input or output, so a conventional sensor still works on an IO-Link master; after a wake-up from the master, the device switches to IO-Link communication.
Three speeds exist: COM1 at 4.8 kbaud, COM2 at 38.4 kbaud and COM3 at 230.4 kbaud, with cycle times of a few milliseconds. That suits presence, pressure or temperature; it is no motion network.
Every device has a description file, the IODD (an XML file identified by vendor and device ID), which tells the engineering tool its parameters and data layout. It plays the role a register map plays for Modbus, except that it is machine-readable and standardized.
A replaced device can be configured automatically: the master stores the parameters and writes them into the new unit, so swapping a failed sensor at night takes no laptop.
Against HART: both add digital data to a simple field connection, but HART keeps the analog 4-20 mA value and overlays slow data on it, typical of process instruments, while IO-Link is fully digital and typical of discrete automation and machine building.