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What Is IO-Link and How Does It Work?
IO-Link is a standard (IEC 61131-9) for digital, point-to-point communication between an IO-Link master and a single sensor or actuator, over the same unshielded 3-wire cable and M12 connector you already use for ordinary sensors. Instead of a single on/off signal, the device can send measured values, accept new settings and report its health.
What IO-Link is, and is not
- It is a link from one master port to one device, up to 20 m long.
- It is not a fieldbus. The master connects to your PLC over whatever network the PLC uses (EtherNet/IP, PROFINET, EtherCAT, Modbus TCP and others), so IO-Link works with any controller brand.
- It is backward compatible. An IO-Link sensor plugged into a normal PLC input behaves like a standard switching sensor (called SIO mode), and a standard sensor plugged into an IO-Link master port works as a normal digital input.
How it works
On an M12 A-coded connector, pin 1 carries +24 V, pin 3 is 0V, and pin 4 is the C/Q line, which carries IO-Link communication or a standard switching signal. The master and device exchange three kinds of data:
- Process data: the live value, sent cyclically (a distance, a temperature, the state of 16 inputs).
- Service data: parameters read or written on demand, such as switch points, filter time or NO/NC.
- Events: diagnostics such as "lens dirty," "over-temperature" or "short circuit."
Each device comes with an IODD file (IO Device Description) that tells the master and configuration software what data and parameters the device has. Communication runs at 4.8, 38.4 or 230.4 kbaud depending on the device.
Masters, hubs and devices
| Component | What it does | Example |
|---|---|---|
| IO-Link master | Multi-port gateway between IO-Link devices and the PLC network | ULK-EIP-4AP6: 4 Class A ports, EtherNet/IP |
| IO-Link hub | One IO-Link device that collects many standard digital or analog signals over a single cable | ULK-1616P-M2P6: 16 digital I/O hub |
| IO-Link sensor or actuator | Smart device with process data, parameters and diagnostics | Distance, pressure, temperature sensors, valve terminals |
Class A vs. Class B ports
- Class A: pins 1, 3 and 4 as above; pin 2 may be an extra digital input or output. Used for sensors.
- Class B: adds a second, galvanically isolated power supply on pins 2 and 5. Used for actuators and valve terminals that need separate, switchable power. Masters such as the ULK-EIP-4A4BP6 offer both types.
Why use IO-Link?
- Less wiring: a hub turns 8 or 16 sensors into one cable back to the master, and analog signals travel digitally with no scaling or noise problems.
- Faster replacement: the master can store a device's parameters and load them automatically into a replacement, so a technician just plugs it in.
- Diagnostics: you know a sensor is dirty, failing or disconnected before the machine stops.
- Remote setup: change switch points or ranges from the PLC or software without touching the sensor.
- Standard cables: ordinary unshielded M12 cordsets.
When is it worth it?
IO-Link pays off on machines with many sensors, analog measurements, frequent changeovers that need new sensor settings, or where downtime from a failed sensor is expensive. For a handful of simple on/off sensors on a small machine, standard PNP sensors wired to PLC inputs are still the cheapest answer.
Frequently asked questions
Does IO-Link need shielded cable?
No. Standard unshielded 3- to 5-wire sensor cable up to 20 m.
Can I use IO-Link with my existing PLC?
Yes. Choose a master that speaks your PLC's network protocol.
Is IO-Link wireless?
A wireless version exists, but most installations use the wired standard described here.
IO-Link from United Industries
We supply Unitronics IO-Link masters, digital and analog hubs, and accessories, including the ADP-ULKCFG USB configurator. Browse Unitronics, call 1-800-724-4209, or request a quote.