An overview of


There’s a communication protocol that is very quietly gaining followers and market share. It’s called IO-Link and it’s designed to facilitate the connection of sensors and actuators at the lowest level of the automation hierarchy. That’s below DeviceNet, below Profibus, and just a step above direct field wiring to the automation controller.

Thirty years ago, sensors like Photoeyes, Prox switches and the rest were extremely simple. Today, like everything else, they have much more sophistication and flexibility. That means more complex configuration and more data available – diagnostics, alarms, and status. IO-Link is designed for these kinds of advanced sensors.

IO-Link is built with some pretty unique features. It uses standardized connectors and cabling, eliminating the troublesome, labor-intensive wiring of sensors and actuators. Devices are connected directly to the IO-Link Master device using point-to-point wiring, not bus-oriented wiring like a Profibus or DeviceNet device. Device replacement is one of its most impressive features. The IO-Link Master records the configuration of your devices and automatically re-parameterizes them when one is replaced.

Traditionally, most digital and analog sensors are connected to controllers using some sort of physical conditioning module that digitizes the discrete or analog signal and makes it available to a controller. Some of these conditioning modules plug into the controller directly, while others connect to some bus coupler that makes the digital data signal available over a DeviceNet, Profibus, EtherNet/IP or Profinet IO device.

You can find a lot of these kinds of modules. They come in every shape, size and configuration because there are an infinite variety of sensors. There’s modules for connecting analog sensors with various kinds of resolutions (8, 10, 16-bit), modules with single-ended inputs, modules with differential inputs, modules with RTD inputs, modules for current, and modules for voltage. Endless doesn’t describe the variety of modules available.

Connecting a sensor to these modules is not without a series of headaches. You’ll need the right number of modules with the right kind of inputs that match your sensor count. Often, you’ll need one more sensor than is available on the modules you have.You’ll have to make sure you have enough wire to connect each sensor to the bus coupler module, and you’ll have to make sure you don’t terminate the sensors to the I/O boards wrong. Of course, you’ll have to watch where you channel the wires. Lay your sensor wires near power or high voltage signals and you’ll risk intermittent data loss.

IO-Link eliminates these problems, especially the signal issues, with analog wiring. The Analog input to Digital conversion (or Digital to Analog Output) occurs right at the sensor, not at the bus coupler or controller. You no longer risk signal interruption or degradation because of your terminations, wire paths, or due to interference issues.

There are many advantages to IO-Link:

  • Fast installation using standard cables and M5, M8 or M12 connectors.
  • Standardized, uniform configuration no matter what kind of IO-Link sensor you are integrating.
  • Automatic device replacement when you replace a sensor with an identical model.
  • Standardized, uniform access to diagnostics, configuration and service data like device identification and serial number.
  • Support for analog inputs, analog outputs, digital inputs, digital outputs and serial devices on the same port of the IO-Link Master.
  • Stable specification, as IO-Link is an open, international standard (IEC 61131-9).
  • Built-in event handling with event transmission occurring independently from the cyclic transmission of normal process data.
  • Plug-n-play operation with the IO-Link Device Master automatically adjusting to the baud rate supported by the sensor.
  • Serial device connectivity through the same connector and cable.
  • Automatic cable break detection.

IO-Link supports three kinds of data exchanges. Process Data with 2 to 32 bytes of data and cycle times as low as 2ms. Service Data: a user can request like diagnostics and detailed device data (identity, serial numbers and more). Lastly, events, alarms or sensor information that needs to be reported immediately without waiting for a service request message.

The IO-Link Master Device

Many vendors offer IO-Link Master devices. These devices connect an IO-Link Sensor device to a controller. IO-Link Masters can be part of a controller or exist as Remote I/O. As Remote I/O, they are offered with various kinds of controller interfaces, including sensor buses like DeviceNet and Profibus DP, and Ethernet networks like EtherNet/IP and Profinet IO.

IO-Link Master devices provide some number of ports, each of which is connected to a single IO-Link sensor device. On a power cycle, the IO-Link Master device establishes communication by attempting connection on all three supported IO-Link baud rates (4.8K, 38.4K, 230.4K) and adapting to the baud rate that is successful.

IO-Link Masters have ports with 3 pins, 4 pins or 5 pins. Standard 3-pin ports are used for typical IO-Link sensor devices. 4-pin devices are used when the device requires power from the IO-Link Master device. Up to 200ma can be provided on that 4th pin. 5-pin ports are used for Actuators. The two extra pins are available for special functions as defined by the IO-Link Master device manufacturer.

Device Description File (IODD)

Each device comes with an IO-Link Device Description File (IODD). The Device Description File contains information about the device, including the ID and name of the manufacturer, the device serial number, the baud rate supported, and a description of the data provided by the sensor or actuator.

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Organization & Conformance Testing

IO-Link is promoted by the IO-Link Consortium ( and exists under the umbrella of Profinet International (PI), the same organization that promotes Profibus and Profinet. The IO-Link Consortium manages the IOL-Competence Centers (IOL-CCs), the centers of technical expertise where first time users can get information, training and support in IO-Link development.

Unlike some of the other protocols we’ve discussed in these pages, IO-Link does not require a certification test. IO-Link products are self-certified by the manufacturer. Automated test systems are available which validate the functions offered by IO-Link Masters and IO-Link devices. These test systems perform the test cases defined in the test specification in a highly automated fashion. Once the tests are completed, a vendor issues a manufacturer’s declaration of conformity to certify a product as conforming to the IO-Link specification.

If you’re looking at I/O in the future you should consider IO-Link.

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