IO Link: Sensor to Automation System Communication

Innovations in sensors such as proximity switches have been impressive. The packages are smaller, yet have longer ranges. Microcontroller-driven “teach-in” processes for precise sensor settings have replaced cumbersome and inaccurate potentiometer synchronization. Diagnostics have expanded to include such things as early warnings against dirt contamination.

By Michael Babb, Control Engineering Europe December 1, 2008

Sidebars: Key technical data of the IO Link interface World’s first plant with IO Link technology

Innovations in sensors such as proximity switches have been impressive. The packages are smaller, yet have longer ranges. Microcontroller-driven “teach-in” processes for precise sensor settings have replaced cumbersome and inaccurate potentiometer synchronization. Diagnostics have expanded to include such things as early warnings against dirt contamination.

Yet, to be able to fully use these new developments in diagnostic and parameter information, they must be made available to the automation system. Beyond the basic on/off switching signal, the interface between the I/O module and the sensor should provide the possibility of exchanging diagnostics and parameter data. Only in this way can the capability of the sensors be optimally utilized.

Until now, the only way to communicate with “intelligent” proximity sensors is with a separate PC directly connected to them, because this newfound intelligence stops at the interface. It goes no farther because there is no way for it to go. Modern automation systems do not have the benefit of standardized, bidirectional communication between the sensors and the control level.

As a result, the sensor’s parameters must be configured directly by hand, either before delivery or during the commissioning phase of a new machine when it is installed.

And that’s exactly where the backup of these sensor parameters stays: on the PC that was used to commission them. It is easy to see the problems that will arise from this. If a sensor (or actuator) needs replacing, woe be it to the technician if he can’t find a copy of the parameter file. He’ll have to enter them all over again in the new devices.

What about sensor diagnostics? Few technicians want to leave their laptops connected to the sensors during operation, so continuous diagnostics are usually not available. While some sensors do provide a diagnostic output signal, this requires a separate I/O line. Most end users understandably don’t want to double the amount of I/O wiring they have on their machines, and without this, no meaningful preventive maintenance on the sensors can be performed.

The packaging line at Dunni is the first implementation of IO Link technology. See box on p. 40. Source: Dunni

Bi-directional communication

To bring intelligent sensors into the automation system, the Profibus organization initiated a new technology called IO Link. Out of necessity, IO Link communication was defined as an “add-on” to the standard binary sensor interface that has been in place for many years. With IO Link, the standard sensor interface receives, in effect, an additional bi-directional communication function that allows users to send parameter and “teach” signals to the sensor, and receive diagnostic information back from the sensor.

This is a big change in the way sensors are handled. In the past, users had to connect their PCs (or some dedicated tool) directly to the sensors to exchange these values. Now they can put their PCs away and communicate directly with the sensors through the automation system.

IO Link allows the user to take advantage of automation solutions with plant-wide, integrated communication up to the individual sensor or actuator. And yet the wiring between the sensor or actuator and the I/O module is maintained as it was before: as a point-to-point connection. It does not involve a new bus system. The sensor power supply is provided through the cable as before. A simple telegram superimposed on the power supply voltage permits bidirectional data transfer between the sensor and the control system.

The IO Link sensors can either be directly connected to a PLC system that has an IO Link card, or connected with a fieldbus system through an IO Link fieldbus module.

IO Link is a low-cost communications mechanism. The developers paid special attention to the economical implementation of the interface in the sensors and actuators as well as the I/O modules. The first components were demonstrated at Hannover Fair in 2006, and today many European manufacturers have their first IO Link sensor and connection modules on the market.


In the installation phase, the work of the engineer is made easier since only one cable must be laid—a standardised unshielded 3-wire cable between the control level and the sensor or actuator. This makes it possible to connect the “old” sensors and the “new” IO Link sensors on the same standardized 3-wire cable. Thus it is possible to have a simple binary switching device and a multi-level signal light on the same cable. The compatibility with existing sensor interfaces still exists, which allows the mixture of IO Link sensors with conventional sensors. They can then be operated on an IO Link master with full IO Link scope of functions, or on a standard assembly as standard digital I/O devices.

The wiring of analog signals also becomes easier since they are no longer connected with a shielded cable, but with the same type of 3-wire cable. This saves warehousing costs for different cable types, and reduces wiring expenses. It also significantly lowers the risk of errors during installation. Since the IO Link interface is derived from the standardized interface for proximity switches (IEC 60947-5-2), no additional tool is required for installation.

Engineering, commissioning

In the past, intelligent sensors and actuators had to be configured and parameterized locally via the manufacturer’s software with a PC. This can now be done for IO Link devices from the PC workstation or control room using a central engineering tool, such as Siemens’ STEP 7 software. This ensures the reproducibility of sensor and actuator parameters since they are stored centrally within a project in the same data format.

If sensor data are not in the central engineering tool, it is possible to import them from the GSD or IODD file provided by every manufacturer. The user then has the option to perform the parameterization with the central engineering tool. Using STEP 7 as an example, this would be done via the port configurator or with the TCI (tool calling interface) with the help of the sensor manufacturer tool. When the parameterization is called via the TCI, the data can be saved in the manufacturer’s format.

IO Link also achieves significant improvements during the commissioning phase. The engineer only has to check the connection of the IO Link during the start phase, since wiring faults can nearly be ruled out due to the standardized 3-wire line. In addition, he or she can test the accessibility of the IO Link device using the engineering tool. If several devices within a machine require the same configuration, it can be duplicated and sent to additional devices.

Commissioning engineers have a high degree of flexibility for device parameterization. They can edit and “teach” the devices directly on site. Later, they can determine which setting data they want to read into the central engineering tool, and save it in the tool.

Diagnostic transparency

To continue increasing the efficiency of machines, it is now possible to perform the configuration of IO Link sensors during runtime. This allows the commissioning engineer to adjust optimizations of systems while they are running—a significant improvement. And, due to direct data access via fieldbus, this also shortens the configuration process since parameters do not have to be adjusted directly at the device. While the operation is running, the user receives all the diagnostic information through the completely transparent system, starting with the control level and going down to the sensor and actuator level.

For example, if a machine fails, it is now possible to recognize at the control level precisely which sensor caused the machine standstill. This is helpful to maintenance engineers, because they can bring along the correct spare part when they come to fix the machine. The newly installed sensor automatically receives its parameter data from the control system. Automatic parameterization and advance information about the defective sensor significantly reduce the down times of a machine.

Author Information

Michael Babb is editor of Control Engineering Europe. This article was compiled from information from PI (Profibus and Profinet International) and the kind assistance of Dipl. Ing. (FH) Reinhard Schlagenhaufer, head of R&D, Simatic sensors at Siemens in Amberg, Germany, and Dipl. Ing. Christian Gemke, Phoenix Contact in Blomberg, Germany.

Key technical data of the IO Link interface

Serial point-to-point connection

3/2 wiring interface oriented at the IEC 60947-5-2

Max cable length: 20 m, unshielded standard sensor cable

Current consumption per sensor/actuator: 200 mA

Communication takes place through the 24 V DC pulse modulation as serial UART protocol

Communication mode

Process data (cyclical):Typical: 2 Bytes input data and 2 Bytes output dataMax.: 32 Bytes input data and 32 Bytes output data

Deterministic time response:Typical: 2 ms cycle time with 16-Bit resolution (process data)

Required data (diagnostics, parameters) noninteracting to the switching signal

Integrity level: 2

Alternative: Switching mode

Switching signals in real-time

Only parameterization possible through communication

World’s first plant with IO Link technology

Dunni GmbH in Bramsche, Germany, is the world’s first plant with IO Link technology. The company, a producer of table decorations, installed it on its packaging line. The system was commissioned by the packaging systems integrator firm meurer Verpackungssysteme.

On the new line, a dozen Sick photoelectric proximity switches communicate with the controller in a Profibus environment. The in-house designed touchscreen interface displays the sensor signals and, according to the operators, the HMI—in combination with the IO Link communication technology—has simplified machine operation to a large extent, providing greater flexibility, operational reliability and availability for the end-user.

Parameters such as the sensor’s scanning distance can be called up from the automation system during operation, transferred to the sensors and, if necessary, displayed on the panel.

During plant operation, there are two areas where IO Link sensor communication becomes important, say meurer engineers: First, when the system is transitioning between different packaging orders and has to make adjustments on the fly; second, for the use of preventive maintenance and the correction of faults.

With the automated system, it is no longer necessary to carry out a mechanical readjustment of the sensors every time the order changes, and their settings are entirely reproducible from order to order. Secondly, if there is a problem, the machine operator can immediately diagnose it by touching the sensor image on the panel and viewing the diagnostic information (which is displayed as the sensor signal quality in figures and as a bar graph), as well as the status of the switching output, the level of contamination, the display of any short circuit, and information on any misalignment or interference from other sensors.