Understanding the IO-Link IO Device Description (IODD) File

The IO Device Description (IODD) is a detailed .xml document that contains a complete description of an IO-Link field device. We can think of the IODD as a complement to the sensor’s physical datasheet. Also, the IODD is like a programmer guide. It provides a detailed description of the protocol required for device specific parameters and settings.

But it’s more than that.

The IODD is a highly structured document that may be machine read. In fact, the IODD is a critical software element required to link the device to the IO-Link master and to the larger network containing the Programmable Logic Controller (PLC). Stated another way, the IODD is the key to unlock the capabilities of an individual IO-Link field device. It identifies the parameters, data types, and boundaries. This information is an essential part of the IO-Link “plug and work” principle for field devices such as the Festo EGSS actuator shown in Figure 1.

DigiKey is a distributor of industrial automation and control components including many IO-Link products. DigiKey’s traditional tools such as the parametric metric search engine and the associated datasheets provide a great deal of information about these products. While this is a good start, you will find that a wealth of data is located in the IODD and nowhere else.

Figure 1: The Festo EGSS mini slide unit is a complex field device featuring an IO-Link interface.

Where can I find the IODD file for an IO-Link device?

The IODD may be found in several different locations including:

  • as a download from the device manufacturer
  • as a download from the centralized database provided by io-link.com

How can I view the IODD file?

Perhaps the best and most natural way to view the IODD is to use the configuration software associated with your PLC and greater industrial network. This is a natural method as the IODD is read by the configuration tools. We then program the greater industrial system to interface with the individual IO-Link device.

To simplify this article, we will sidestep the vendor specific software and use the IODDviewer provided by io-link.com. This is an excellent human readable way to view the IODD information as the raw .xml documents are difficult to manually decipher.

Locating a specific IODD

For the remainder of this article we will focus on the Fest EGSS mini slide unit as pictured in Figure 1. It’s a complex ball screw drive that provides a good case study for the IO-Link IODD material.
As a starting point, we can review the material on the DigiKey site including an abbreviated datasheet. We can then explore the associated Festo portal for a complete description of the device. This includes essential operating instructions that shows how to wire the device in an IO-Link configuration as shown in Figure 2. The IODD file is available on this page. It is also available from io_link.com.

Figure 2: Power and IO-Link connections for the Festo EGSS mini slide unit. Note the crossover for pins 3 and 4.

Exploring IODD content

A complete review of an IODD is beyond the scope of this introductory article. However, we can gain an understanding of the IODD by exploring Figures 3, through 5. These sections were chosen as they represent the types of things you would like to know or control in a motion control system.

Tech Tip: The IODD is presented in a highly structured .xml format. The raw form is difficult to read. The IODDviewer provided by io-link.com greatly simplifies the process by presenting the material in a human readable form.

Basic information

Figure 3 presents the top of the human readable IODD as read by the io-link.com IODD reader. Here we see basic information about the IO-Link field device including manufacturer and model number. The IO-Link version is an important metric as is the communications speed for the device. Not show are related fields to capture the characteristics of variant family members.

Figure 3: Initial information for an IODD as viewed by the IODD Viewer. This includes basic information for the field device such as vendor, make and model of the field device, and high-level communication settings.

Tech Tip: We should not expect to find serial number information in an IODD. Just like we should not expect to locate an individual serial number on a datasheet. Remember that we are looking at a general file describing the field device. On a related note, we can expect to see the serial number for a field device if we are using a software configuration tool attached to your industrial control network.

Serial number

The serial number for an individual field device is not contained within the IODD. However, the IODD tells us where to look for the serial number. As shown in Figure 4, we see that the serial number is 128 bits long (16 bytes).

The serial number is an important field from a maintenance perspective. It allows a properly configured system to detect that a field device has been replaced. The system may then automatically program the new IO-Link device. Ideally, this action will immediately restore the system to full operation with minimal interaction from the technician. Similar field can be also be used to verify that the correct sensor type has been installed.

Figure 4: The serial number is a 128-bit code held at index 21.

Representative data fields

Figure 5 shows some of the fields associated with the operation of the Festo motor drive. This feels like reading the datasheet for a Serial Peripheral Interface (SPI) device. A large part of the IODD is a mapping process. Where we identify a parameter of interest, declare a data type, and then describe the location by name or address.

In this example we see fields for motor position, speed, and force. These are essential elements required to control the motor drive. Many additional fields such as voltage, current, temperature, and even milage are contained within the IODD mapping. With a fast communication link and a fast PLC, we can monitor the IO-Link device in near real time. If the system is fast enough, we can even perform closed loop control.

Figure 5: Dynamic operating conditions such as position, speed, and force may be read from the Festo mini slide unit.

Parting thoughts

This article provides a quick review of the IODD that accompanies every IO-Link field device. We now know where to obtain these critical documents and have a better appreciation for how they are used within the IO-Link ecosystem. Specifically, we know that the IODD is a critical link in the chain of “plug and work” functionality. It serves as a memory map for the individual field devise effectively telling the greater system how to interface with the device. Finally, we explored the importance of the IODDviewer and how it converts the .xml document into a human readable document.
There is certainly more to learn about IO-Link as we have hardly scratched the surface. Still, I hope you found this information useful. Please leave your comments, questions, and concerns in the space below. We will do our very best to answer IO-Link or any related industrial control and automation questions.

Be sure to test your knowledge by answering the questions and critical thinking questions at the end of this post.

Best Wishes,


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About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 144 unique posts and provided an additional 473 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

Connect with Aaron Dahlen on LinkedIn.


The following questions will help reinforce the content of the article.

  1. Define field device.

  2. T/F: An IO-Link master is typically situated between a PLC and the IO-Link field device.

  3. What is the purpose of an IODD?

  4. Why are all IODDs strictly formatted with the same structure?

  5. What is the significance of human readable?

  6. What is the relationship between the IODD and the serial number for an individual IO-Link device?

  7. How many bits are used for an IO-Link serial number? How many unique devices are allowed using this numbering scheme?

  8. How could the serial number be used as part of corrective maintenance. Hint Automatic.

  9. Why does the EGSS mini slide require two power supplies? Identify the maximum electrical current required for each supply.

  10. Research the Festo EGSS mini slide unit. Locate the DigiKey part numbers for:
    A) Power cable with molded connector and flying leads
    B) Primary DC power supply
    C) Cable to cross the 8-pin IO-Link to the 5-pin M12 cable found on most IO-Link Master units.

Critical thinking questions

These critical thinking question expand the article’s content to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.

  1. The Festo mini slide unit offers a good case study from which to explore device documentation. Summarize the information that is available for each resource: DigiKey parameters, DigiKey linked datasheet, Festo EGSS page, and the IODD. For full credit describe the likely audience for each resource.

  2. Miles traveled is a field contained within the Festo mini slide. How can this be used as part of a preventative maintenance routine? Is this a reliable metric? Hint: Consider a heat map showing where the actuator spends most of its life.

  3. Computer networks may be formed using configurations such as ring, star, and mesh. How are IO-Link networks constructed. Hint 1: The answer depends on which side of the IO-Link master we on. Hint 2: Does the IODD say anything about addressing an IO-Link device?

  4. Latency is a perpetual problem associated with real time systems. Review the contents of a few IODD files and then provide at least three latency reducing recommendations.