IO-Link Glossary: 25 Important Terms to Know

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According to the IO-Link website, IO-Link is the first standardized IO technology worldwide for communication with sensors and actuators.

If you’re new to IO-Link or are looking for a great refresher on the fundamentals, check out the guest post by Comtrol, the North American IO-Link Competency Center: IO-Link 101: 8 Common Questions Answered.

Below are 25 terms you need to know as you’re working with IO-Link. The terms are organized in alphabetical order:

1. Acyclic Data

Data that is transmitted from the PLC or controller by a request. Could also be considered as data not relative to the process data the sensor outputs to the PLC or controller (e.g., manufacturer name, model number, parameter values, etc.)

2. COM 1-3

IO-Link data transmission rates

3. Cyclic Data

Data that is regularly and automatically transmitted by the PLC or controller. Could also be considered as information in which the device is intended to deliver or measure (e.g., process data, value status, temperature, distance, weight, etc.)

4. Event Data

When specified critical events occur, “event flags” are transmitted to the PLC (Programmable Logical Controller) or controllers for maintenance (e.g., open circuits, communication aborts, overloads, etc.)


An Industrial Ethernet network that combines standard Ethernet technologies with CIP (Common Industrial Protocol) and is widely used in a range of industries, including factory and process.

6-8. FB, DB, and FC – Siemens-specific

FB stands for “Function Block” and requires a DB (“Data Block”) to work properly. An FB is able to save data over multiple PLC cycles in the corresponding Data Block. FC stands for “Function.” Results of this module are only accessible in the same cycle.

9. Gateway

See Master

10. GSD (Generic Station Description) – Siemens specific

The GSD is an interface file for PROFIBUS/PROFINET to communicate with a master PLC. Every device like a light grid or an IO-Link Hub, for example, should be integrated in a PROFIBUS/PROFINET network and needs a .GSD File.

11. IEC 61131-9

International standard that deals with the basics of programmable controllers. Part 9 describes IO-Link under the designation of Single-drop digital communication interface for small sensors and actuators (SDCI).

12. IODD (IO Device Description)

Each IO-Link device has an IODD (IO Device Description). This is a device description file which contains information about the identification, device parameter, process- and diagnostic- data, communication properties, and the structure of the user interface. This information can easily be read and processed by the user. Each device can unambiguously be identified via the IODD as well as via an internal device ID. An IODD consist of a main file (in .xml Format) and optional graphics (.png format).

13. IODD Interpreter

If an FDT container or Device DTM are being used, the IODD Interpreter translates the IODD and provides the information to the FDT Frame Application, so that the parameter of a Device DTM for example will be available.

14-15. ISDU (Index Service Data Unit) and SPDU (Service Protocol Data Unit)

Service data of the Sensor can be retrieved acyclically by the PLC to read out or to change parameters in the sensor. The process data, which is separate from the service data, is retrieved cyclically.

16. Master (also referred to as Gateway)

Represents the connection between a higher-level BUS network and IO-Link devices. Monitors and controls the IO-Link devices.

17. Parameter Assignment Server

An IO-Link master according to IO-Link Specification 1.1 can act as a parameter assignment server for the IO-Link device

18. Port

An IO-Link communication channel

19. Process Data

See Cyclic Data


PROFIBUS means Process Field Bus and is a standard for the Fieldbus-Communication in automation engineering.


PROFINET is a standard for industrial automation using a computer network. PROFINET uses standards such as TCP/IP and Ethernet. PROFINET’s modular structure allows users to select only needed functions for different requirements.

22. SDD (SOPAS Device Driver) – SICK specific

SDD stands for SOPAS Device Driver and is a software representative of the device. A SDD contains all the information about parameters, methods and events which supports the device. Additionally it also provides a Graphical User Interface (GUI) to make the handling of the information easier. These drivers can only be shown and executed in SOPAS.

23. Service Data

See Acyclic Data

24. SIO (Standard Input Output)

Traditional operating mode of basic sensors without IO-Link capabilities

25. SOPAS – SICK specific

SOPAS stands for “SICK Open Portal for Application Specific Software” and is primarily a configuration tool. Depending on which sensor is being used, the appropriate SDD needs to be installed and loaded into SOPAS. With this tool, you also have the ability to supervise and/or change parameters in a SICK sensor according to the requirements of the application.

Jill Oertel

Jill is a Product Manager covering electromagnetic sensors and connectivity. She had enjoyed working for SICK as an Applications Engineer for the past four years. She lives in the Twin Cities and spends her free time with her husband, 2-year-old son and new baby girl (very busy)!


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