IO‚ÄêLink Sensor Communications Standard Growing

  • March 04, 2013
  • Feature

March 2013

By Bill Lydon, Editor

IO-Link is gaining momentum as the first open standard for smart sensor I/O. IO-Link is a point to point network communications standard and protocol that is an IEC standard (IEC 61131-9) for sensors and actuators. IO-Link enables smart sensors to communicate information including status, events, and configuration parameters.

IO-Link allows easy replacement of end devices. The master downloads parameters into the replacement device which simplifies maintenance tasks since no additional configuration is required. Available IO-Link sensors include binary sensors, measurement sensors, photoelectric sensors, RFID readers, laser distance sensors, and color sensors. IO-Link is managed by an open consortium of manufacturers. The organization has a convenient PDF document that list vendors offerings.

The IO-Link physical communications is a three conductor wire connection and standard connectors such as M12, M8, and M5 are supported. An IO‚ÄêLink system consists of IO‚ÄêLink devices including sensors and actuators and a master device. Since IO-Link is a point to point architecture only one device can be connected to each port on the master.

An IO-Link master can take many forms including a card in a PLC rack or a standalone device that interfaces to standard fieldbus communications. For example, there are IO-Link Proxies available for PROFIBUS and PROFINET. The PI North America organization allocates time during their free PROFINET trainings in the U.S. and Canada to describe IO-Link. The ODVA has formed a new Special Interest Group SIG (SIG) for a Single-drop Digital Communication Interface (SDCI) for IEC 61131-9; frequently referred to as “IO-Link.”


IO-Link is a simple serial point to point communication between two devices typically at transmission rates of 4.8 and 38.4 k baud. Examples of IO-Link smart devices include I/O blocks, valve bank connectors, measurement sensors, color sensors, and RFID systems. At first, connection devices are always in the SIO mode (standard I/O mode) and the master acts on this port like a normal digital input/output. If the port is set to communication mode, the master tries to find the connected IO‚ÄêLink device to initiate a wake‚Äêup. During wake‚Äêup, the master sends a defined signal and waits for the response from the device. The master tries to do this with the highest defined baud rate. If this is unsuccessful, the master tries the next lower baud rate. With every baud rate, the master tries to address the device three times. If the master receives a response (i.e. the device was woken up) both will start communication.

The first communication with a device is an exchange of communication parameters and subsequent communication are exchanges of cyclic process data. If the device is removed during operation, the master detects the communication abort and reports it. The master communicates cyclically to wake‚Äêup the device when it returns to service. After a successful wake‚Äêup the communication parameters will be again validated and cyclic data exchange will be started. If the master terminates communication, both the master and the device return to the original mode - the SIO mode also known as fall back.

The IO-Link protocol supports various types of data - cyclic data (process data), acyclic data, service data, and events. The IO‚ÄêLink device sends data only after a request from the IO‚ÄêLink master. Acyclic data and events are requested explicitly by the master and cyclic data is sent after the IDLE telegram of the master.

IO‚ÄêLink Device Description (IODD)

The IO‚ÄêLink Device Description (IODD) is an electronic data definition that is one or several XML files describing the device, manufacturer‚Äêspecific properties, and image files in the "png" format. The IO-Link organization states this information can be easily read and processed by the user since it is in XML.

Quality of Service

If communications fails, the master’s request is repeated twice. Only when the second attempt to send data fails again, will the master detect a communication abort and report it to the higher‚Äêlevel control system. The master measures the quality of the transmission (QoS = Quality of Service) with the number of telegram retries.


The IO-Link master provides a gateway function to exchange data between an IO‚ÄêLink device and a PLC by mapping. If the IO‚ÄêLink master is directly connected with a PLC over a proprietary backplane bus, the IO‚ÄêLink data is mapped to this bus and transmitted to the PLC or from the PLC to the IO‚ÄêLink master. IO‚ÄêLink maps have already been specified for PROFIBUS, PROFINET, INTERBUS, AS‚Äêi and EtherCAT.

IO Link Overview

A detailed IO-Link overview is available at:

Thoughts & Observations

The IO-Link organization describes their goal as “bridging the last meter” to intelligent sensors and actuators. The fact that it is point to point means the wiring is the same as hardwired basic sensors to controllers.

Using a multi-drop bus network rather than point to point would save wire, commissioning time, and installation labor. One problem with IO-Link has been that the configuration of these networks is complex since they have not supported or fully embraced device self-discovery, so are dependent on configuration files.

IO-Link is another step towards the refinement of smarter end device interfaces.

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