What is the difference between a Class A and a Class B IO-Link port?

The Class A IO-Link port is designed for low powered 24 VDC sensors requiring up to approximately 500 mA. The Class B port is for high powered actuators with current demands up to 2A.

This engineering brief is focused on the IO ports and the distinction between Class A and Class B. The example shown in Figure 1 features four Class A inputs ports and four Class B output ports. We will show that the class designation is concerned with power delivery to the field devices. We will also show that the design includes a measure of backwards compatibility allowing a sensor to be used in the traditional digital (on-off) configuration or the new serial protocol associated with IO-Link.

A brief introduction to IO-Link

The hub and spoke model represents the traditional method of configuring an industrial control system. The Programmable Logic Controller (PLC) serves as the center of the hub with many parallel wires radiating out to various field devices. The model was expanded with the introduction of Remote Input Output (IO). However, there were still many spokes, one for each sensor and actuator. This model has changed once again with the introduction of IO-Link. Devices such as the Weidmüller IO-Link master pictured in Figure 1 may be used to replace the traditional remote IO. The hub and spoke model is no longer strictly necessary as devices such as the IO-Link master in Figure 1 are designed to be daisy chained together.

Smart sensors are an essential component to the IO-Link system. These sensors contain additional intelligence, along with a serial interface. This allows the sensor to communicate more than the traditional binary signal. The sensor can also be configured via the interface. For example, a proximity sensor may be remotely configured to detect objects at a specific distance. The PLC associated with this sensor can detect and then program a direct replacement sensor with the same information. The combined result is a reliable system with simplified repair procedures with the potential to reduce costly down time.

Figure 1: Picture of a Weidmüller IO-Link master module featuring four class A inputs and four Class B outputs. The lower left of the image shows the pin assignments for the M12 A-coded connectors.

Backwards compatibility of the M12 A-coded connector

As we explore the IO-Link system, it’s good to look back and consider the conventional pin assignments for the 5-pin M12 connector. While there have been many unique configurations over the years several patterns have emerged:

  • Pin 1 is reserved for the 24 VDC supply
  • Pin 3 is reserved for the 24 VDC return
  • Pin 2 is generally used for a digital (PNP) signal traveling from sensor to controller
  • Pin 4 is generally used for a signal traveling from controller to actuator
  • Pin 5 may be used for additional IO or even ground (shield)
  • If a sensor has two outputs, pin 2 is for signal 1 and pin 4 is for signal 2
  • If an actuator has two inputs, pin 4 is for control signal 1 and pin 2 is control signal 2
  • If a device has both an input and an output, the sensor to controller signal is carried on pin 2 while pin 4 carries the controller to actuator signal.

Tech Tip: The IO-Link master featured in Figure 1 contains a variety of M12 connections. Close inspection shows that three different keyings are used to prevent inadvertently plugging a cable into the wrong port. For example, we would not want to plug a DC power source directly into a communications port. From experience, this releases the magic smoke.

By convention, the M12 port keying is known as a code. The Figure 1 IO-Link master includes eight black A-coded 5-pin female ports, two green D coded ports for communications, and 2 silver ports (one male and one female) for power. There is also a black plug covering a micro-USB port (not to be used by the customer).

From a DigiKey perspective, we often see the code reflected in the orientation parameter in the component search such as shown in Figure 2.

Figure 2: The M12 connector code is specified in DigiKey as the Orientation parameter.

An interesting case study is the Banner K50 illuminated touch button as described in this article. The K50 features a M12 connector with one binary output (pin 4) and two binary inputs (pins 2 and 5). Be sure to compare the K50’s IO assignments to the generalized list. Note that pin 2 may be programmed to serve as an output. When reprogrammed, the K50 is now a 2-output device as described in our guidelines.

Closely related to the Banner K50 shown in Figure 2 are a series of IO-Link specific devices. While this is beyond the scope of this article, we should mention the information included in the IO Device Description (IODD) file. The IODD is a structured file that provides a detailed and device specific description of the IO-Link interface. This may make a good future article. A tiny section of the structured IODD is included as Figure 4.

Figure 3: The Banner K50 as shown in this picture features one digital output (pin 4) and 2 digital inputs (pins 2 and 5). Note that pin 2 may be programmed as a second output.

Figure 4: A tiny portion of the IODD XML file for the Banner K50 IO-Link touch button.

Tech Tip: An IO-Link master such as the one shown in Figure 1 usually contain 5-pin M12 connectors for the field devices. This is convenient as the 5-pin connector is plug-in compatible with 3 and 4 pin cables. This can simplify and lower the cost for devices that do not require all 5 pins.

What are the Class A and Class B IO-Link pin assignments for the 5-pin M12 connectors?

Now that we have considered the backwards compatibility we can address the pin assignments for IO-Link and then answer our primary question about Class A and Class B ports.

A Class A IO-Link pin assignment is defined as:

  • Pin 1 is reserved for the 24 VDC supply
  • Pin 3 is reserved for the 24 VDC return
  • Pin 4 is used as bi-directional serial data communication between the IO-Link master and the field device
  • As a legacy option, Pin 2 may be used for a digital (PNP) signal traveling from sensor to controller

A Class B IO-Link pin assignment is defined as:

  • Pin 1 is reserved for the 24 VDC supply
  • Pin 3 is reserved for the 24 VDC return
  • Pin 4 is used as bi-directional serial data communication between the IO-Link master and the field device
  • Pin 2 is used to reinforce the +24 VDC supply in parallel with pin 1
  • Pin 5 is used to reinforce the +24 VDC return in parallel with pin 3

Returning to the Weidmüller IO-Link master in Figure 1, we note that the ports dedicated to sensors input are Class A while the high energy actuator ports are class B. The current supplied by a Class A port ranges from 200 mA to 500 mA, while the power available from a Class B port is approximately 2 A.

While it is beyond the scope of this article, we should mention that there are two operating modes for IO-Link devices. The first mode is the IO-Link which allows a single wire (pin 4) bidirectional serial interface between the IO-Link mater and the field device. The second operating mode is a Standard Input/Output (SIO) mode. This is like a fallback mode for the field device similar to the digital output used in the previous generation of digital field devices.

Tech Tip: Careful inspection of Figure 1 shows that the power connection to the IO-Link master is a stout 5-pole L-coded M12 connector. This connector is critical, as it must handle the full load current of the IO ports plus any daisy chained IO-Link nodes. With maximum of 2 A for each Class B port plus 500 mA for each Class A port, the calculated input current is slightly over 10 A with a small current to power the controller logic. This demands a 240 watt, 24 VDC supply plus a safety margin. It is also prudent to consider the power budget for the IO-Link system to ensure system reliability.

Parting thoughts

I’ll confess that IO-Link is a difficult system to learn as it encompasses so many different aspects of electronics. It’s a unique technology as we find ourselves talking about current limitations with hints of Ohm’s Law and power budgets in the same paragraph as communications protocols. This is all situated on top of a mountain of information about PLC and their associated field devices.

This article is focused on a small aspect of IO-Link emphasizing the Class A and Class B port designations. Personally, I found it useful to take a historical approach and examine the M12 wire assignments for conventional as well as IO-Link sensors. Oh, and let’s not forget, while IO-Link is gaining in popularity, it is unlikely to unseat those conventional sensors as the additional functionality is not required for all applications. Consequently, an understanding of the wire and color conventions can save heartache as we troubleshoot and maintain these systems.

Please leave your comments and questions in the space below. Also, you are encouraged to answer the questions and critical thinking questions that appear at the end of this note.

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

Leveraging his military engineering experience, Dahlen provides unique insights into rugged and reliable electronics solutions suited for extreme environments. His articles often reflect the practical, hands-on knowledge gained from his time in the U.S. Coast Guard. You can find over 75 of Dahlen’s industrial control and automation articles in this index.

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  1. What is the fundamental difference between the IO-Link Class A port and the Class B port.

  2. How is the electrical current capacity increased when we shift from Class A to Class B? Hint Two is better than one.

  3. Are the benefits of a Class B port available when a 4-pin cable is used? Hint: There are multiple answers depending on which pin is omitted. However, for consistency, assume a conventional 4-pin cable without the center pin.

  4. User standard condition for an IO-Link system, calculate the power deliver by a Class B port.

  5. A valve island is a collection of air or hydraulic solenoids on a common manifold. Suppose each Directional Control Valve (DCV) features a 3 W solenoid. In an ideal situation, how many solenoids could be controlled via a Class B IO-Link connection. In practice, how many would you recommend? Hint: What is an acceptable safety margin?

  6. Use the DigiKey search tools to locate a suitable L-coded connector to connect the Figure 1 Weidmüller IO-Link master to a DC power supply. For full credit, locate a molded connector with a length of at least 1 meter with flying leads.

  7. What type of data is transferred to a field device on pin 4 of the IO-Link port?

  8. Present a table showing the wire color convention for the 5-pin M12 connector.

  9. Research and then identify the limitations for cable length in the IO-Link system. For full credit, account for the Ethernet communication length, the length of power supply cables, and the IO-Link mater to field device length.

  10. The IO-Link devices are rated an Ingress Protection (IP-67) when properly sealed. Locate the DigiKey part number for the protective bort covers for the featured Weidmüller IO-Link master.

  11. Locate the datasheets for several IO-Link sensors including a proximity sensor, an infrared heat sensor, and an ultrasonic distance sensor. For each sensor:
    A) provide a brief description
    B) identify configurable parameters
    C) verify the pinout

Critical Thinking Questions

  1. Use the DigiKey search tools to locate a suitable power supply to match the Figure 1 Weidmüller IO-Link master. Fur full credit, state your assumed design requirements.

  2. Many IO-Link devices may be daisy chained together. For example, the featured Weidmüller device has dual ports for power and fieldbus. Provide guidelines for using the daisy chain connections?

  3. Does the port classification impact data rates?

  4. The IO-Link IEC 61131-9 standard has been described as a “last meter” solution. Describe the ways in which IO-Link is advantageous to the traditional PLC / remote IO systems.

  5. Fault management is an important consideration for 24 VDC industrial systems. A central tenant is that a short circuit be isolated to a particular branch so that the remainder of the system can operate or perform an orderly shutdown. How are faults handled in an IO-Links system? Hint: Start with an exploration of the Weidmüller datasheet.