Empowering Smart Automation with IO-Link

Empowering Smart Automation with IO-Link

By Brian Condell & Michael Jackson

As industrial processes become more complex, field instruments like sensors and actuators have evolved to multitask, requiring regular communication with a process controller. This imposes additional overhead on the device microcontroller that must be carefully managed. Lost process data can lead to production downtime, the very thing that modern industrial communication protocols aim to reduce.

IO-Link Timing

IO-Link is a 24V, 3-wire industrial communications standard that enables point-to-point communication between industrial devices and an IO-Link master which, in turn, communicates with higher level process control networks (Figure 1). A transceiver acts as the physical layer interface between a microcontroller running a data-link layer protocol (stack) and the 24V IO-Link signal line.

 IO-Link master/device communication

Figure 1 IO-Link master/device communication

IO-Link communication involves several different data types including process data, value status, device data, and events. If an error occurs, this information can help identify, trace, and address the problem quickly, thereby helping to reduce downtime.

Communication between an IO-Link master port and a device is subject to several timing constraints and takes place in a fixed schedule called the M-sequence time. An M-sequence message includes a command or request sent from the IO-Link master to a device and the reply message from the device (Figure 2).

  M-sequence timing in IO-Link communication

Figure 2 M-sequence timing in IO-Link communication

The device must begin to respond to the master within the response time of the device, tA, which ranges from 1 TBIT to 10 TBIT (TBIT = Bit Times). For a COM3 baud rate, tA should be between 4.3µs and 43µs. If the device response is delayed for more than tA, data is dropped, and a communications failure occurs.

When Punctuality Slips

An IO-link device microcontroller must perform multiple tasks simultaneously. This can create difficulty in responding to a request before tA elapses, especially if performing a task that cannot be interrupted – known as a non-maskable interrupt (NMI). If the device does not respond within the specified time window, communication breaks down and must be reinitiated.

Continuously processing data samples leaves little time for a microcontroller to manage the IO-Link data link layer communication tasks. This can lead to considerable variations in device response time. In extreme cases, it may not even be possible to meet the timing requirement for tA.


Managing the Data Link

Timing issues caused by NMIs cannot always be addressed by simply using a faster microcontroller with more features. Using a second microcontroller to manage the IO-Link communication can help maintain a more constant response time interval between the device and the IO-Link master, but it is a highly inefficient approach. It uses more power, requiring a larger PCB and therefore a larger sensor enclosure.

A new alternative is to use a transceiver to manage both the data link and physical layers in the communication pathway. Figure 3 shows how the new MAX22516 reduces the need for a second microcontroller, because its integrated data-link controller manages the task of communicating with the IO-Link master.

 Using MAX22516 saves cost of additional microcontroller

Figure 3 Using MAX22516 saves cost of additional microcontroller

Improving TA with Integrated Functionality

The MAX22516 IO-Link data controller (Figure 4) a full-feature data-link controller to fully manage the timing of IO-Link data communication. It integrates all the functionality commonly found in IO-Link device transceivers, including the 24V C/Q, an integrated step-down DC-DC, and 5V and 3.3V linear regulators, with the added ability to manage IO-Link communication between a device and an IO-Link master.

 MAX22516 IO-Link Data Link Controller with Transceiver and Integrated DC-DC converter

Figure 4 MAX22516 IO-Link Data Link Controller with Transceiver and Integrated DC-DC converter

The MAX22516 autonomously handles communication with the IO-Link master for requests including configuration and maintenance requests. Offloading this task allows the microcontroller to increase its required response time from 43µs to a generous 5 seconds (assuming a COM3 data rate), an exponential improvement!

The MAXREFDES281 IO-Link device reference design (Figure 5) features the MAX22516 and can be used to verify the timing performance of the MAX22516 when used with different types of IO-Link sensors.

 MAXREFDES281 IO-Link device reference design

Figure 5 MAXREFDES281 IO-Link device reference design



The requirement for microcontrollers to manage multiple tasks simultaneously means they sometimes struggle to meet the timing specifications for IO-Link data communications. This means that some equipment manufacturers are left with the unpalatable alternative of using a second microcontroller to offload management of the IO-Link stack.

This two-microcontroller approach is no longer necessary because the MAX22516 IO-Link transceiver integrates a data link controller that can manage all IO-Link communication, freeing up the main device microcontroller to perform other time-critical tasks.