Launched eleven years ago, IO-Link is now widely established in the industry. It is not surprising that sooner or later users will want to be able to make machines safe with the communicative interface. The IO-Link community has responded to this and launched a corresponding safety specification. At the same time, Balluff - one of the pioneers and the driving force behind IO-Link development from the very beginning - presented its own solution for safety-related communication based on IO-Link at SPS IPC Drives 2016 with 'Safety over IO-Link'.

With twelve safe inputs and two safe outputs in a robust metal design and pluggable throughout, this safe I/O module from Balluff can be used anywhere.

© Balluff

The safe I/O module is able to safely process all signals such as switching contacts or OSSD signals. It monitors the connected sensors, transmits their status to the higher-level Profisafe control system and can safely switch off actuators in the opposite direction. Another special feature is that the device can also accommodate simple standard sensors and actuators, making an additional standard sensor/actuator hub superfluous if necessary. Safety requirements up to PLe/SIL3 can be realized with this approach.

A characteristic feature of the safety solution is that the IO-Link master used is and remains a non-safety-related standard device. In other words, safety-related signals are tunneled untouched through the master across all intermediate levels to the control level and only processed there. The fact that an existing IO-Link master is used for this, which is not itself a safety device, proves to be a cost advantage.

As mentioned at the beginning, 'Safety over IO-Link' from Balluff is not the only IO-Link safety standard. The IO-Link community has also opened a new chapter: with the release and publication of the IO-Link safety specification by the IO-Link community and the successful concept assessment by TÜV Süd, nothing stands in the way of its implementation in systems and devices. However, according to Andreas Glasenapp, an operational device portfolio based on this standard will not be available until the end of 2018 at the earliest.

As is well known, Balluff is also a member of the IO-Link community and has been contributing to the creation of universal standards for years. A contradiction? "Not at all," emphasizes Matthias Bristle, Product Manager Safety at Balluff, and adds: "We see Safety over IO-Link as a supplement to the community solution, which is not yet ready for use."

Two safety approaches - what's the difference?

Technically, 'IO-Link Safety', which is not yet available on the market, is structured somewhat differently to Safety over IO-Link: It is not based on the tunneling principle and does not use existing IO-Link masters, but requires a new type of integrated IO-Link safety master. Depending on the configuration of the safety master, the solution can also be adapted to all higher-level fieldbus safety protocols. IO-Link Safety therefore essentially follows the IO-Link principle of fieldbus independence. In contrast, Safety over IO-Link is - at least at present - bound to the Profisafe/Profinet protocol and to controllers that cooperate with these standards.

As far as the configuration effort for IO-Link Safety is concerned, authentication is based on the assignment to the master port and the monitoring time for each device is set automatically. As with IO-Link, devices can be replaced without the use of an engineering tool. A replaced device automatically receives the saved parameters of its predecessor after restarting. Authentication also ensures that mix-ups and tampering are ruled out.

Matthias Bristle, Balluff: "We are convinced that the coexistence of both systems will initially be good for the further development of IO-Link."

© Balluff

The open and safe parameterization of safety-related components is a major challenge. IO-Link safety devices always have a device description - a so-called IODD - which contains the communication properties, identification, parameterization and diagnostics in full. However, the relevant standards require a 'Dedicated Safety Tool' to prevent manipulation. For this reason, there is a software interface for integrating the dedicated tools belonging to the devices into the IO-Link engineering tools. According to the IO-Link community, the 'Device Tool Interface' (DTI) is kept very simple and therefore ensures that integration into the existing IO-Link engineering tools is not a problem and that existing safety-related device software can also be easily adapted and reused on the device side. What is important here is that the package consisting of IO-Link safety device, IODD and dedicated tool can be used unchanged in all system environments worldwide.

The safety master, which is ultimately technologically more sophisticated and probably significantly more expensive than the safety hub from Balluff, therefore allows the implementation of a standardized, lean protocol instead of different tunnels. When questions about AS-i occasionally arise in this context, Andreas Glasenapp shakes his head: "This is comparing apples with oranges, because AS-i is a fieldbus, whereas IO-Link is a point-to-point connection. Both concepts have their place on the market and different advantages. Especially when it comes to data transparency, parameterization and diagnostics - which are ultimately essential prerequisites for use in Industry 4.0 applications - IO-Link offers many advantages."

The fact is that manufacturers can now start integrating IO-Link Safety into their systems on the basis of the recently adopted specification. At the same time, the community is working hand in hand to prepare the test specification and test systems as well as certification. However, there are currently no components for IO-Link Safety. However, if you want to add the safety factor to your existing IO-Link infrastructure today, Safety over IO-Link is an immediately available technology in a system. "Nobody can predict today whether the two systems will coexist or merge in the medium to long term," summarizes Matthias Bristle. Device manufacturers will, of course, be interested in both concepts being compatible with each other. According to Bristle, it remains to be seen in what form this will become reality. In any case, Balluff will support both concepts and thus ensure the greatest possible compatibility.

And the same applies to both concepts: What users have appreciated about IO-Link for years - namely simplified installation, more space in the control cabinet, universality, unhindered data exchange, simple and transparent parameterization and diagnostic concepts with a view down to the process level - will also apply to the safety-relevant level in the future. By merging standard automation and safety in a simple and transparent way or making them scalable and mixable as required, users save time, space and money because complex and parallel wired safety solutions are no longer necessary. All in all, the highly integrative system infrastructure provides a better overview and greater flexibility, as systems can be modified right up to the last minute if necessary - as is increasingly required in the context of Industry 4.0.

What is IO-Link?

IO-Link is a standardized IO technology (IEC 61131-9) for communicating with sensors and actuators.

The point-to-point communication is based on the familiar 3-wire sensor/actuator connection. The connection between the IO-Link master and device is made via a maximum 20 m unshielded cable. The pin assignments based on M5, M8 and M12 are standardized. The vast majority of IO-Link devices are equipped with an M12 plug connection, which can be used without restriction for switching and communicative IO-Link operation. Each port of an IO-Link master is able to process binary switching signals and analog values (e.g. 8 bit, 12 bit, 16 bit). Serial IO-Link communication takes place via the same port.

By default, 2 bytes of process data are available per cycle. 400 μs are required for transmission between the IO-Link master and device at a speed of 230 kbaud. The user can also select larger data packets (frame types) - with a correspondingly slower cycle time, process data with a length of up to 32 bytes can be transmitted.

Each IO-Link device has an IODD (IO Device Description). This is a device description file that contains the manufacturer, article number, functionality, etc. This information can be easily read by the user. This information can be easily read and processed by the user. Each device can be clearly identified both via the IODD and via an internal device ID.