IO Link—Communication for sensors and actuators on the way to becoming a standard

01 October 2006

Minimising downtime and maintenance can be achieved through systematic diagnostics and centralised handling of parameter data. To accomplish this, automation engineers need powerful, intelligent sensors and actuators with comprehensive diagnostics and precise adjustment options.

Innovations in sensors such as the proximity switch have been impressive. The packages are smaller, yet have longer ranges. Microcontroller-driven ‘teach-in’ processes for precise sensor settings have replaced cumbersome and inaccurate potentiometer synchronisation. Diagnostics have expanded to include such things as early warnings against dirt contamination.

Yet, to be able to fully use these new developments in diagnostic and parameter information, they must be
made available to the automation system. Beyond the basic on/off switching signal, the interface between
the I/O module and the sensor should provide the possibility of exchanging diagnostics and parameter data. Only in this way can the capability of the sensors be optimally utilised.

Incorporating this ‘final metre’ into an integrated communication solution without losing the advantages of the economical point-to-point sensor connection is the technological leap that gives the user the opportunity to lower costs.

Well-known German manufacturers of sensors and automation technology have combined as an independent research group called the Research Group IO Link to develop the necessary specifications.
The group operates under the umbrella of the Profibus Nutzerorganisation (PNO, Profibus User Organisation).

IO Link
To retain the well known and accepted topology for the sensor/actuator connection and to ensure the
compatibility with today’s interface, IO Link communication had to be defined as an ‘add-on’ to the Binary Standard Interface. This familiar interface therefore receives, in effect, an additional communication function.

Through this expansion, the user can take advantage of automation solutions with plant-wide, integrated
communication up to the individual sensor or actuator.

In doing so, the wiring between the sensor or actuator and the I/O module is maintained as a point-to-point
connection (see illustration on page 50).

Since IO Link is a true expansion of the existing binary interface by a communication function, an easy and
continuous switch to the new technology, as IO Link components become available, is made possible: IO
Link sensors can be operated at IO Link modules as well as at today’s I/O modules. And the reverse is true: IO Link modules can use IO Link sensors as well as today’s standard sensors.

Furthermore, IO Link is a low-cost interface. The developers of IO Link paid special attention to the economical implementation of the interface in the sensors and actuators as well as the I/O modules.

In the long run, only open and standardised interfaces will be successful on the market. Hence, IO Link was
designed as an open interface with a freely available specification. IEC standardisation is in the works. IO Link can be integrated into all current fieldbus and automation systems.

High availability
The powerful IO Link interface results in new possibilities for the remote retrieval of diagnostics information, such as dirt contamination indicators on photoelectric sensors. It also makes possible in machines or plants the remote parameterisation of the sensor or actuator, allowing the exchange of parameters while the machine and control system are in operation.

With IO Link, the parameters of simple field devices no longer have to be entered manually or via a separate tool. Rather, all specified parameters are downloaded from a parameter record—which only has to be defined once—at the start of the system. This considerably reduces commissioning times.

This mechanism also shortens the changeover times, if the machine or plant has to be converted from product A to product B or sensors have to be replaced in case of repairs. A sensor can be exchanged for a new one and the parameters downloaded directly from the system into the new sensor, instead of having a technician enter the data by hand. In this way the new sensor ‘learns’ to do exactly what the previous sensor did.

Since sensor diagnostics are transparently mirrored into the respective network via IO Link, the system status as well as the status of the communication route can be displayed within the framework of the respective visualisation solution. Possible failure causes—such as increasing dirt contamination at light barriers—are thus detected early and can be corrected within the scope of the next preventive maintenance activity. This productive activity becomes positively noticeable in the increased availability of the plant. And the best thing of all is, for this additional information, no separate tool is required.

Freely available
Besides the description of the communication specification, which includes the transmission physics and
the protocol, the IO Link specification also contains a field bus-neutral data interface for process and required data as well as information on the system integration. In the first step, the mapping to the field bus systems Profibus DP, Interbus, and AS-I, as well as the mapping to the Ethernet protocol Profinet have already been developed.

Since IO Link is supported by prominent manufacturers of sensors and actuators and automation technology, the concept, thanks to the extensive availability of corresponding products, possesses all the necessary prerequisites to establish itself as the leading system on the market.

With it, the problem of ‘the final metre’ has been solved.

Key Technical Data of the IO Link Interface

Serial point-to-point connection
....3/2 wiring interface oriented at the IEC 60947-5-2
....Max cable length: 20 m, unshielded standard sensor cable
....Current consumption per sensor/actuator: 200 mA
....Communication takes place through the 24 V DC pulse modulation as serial UART protocol

Communication mode
....Process data (cyclical):
Typical: 2 Bytes input data and 2 Bytes output data
Max.: 32 Bytes input data and 32 Bytes output data
....Deterministic time response:
Typical: 2 ms cycle time with 16-bit resolution (process data)
....Required data (diagnostics, parameters) noninteracting to the switching signal
....Integrity level: 2

Alternative: Switching mode
....Switching signals in real-time
....Only parameterisation possible through communication

Dipl. Ing. (FH) Reinhard Schlagenhaufer is Head of Research and Development, Simatic Sensors at Siemens A&D in Amberg and Dipl. Ing. Christian Gemke is in product marketing with Phoenix Contact in Blomberg.

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