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IO-Link brings greater intelligence to sensor level

IO-Link brings greater intelligence to sensor level

Jacqui Hanbury of Festo presents an introduction to IO-Link - a control network that doesn't replace existing fieldbus or industrial Ethernet networks, but sits below them at the sensor level.


Whilst industrial Ethernet and fieldbus communications have allowed direct access and control of a modern factory there is still a missing 'last mile' to the sensors used on the machine where diagnostics and real time communications cannot go. It is for this reason that industry has defined the new IO-Link standard; a generic interface for connecting sensors and actuators to a master unit and allowing them to become accessible to higher level networks.  

IO-Link is an intelligent, point-to-point serial communication protocol allowing fast set up and maintenance of the sensors and actuators. It provides diagnostic data back to the master, which typically would have a gateway application built into it so connecting it to the master PLC.

Growing initially from a group of European PLC, sensor and actuator manufacturers, IO-Link is compatible with the PLC standard IEC 61131. As manufacturers such as Rockwell and Mitsubishi Electric have joined Siemens and others in the IO-Link consortium the many advantages it offers over standard analogue I/O will be promoted and used on a global basis.

The physical topology of IO-Link is point-to-point from each device to the Master using three wires over distances of up to 20m with transmission rates of 4.8kbit/s, 38.4kbit/s and 230.4kbit/s supported. IO-Link is IEC 61131-2 compatible and supports the digital input functionality SIO (standard input output), as well as communication on sensor devices and also on masters. A master is able to switch at any time from SIO mode to communication mode with the different transmission rates COM1, COM2 and COM3 and vice versa. While a device supports just one transmission rate, the master supports all three rates.

Once activated, the IO-Link mode supports parameterisation, cyclic data exchange, diagnosis reporting, identification and maintenance information, and external parameter storage for device backup and fast reload of replacement devices.

IO-Link is not a fieldbus yet brings a whole new level of intelligence down to the sensor level, enabling bi-directional communication for diagnostics, with an intelligent menu structure for monitoring maintenance and failure events. Its applications therefore are wide ranging; anywhere that a 20m cable is long enough to connect to a sensor, system builders can make use of the diagnostic and setup functions to reduce both build and maintenance times. The IO-Link set-up requires an IO-Link master and a number of connected devices, which are mostly sensors, as well as an integrated gateway/master. If the device is in communication mode, the master will try to find each device; during this wake up mode, the master sends information and waits for answers - if wake up is successful, cyclic data communication is then initiated.

Increased control, reduced downtime

IO-Link's ability to identify devices and provide access to the parameters increases operating efficiency to a whole new level of control over conventional sensors and actuators. It is also very easy to change a sensor, as its settings and parameters are held in the main PLC - greatly reducing the downtime associated with manual reconfiguration especially in a breakdown situation.

IO-Link allows three types of data to be exchanged: process data, service data and events. The process data indicates the current state of the sensor, or communicates the desired state of the actuator. Each communication cycle lasts approximately 2ms and can contain up to 32 bytes of data.

For detailed information, the Service Protocol Data Unit (SPDU) gives basic device information (version, type, serial number) as well as more advanced details (configuration, detailed diagnostics or status). Most parts of this information are standard to the protocol but, depending on the device manufacturer, additional information can be made available.

For events that occur too rarely to be included in the basic process data exchange but should be reported immediately, the IO-Link's event facility can be used. Because this information is delivered as the event is encountered, it helps pinpoint issues or potential process errors before they occur, facilitating much improved maintenance regimes.

Standard IO-Link components, such as connectors and cabling, also help ease installation and commissioning of the system, making it much less demanding, for both new machine builds and retrofits, than traditional machine installations, as well as reducing wiring complexity.

Within the IO-Link standard, there are two types of definition for the supply of power. The majority of IO-Link masters are Type A, which supports enough power for a sensor supply. This power is limited though and is not enough for products such as valve terminals, so additional power is added using the functionality in Type B. In order to achieve this, Festo has developed specially designed T-pieces for use with its IO-Link valve terminals to separate the I/O and power cabling.

All Festo valve terminals, which have the Festo I-Port connection integrated, can be used in an IO-Link mode when used with an IO-Link description (IODD) file - this is freely available for download from the website. Alternatively, the same IO-Link valve terminals can be connected, via a Festo CTEU fieldbus node, to many Fieldbus and Industrial Ethernet systems as required.

The future for IO-Link is interesting. With its adoption by not only Siemens but by other PLC manufacturers, it looks set to become a worthwhile alternative to a standard analogue signal or a group of digital signals. However, this depends on the value that the system builder and end user place on the faster set-up and maintenance. With current prices for IO-Link masters typically standing at 50% higher than a typical analogue channel, there will be some hard discussions between purchasing and build and maintenance departments to ensure the value of IO-Link is taken on board.
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