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Definition
IO-Link is a new point-to-point communication and connectivity protocol between a master and a slave (which can be a sensor/actuator), using which it is possible to transfer serial data by way of switching output, using only 3 wires.

The first IO-Link enabled product was displayed publicly at the Hannover Messe 2008, Hannover, Germany. The first IO‐Link Communication Specification, Version – 1.0, (Document identification:IOL‐09‐0001) was released publicly in January 2009 and is now available for free download from http://www.io-link.com/en/index.php.

Background
In an automation environment, modern fieldbus devices do not only receive measured values, but are also equipped to output and configure data relating to processes, diagnostics, and parameters. Unfortunately, these data are not accessible through existing interfaces or, if it is, then only with great difficulty. The density of information provided and communication requirements are continually on the rise. The process of signal connection should therefore become more intelligent without needing to change the topology and wiring technology involved, and it should be compatible to all fieldbus systems. This is where the IO-Link communications standard shows its value and aims to revolutionize the sensor and actuator interface. It is a new and open standard being adopted by leading manufacturers in the fields of sensors and automation.

25 renowned automation technology manufacturers joined together behind the doors of Profibus and Profinet International to form a working group aimed at defining this new standardized interface called IO-Link. For more details on the working group, please visit http://www.io-link.com/en/WirUeberUns/Gemeinschaftslogo.php?thisID=42.

Topology & Connection
Think of this protocol as a connection definition between a master and a slave. A master contains many IO-Link ports. A slave contains only one IO-Link(IOL) port. So it is possible for the master to be connected to many slaves at the same time and exchange information. This connection is via 3 unshielded wires. One wire supplies +24V of power and L+ symbol is used for its nomenclature while another one provides the ground connection and is L-. The 3rd wire has the nomenclature C/Q and is used for carry a logical high or logical low value (essentially transmits either a '0' or a '1'). Generally a stream of combination of 0's or 1's is sent as a telegram from the master to the slave or vice-verse, which conveys specific information. The length such a wired connection can be upto 20 meters (as defined in the IOL tech. spec.).

The main features of the IO-Link system are as given below: 1> A master and a slave communicates in half-duplex mode. This means that either the master or the salve, at any given instant of time is either transmitting or receiving data. 2> Three baud rates of data transfer are supported by the master. They are 4,800 bauds, 38,400 bauds and 230,400 bauds. Support of 230,400 bauds is optional for the device. The master supports all three. This also means that one master can be communicating simultaneously with several slaves, each slave supporting a different baud rate. So comes the significance of a point-to-point connection. 3> The physical connection is provided by an unshielded 3 wire system.

The master and the device follow a layered structure as per the ISO/OSI reference model. The most important layers here are the Application Layer, the Data Link Layer and the Physical Layer. There three layers are in the order they are supposed to be and communicates accordingly. As a top down approach, the Application Layer comes first followed by the Data Link layer and then comes the Physical Layer. The Physical Layer is the point of termination of the 3 wire system both for the master and the slave.

In a complete communication cycle, from a master point of view, a master will send out some data and will receive a reply from the device after a certain elapse of time. From a slave point of view, the slave receives some data from the master, and accordingly sends out some data as a reply. This complete cycle is one frame. This frame is further broken down into master-frame or slave-frame. A master-frame or slave-frame is in turn composed of one or many packets of data called frame-bytes. One frame byte is nothing but a single UART character consisting of 11 bits. The UART format is used for character by character encoding.

The IO-Link protocol defines many frame-types. The frame-type used for communication depends on the complexity of data exchange between the master and the sensor/actuator (slave).

Implementation
The most fundamental way of implementing the IO-Link protocol in a device would be to use it as a slave. Of course a master can also be constructed, but it has its complexities.

Actually IO‐Link is a slow protocol and does not have harsh timing constraints or huge resource requirements. The development of IO‐Link enabled products is not a very difficult task and most part of the protocol implementation can be done with the use of software. The first prototype development will take some time, but when that is perfected, rapid prototyping is possible. There are companies which provide complete development kits/packages along with the software stack. While there are also some which provide only the Physical layer electronics, still others are there which provide solution of the master side. One may look into the following links to get a better idea:

http://www.mesco-systems.de/en/io-link

http://www.elmos.de/produkte/assps/bus-ics.html?tx_pmproductlist_productlist%5Bproduct%5D=7&tx_pmproductlist_productlist%5Baction%5D=showProduct&tx_pmproductlist_productlist%5Bcontroller%5D=Product&cHash=60978be4fb9b1850a8d6e1e6412961c9

http://www.ichaus.de/product/iC-GF

http://www.balluff.com/Balluff/de/ProductsChannel/Overview/de/Networking+IO-Link.htm

However a fundamental way to make an existing device IO-Link enabled, would be to add to it, the Data-Link layer and Physical layer. So information can be obtained from the tech. spec. and an experienced engineer can proceed to the steps of integration. Stand alone master device can be obtained from the market as one may test whether his slave is conforming to the IO-Link protocols.