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IO-LINK as a communication interface for distributed I/O.


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  Automation > Resources > Technologies > Networks > Sensors > IO-link  

  Sensors and actuators networks

Wireless networking  


What is IO-Link ?, main characteristics, architecture of the system, protocol, operating IO-link with IODD, web resources.

A Standard Communication Interface For The Field Devices

IO-link is a standard interface assigned to the world of industrial automation allowing the communication with sensors and actuators down to the lowest field level.

One of the main objectives of IO-link is to simplify, to make easier, and to standardize the communication with smart sensors and actuators, adding more functionalities to the users in operating and maintenance, and reducing the costs from the design.

Basically, an IO-link system is composed of IO-link instruments, at least one IO-link master module and 2 or 3 wires standard cables to establish a point to point link between the master module and the other devices.

IO-link is not really a fieldbus nor a sensor and actuator network, but sooner an extension filling a gap between the industrial networks and the inputs and outputs, adding new functions such as diagnostic or dynamic adjustments to the low field level devices inside a standardized system.

The interface carries additional functionalities such as diagnostic functions or adjustment functions with the lowest field devices, while taking advantage of a standard environment.

An industrial automated plant takes advantage of all the capacities provided by IO-link when IO-link is jointly used with IO-link interfaced sensors or actuators (depending on the functions implemented by the builder of the device).

Thus, IO-link is compatible with any standard sensor (non IO-link), to allow a smoother management of the whole installed devices, systems and networks and to make easier maintenance operations.

A progressive migration from standard device to IO-link devices is easier too, while IO-link runs simultaneously with both IO-link and non IO-link devices in a same system.

IO-link Main Features And Performances

IO-link System Architecture

Basic architecture, IO-link master module, Direct inputs and outputs, Distributed inputs and outputs.

IO-link - Architecture And Basic Components
IO-link - Basic architecture

Operating IO-link needs fundamentally :

The IO-link Master Module

The function of the master module is to establish the communication between the devices and the gateway.

In this way, the IO-link master module is a manager, a go-between and an "interpreter".

Io-link - Functions of communication and gateway network / PLC

The IO-link master module assumes mainly two functions :

IO-link Master Module Composition and main features

Io-link - Master module - Features and composition

An IO-link master module can have only one port or several ports.

Each port can only be connected to one instrument (the link between a device and the IO-link master module is a point to point link).

An IO-link master can mix various port types inside the same module : binary, analogical, inputs, outputs, IO-link and non IO-link devices.

Connecting Inputs And Outputs Directly To A Control System
IO-link - Connecting inputs and outputs directly to a control system

An IO-link master module can be an extension card of a programmable logic controller(PLC) directly connected to its system bus, such as a classical inputs or outputs extension card.

In this case, it is an interface between the processor of the PLC and each instrument.

For each type of connected IO-link devices, the development software need the related device description files (IODD).

Connecting Distributed IO Using A Fieldbus
Io-link - Master module with distributed IO
Io-link - Network architecture

While working with distributed inputs and outputs, the IO-link master module can be connected to a fieldbus (ModBus TCP, Profibus), to a sensors / actuators network (ASI, Interbus, DeviceNet) or to an industrial network (ethernet).

The master module is used as an interface between the devices and the network.

The gateway installed in the master module is specific to the network connected with.

The maximum number of IO-link master modules (the boxes connecting the inputs and the outputs) depends only of the features of the network used.

To know how to communicate with them, we need to import the device description files (IODD) in the development software or in the maintenance software.

IO-link Communication Protocol And Data Exchanges

Communication modes, Protocol - Basics, Process data, Service data, Events data.

The Switched Communication Mode (SIO) And The Serial Mode
Io-link - Communication modes

An IO-link master module can communicate with a sensor or an actuator using two communication modes : the "SIO" mode and the "communication" mode.

The SIO mode (standard inputs / outputs) is the mode used to communicate with the standard IOs and when powering up.

The communication is unidirectional, and consists to switch the signal between 0 volt and 24 volts depending of the state to send (such as a classic binary input or output).

The communication mode is a data transfer using serial link according to the IO-link protocol.

It allows the exchange of digital data (configuration, adjustment, diagnostic) between a device and a control system or a SCADA system. In this case, the exchanges are bidirectional.

The IO-link Communication Mode Protocol And Basics

To communicate between an instrument and an IO-link master module in communication mode, IO-link uses an standard asynchronous communication interface (UART) in "half duplex" mode.

An IO-link device is a slave and can only answer to the master module, that means it can not initiate the communication, such as any slave in a master slave system.

The data are encapsulated in telegrams. There are several kinds of telegrams depending on the data length and on the number of data to exchange.

When the telegram length exceeds two bytes (sixteen bits), the telegram is sent over several cycles.

Data transfer parameters between an IO-link master and a device.

The Data transfer rates available between an IO-link master and a devices.

The mean time to read or write a 16 bits data is typically 2 milliseconds for a data transfer rate of 38400 Bps.

Three types of data can be exchanged.

The Process Data

The "process data" are the measurement values or the command values (detecting signal for a digital sensor, analogical value, speed setpoint for an inverter).

The process data are exchanged cyclically.

The period of the cycle is individually adjustable for each instrument.

A diagnostic message is available when a process data is unavailable.

The Service Data

The "service data" are only required and initiated by the master module and the client applications.

They are typically configuration data, parameters for adjustments or diagnostic.

The service data are acyclic.

The Event Data

The "event data" are exchanged acyclically and asynchronously, a little bit like the interruptions for the microprocessors.

They inform about failures in the device : short-circuit, overheating, overload, communication failure or broken circuit.

The event data have the highest priority, and any service data nor any process data can be exchanged while all the event data are not been read.

Operating The IO-link Devices By Using The IODD Files

IO-link Operating with an IODD device description file.

The automation engineer has to import a device description file (IODD) in the development software used to program the applications in the control systems (PLC) or in the visualization devices (MMI, SCADA).

All the files for all the references of sensors and actuators that the engineer wants to access to has to be imported.

(If five IO-link sensors of the same model are used for instance, the related file has to be imported only once)

A user who wishes run a software connected to IO-link devices like a configuration software, an adjustment software or a maintenance software has to import the description files (IODD) of all the models of devices he wants to access with this software.

The device description file (IODD) is XML based. It respects the ISO 15745 standard (ISO 15745-1 for the last version). IODD technology is compliant with FDT technology by using an adapted DTM intepreter.

For each type of device, it describes all the available features, data, commands and parameters, and how to access them.

The complete IODD standard specifications are available freely from the IO-LINK official web site.

IO-link Technical Resources

IO-link Specifications, Technical Documentation And Tools  

You will find detailed and significant technical informations by browsing the IO-link consortium website.

Technical informations to read about IO-link.

Useful informations about IO-link to download.

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