IO-LINK - A NETWORK INTERFACE FOR FIELD SENSORS

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 its main objectives is to simplify, to make easier, and to standardize the communication with smart sensors and actuators, adding more functionalities and more flexibility to the users in operating and maintenance while reducing the costs as soon as the design of an application is starting.

Strictly speaking, this digital communication system is not really a field bus 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 environment. Such a system is basically composed of some instruments, of at least one master module and of two or three wires standard cables in order to establish a point to point link between the master module and each of the other devices.

An industrial automated plant takes advantage of all the capacities provided by such a networking system when it is jointly used with dedicated interfaced sensors or actuators ( depending on the functions implemented by the manufacturer of the device ). Thus, this technology is compatible with any non IO-link standard sensor to allow a smoother management of the whole installed devices, systems and networks and to make maintenance operations easier. This flexibility, in association with a large compatibility with other data transmission standard technologies, makes a progressive migration from standard device to IO-link devices easier since the system runs simultaneously with both IO-link and non IO-link devices in a same installation.

• IO-link is compatible with any standard device ( 2 or 3 wires sensor etc. ) in SIO mode, that is the « standard inputs outputs » mode.
• The system is compatible with the most widespread industrial networks, field buses or sensors and actuators networks.
• It allows the communications with the devices down to the lowest field level.
• The signals are transmitted by voltage modulation through a standard asynchronous communication interface ( UART ) over non shielded cables.
• The data exchanges are robust. The communication channel is a point to point link between one master module and each of the sensors or actuators ( the slaves ) it is connected with. Detection and transmission of errors are available.
• Several data transfer rates are available for a maximum length of about 20 meters.
• The mean cycle time is about 2 ms.
• The data exchanges combine both binary and analogical signals.
• The data storage is permanent for the parameter data. These data can be static or dynamic, that means they can be changed and taken in account while operating.
• Cables and connections are simplified and reduced, in particular for analogical signals.

Operating an IO-link network needs fundamentally owning :

• One IO-link master module ( also assuming a function of gateway by interfacing PLC, MMI or industrial networks ).
• One instrument acting as a slave ( one standard sensor or actuator, or one sensor or actuator providing IO-link functionalities ).
• One device description file ( IODD ) when an instrument is supplied with an IO-link interface, so that the master can take in account the features of the device and which data to access.
• One standard non shielded 2 or 3 wires cable ( depending on the technology applied to the connected device ), to connect the device to the master module.

The role of the master module is to establish the communication between the devices ( the slaves ) and to act as a gateway with the external partners. This way this module is a manager, a go-between and an « interpreter ».

Functions

• It manages the communication and the data exchanges between each port and each connected device.
• It assumes the role of gateway to interface with higher level systems ( programmable controllers, field buses, sensors and actuators networks, Ethernet ).

Communication ports

The ports are the communication channels. Each of the port is in association with one and only one data transmission electrical connection onto its hardware interface. The word « port » is currently employed to point the logical communication channel out and to point the physical location of the connections out.

• Such a module must have at least one port, that means it must be connected at least to one instrument.
• Each port can only be connected to one instrument.

  ( The link between a device and the master module is a point to point link. )

• A master can mix various port types together inside the same module : binary, analogical, inputs, outputs, IO-link and non IO-link devices, while they are all independent from the others.

A master module can be an extension card of a programmable controller ( PLC ) directly connected to its system bus, such as a classical input or output extension card.

In this case, the module is acting as an interface between the processor of the PLC and each slave instrument.

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

While working with distributed inputs and outputs, the master device can be connected to a field bus ( ModBus TCP, Profibus ), to a sensor / actuator network ( AS-I, Interbus, DeviceNet ) or to an industrial network ( ethernet ).

In this case, the master is also used as an interface between the devices and the network.

This interface acts as a gateway implemented into the master module. It is specific to the network it is connected with.

The maximum number of masters ( the boxes connecting the inputs and the outputs ) depends only of the features of the network to be 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.

A master can communicate with a sensor or an actuator by choosing between two possible data transmission modes : the « SIO » mode or the « communication » mode.

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

The transmission is unidirectional and consists in switching the signal between 0 volt and 24 volts. The voltage is related with the state to be sent ( such as a classic binary input or output ).

The communication mode - also called the serial mode - is a data transfer using a bidirectional serial link in the respect of the IO-link protocol.

It allows the exchange of digital data such as configuration data, adjustment data or diagnostic data, between a device and a control system or a SCADA system.

To communicate between an instrument and a master in serial mode, the IO-link system uses a standard asynchronous communication interface ( UART ) in « half duplex » mode.

An IO-link device ( an instrument ) 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 be exchanged. When the telegram length exceeds two bytes, the telegram is sent over several cycles.

Data transfer parameters between a master module and its slave devices.

• 1 start bit.
• 8 data bits.
• 1 parity bit.
• 1 stop bit.

The Data transfer rates available between a master module and its slave devices.

• 4800 Bps ( COM1 ).
• 38400 Bps ( COM2 ).
• 230 400 Bps ( COM3 ).

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

Three types of data can be exchanged.

• The process data ( cyclic and synchronous exchanges ).
• The service data ( acyclic and synchronous exchanges ).
• The event data ( acyclic and asynchronous exchanges ).

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

• They 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 » 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 » are exchanged acyclically and asynchronously, a little bit like the interruptions for the microprocessors and they have the highest priority and any service data nor any process data can not be exchanged while all the event data are not been read.

These data always inform about failures in devices : short-circuit, overheating, overload, communication failure or broken circuit.

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 ( HMI or SCADA ).

All the files for all the references of sensors and actuators that the engineer wants to access to have to be imported. ( If five IO-link compliant sensors of the same model are used for instance, the related file has to be imported only once )

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

The device description files ( IODD ) are XML based. They respect the ISO 15745 standard ( indexed 1 for the last version ). The IODD technology is compliant with the FDT technology by using an adapted DTM interpreter. 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 official web site ( please, see below ).

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

Technical informations to read about IO-link.

• The « technology » section presents an overview about the IO-link system.
• The « FAQ » section with the most common questions asked about IO-link.
• The « download » section where you can freely download some pdf formatted documents.

Useful informations about IO-link to download.

• An Io-link overview.
• The specifications of the standard to communicate between IO-link devices.
• The specifications of the standards to describe the IO-link devices and how the IODD files are structured .
• A software tool to check the device description files : « IODD checker ».

https://io-link.com/