IO-LINK - Sensors and actuators communication interface - Technology basics
IO-link is a standard interface assigned to the world of industrial automation allowing the dialog with sensors and actuators down to the lowest field level, and its usage could be very useful to an automation engineer or a maintenance team.
The goal 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 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 system.
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 master module | Input and outputs connections
Operating IO-link needs fundamentally :
Master module functions | master module architecture
The IO-link master module assumes mainly two functions :
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, analogic, inputs, outputs, IO-link and non IO-link devices.
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 softwares need the related device description files (IODD).
While working with distributed inputs and outputs, the IO-link master module can be connected to a field bus (ModBus TCP, Profibus etc ...), to a sensors / actuators network (ASI, Interbus, DeviceNet etc ...) 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 maximul 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 dialog with them, we need to import the device description files (IODD) in the development softwares or in the maintenance softwares.
Communication modes | Protocol - Basics | Process datas | Service datas | Events datas
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 dialog with the standard IOs and when powering up. The dialog 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 and allows to exchange numerical datas (configuration, adjustment, diagnostic, etc ...) between a device and a control system or a SCADA system. Then, the dialog is bidirectional.
To dialog 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 dialog, such as any slave in a master slave system.
The datas are encapsulated in telegrams. There are several kinds of telegrams depending on the data length and on the number of datas to exchange.
When the telegram lenght exceeds two bytes (sixteen bits), the telegram is sent over several cycles.
Data transfer parameters between an IO-link master and a device :
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 millisecondes for a data transfer rate of 38 400 Bauds.
Three types of data can be exchanged :
The process datas are the measurement values or command values (detecting signal for a digital sensor, analog value, speed setpoint for an inverter, etc ...).
The process datas are exchanged cyclically.
The period od the cycle is individually adjustable for each instrument.
A diagnostic message is available when a process data is unavailable.
The service datas are only required and initiated by the master module and the client applications. They are typically configuration datas, parameters for adjustments or diagnostic.
The service datas are acyclic.
The event datas are exchanged acyclically and asynchronously (a little bit like the interuptions for the microprocessors) to inform about failures in the device : short-circuit, overheating, overload, communication failure, broken circuit etc ...
The event datas have the highest priority, and any service datas nor any process datas can be exchanged while all the event datas are not been read.
The automation engineer has to import a device description file (IODD) in the development sofwares 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 acceed 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 operating 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 acceed with this software.
The device description file (IODD) is based on the FDT standard specification. It describes for each type of device all the features, datas, commands and parameters available and how to access them.
You will find detailed and significant technical informations browsing the IO-link consortium website www.io-link.com/en/.
The technical informations to read about IO-link :
The useful informations about IO-link to download :
