Principles of the IO-Link specification

According to the IO-Link specification, communication functions as follows:

• Transmission takes place via an unshielded three-wire cable no more than 20 m long, of the kind normally used for standard sensors.
• Digital communication from 0 to 24 V on the so-called C/Q cable
• Most of the values transmitted are measured values from the sensors which include the units.
• The sensors and actuators are described by the IO Device Description (IODD).
• As a matter of principle, one IO‑Link device can be connected to one IO‑Link port of the master (point-to-point connection).
• The transmission rates between the IO-Link master and the devices are as follows:
  • Via COM1: 4 800 Bd
  • Via COM2: 38 400 Bd
  • Via COM3: 230 400 Bd
• The average cycle time is 2 ms for the reading/writing of 16 data bits at a transmission rate of 38 400 Bd.

IO-Link protocol

The IO-Link protocol supports both the Standard IO mode (SIO) and the IO-Link communication mode (COM).

The design of the protocol and its message frames depends on the types of data to be transmitted.

Data types

The IO-Link specification makes a distinction between the following data types:

Process data

The process data of the devices are transferred cyclically in data frame, with the process data width defined by the device. Process data of 0 to 32 bytes are possible per device (input and output in each case). The width of the transmission consistency is not fixed and therefore depends on the master.

Value status

Each port has a value status (PortQualifier). The value status shows if the process data is valid or invalid. The value status can be transferred cyclically with the process data.

Device data

Device data can be parameters, identification data and diagnostic information. Device data replacement is acyclic and in response to an inquiry from the IO-Link master. Device data can be written to the device (Write) and read from the device (Read).

Events

In case of an event, the device signals the occurrence of the event to the master. The master then reads out the event. Events can be fault messages (e.g. short-circuit) and warnings/maintenance data (e.g. contamination, overheating). Fault messages are transferred from the device via the IO‑Link master to the controller or HMI. The IO‑Link master can also transfer events and states. Events include, for example, cable break or communication breakdown.

The transmission of device parameters or events takes place independently of the cyclic transmission of the process data. Transmissions do not influence or impair each other.

Data storage

As of Specification V1.1, a data storage concept has been created for IO-Link. In this concept, the IO-Link device initiates storage of its data on a higher-level parameter server. In the event that a device is replaced, the parameter server can restore the original parameterization. It is therefore possible to replace the devices without re-parameterization.

The IO‑Link master contains the parameter server. The parameter server can also be implemented centrally in the PLC or in a system server. In this case the data must be downloaded to the control system by means of the function blocks provided.

IO‑Link master

The IO-Link master is the interface to higher-level control systems. The IO‑Link master presents itself to the fieldbus as a normal fieldbus node, and is integrated into the appropriate network configurator via the relevant device description (GSD file).

IO Device Description (IODD)

The IO Device Description (IODD) was defined to provide a full, transparent description of system characteristics as far as the IO‑Link device.

The IODD contains information on communication characteristics, device parameters, identification, process and diagnostic data, and is supplied by the manufacturer. The design of the IODD is the same for all devices from all manufacturers, and is always presented in the same way by the IODD Interpreter Tools. This therefore ensures that the handling is the same for all IO-Link devices, whatever the manufacturer.

New in IO-Link Specification V1.1

The IO-Link Specification is currently available in Version 1.1, and is currently standardized according to IEC 61131‑9.

Specification V1.1 offers the following new features compared with the previous Specification V1.0:

• Transmission of up to 32 bytes of process data in one cycle
• Parameter server function