The Role of IO-Link (Single Drop Communication Technology) in Industrial Networking

The increasing application of microcontrollers in low cost sensors and actuators has provided opportunities for adding diagnosis and configuration data to support increasing function requirements.

The chief reason for the development of a new technology called IO-Link has been the need for these low-cost sensors and actuators to exchange the diagnosis and configuration data with a controller (either a PLC or PC) using a low-cost digital communication technology while maintaining backward compatibility with the current digital input and digital output (DI/DO) signals.

Another reason is the cost reduction and substitution of error-prone analog transmission such as the 0-10 V. Employing the IO-Link avoids digital/analog conversion on the sensor side and analog/digital conversion on the controller side.

In fieldbus systems, the IO-Link defines a generic interface for connecting sensors and actuators to a Master unit, which may be combined with gateway capabilities to become a fieldbus remote I/O node. Any IO-Link compliant device can be attached to any available interface port of the Master. Devices perform physical-to-digital conversion in the Devices and then communicate the result directly in a standard format using coded switching of the 24 V I/O signaling line, thus doing away with the need for the different DI, DO, AI, AO modules, and variety of cables.

The physical topology is point-to-point from each Device to the Master using three wires over distances up to 20 m (Shielding is not required). The IO-Link physical interface is backward compatible with the 24 V I/O signaling specified in IEC 61131-2. Transmissions rates of 4.8, 38.4, and 230.4 kbps are supported.

The Master of the IO-Link interface detects, identifies and manages Devices plugged into its ports. Tools allow the association of Devices with their corresponding electronic I/O device description (IODDs) and their subsequent configuration to match the application requirements.

The IO-Link technology specifies three different levels of diagnostic capabilities:

• For immediate response by automated needs during the production phase.
• For medium-term response by operator intervention.
• Lastly, for longer-term commissioning via extended diagnosis information.

The Basic Features of IO-Link Technology

Let’s consider the figure below:

The single-drop digital communication interface technology for small sensors and actuators IO-Link (also known as SDCI in IEC 61131-9) defines a migration path from existing DI/DO interfaces for switching 24 V devices as defined in IEC 61131-2 toward a point-to-point communication link. Hence, for example, digital I/O modules in existing fieldbus peripherals can be replaced by IO-Link Master modules providing both the classic DI/DO interfaces and IO-Link.

Analog transmission technology can be replaced by IO-Link combining its robustness, parameterization, and diagnostic features with the saving of digital/analog and analog/digital conversion efforts.

The Domain of the IO-Link Technology within the Automation Hierarchy

IO-Link technology defines a generic interface for connecting sensors and actuators to a Master unit, which may be combined with gateway capabilities to become a fieldbus remote I/O node or via an adapter to a PC or drives.

The beginning point for the design of IO-Link is the classic 24 V DI defined in IEC 61131-2 and output interface DO. Therefore IO-Link provides connectivity of classic 24 V sensors (switching signals) as a default operational mode. Additional connectivity is provided for actuators when a port has been configured into a single-drop communication mode (coded switching).

Presently, most sensors and actuators are fitted with microcontrollers offering a universal asynchronous receiver transmitter interface that can be extended by the addition of a small number of hardware components and protocol software to support IO-Link communication.

The second operational mode employs coded switching of the 24 V I/O signaling line. Once activated the IO-Link mode supports parameterization, cyclic data exchange, diagnosis reporting, identification & maintenance information and external parameter storage for device backup and fast reload of replacement devices. To improve start-up performance these devices, usually provide non-volatile storage of parameters.

Related articles:

• Basic Features of Foundation Fieldbus (FF)
• How RS-232, RS-422 and RS-485 Networks are applied in Instrumentation

The configuration and parameterization of the devices are supported through an XML-based device description similar to fieldbus electronic device descriptions.