Safe communication in distributed control systems

The publisher/subscriber communication principle is used universally on SafetyNET p. To enable the publisher/subscriber approach to also be used for safe communication, some new security mechanisms have been developed for SafetyNET p. For example, telegram delays can be managed by a runtime measurement initiated by the receiver. The advantage over previous standard solutions is that the transmitter of the message does not need to know the receiver. So the publisher/subscriber approach can also be applied in safety technology, which enables distributed, safe control systems.

Safe Ethernet communication with SafetyNET p®

CANopen application layer

The application layer of SafetyNET p adapts the mechanisms of CANopen to the conditions of SafetyNET p. CANopen is an open, manufacturer independent fieldbus standard specified/standardized by CiA ( CAN in Automation). SafetyNET p therefore has a standardized application layer for industrial applications. This includes the standardization of communication, i.e. the technical and functional features used to network distributed field
automation devices and standardize application objects via device profiles.

The SafetyNET p application layer is largely based on the CANopen standard. The changes that have been made are mainly in the communications area and in the way safe application data is handled. The key element in CANopen is the object directory, which acts as the interface between the application and the communication subsystem. Essentially it is a grouping of objects and functions, which can then be stored and called up as  application objects. The integration of safety functions into the application layer means that the object directory, as the interface to the safe application, needs to be redundant in design.

Generally there are two possibilities for communication between devices: Application data can be merged into process data objects/PDOs (mapping) and then published via the communication system. This is achieved via the cyclical data channel in SafetyNET p. The second possibility is the SDO ( service data object), which is used for acyclic data and is applied when setting control system parameters, for example.

A wide range of device profiles have been developed for CANopen. For example, profiles for digital and analogue I/O devices or drives. By using the CANopen application layer it is possible to use these in SafetyNET p.

Hard real-time communication with RTFL

The RTFL transport layer of SafetyNET p is optimized for the fastest real-time applications. Typically the devices are networked in a linear structure, as with traditional fieldbus systems. All the bus subscribers have equal rights. Data is exchanged in accordance with the publisher/subscriber principle. As a publisher, each device can provide data to the other devices (subscribers) via SafetyNET p. In turn these subscribers can read
the published data from individual subscribers or all subscribers. This way it is possible to exchange data efficiently between all the subscribers. The communication mechanism used by RTFL is a very fast cyclical data transfer in one single Ethernet data frame or multiple data frames per cycle. Communication is initiated by a  special device called the Root Device (RD). The Ethernet frame generated within the Root Device is then  transferred to the other devices (OD – Ordinary Device). The ODs fill the Ethernet frame with data to be  published and extract from the Ethernet frame the data to be read. The devices are addressed via their MAC address. Each RTFL network requires just one Root Device. Each RTFL device has two Ethernet interfaces, which enables the familiar daisy chain wiring often found on fieldbuses.