This system, a real-time Ethernet, was developed in order to be able to transmit data in the microsecond range. It is used mainly for the transmission of process data in automation technology. It was developed first by B&R, and today it is specified as a standard by the open user group EPSG, the Ethernet Powerlink Standardization Group.
Developed for standard conformity, it introduces mixed polling and time slice mechanisms for the deterministic transmission of data. This way, amongst other this, a guaranteed transmission of time-critical data is achieved in short cycles with configurable time behavior. Furthermore, a very accurate time synchronization of all of the connected networks is possible. Additionally, there is transmission of less time-critical data in the asynchronous channel. With the instantaneous implementation of Ethernet POWERLINK, it is possible to achieve times of under 200 microseconds and a time precision (jitter) of less than one microsecond. It also specifies a communication protocol for payload exchange with nodes in the network. Both are handled by a POWERLINK protocol stack. However, no special hardware is required for this. As such, the master and slave nodes can be realized with simple Ethernet components. Open-source master and slave stacks are also available for different operating systems.
The data transmission of Ethernet POWERLINK
As the link on layers two and seven exists in the OSI layer model, it is normally independent of the physics that are applied. In practice, it is often used with twisted-pair cables to the fast Ethernet. With this, commercial 8P8Cs are approved as M12 plug connections. The use of optical waveguides is also possible, although additional delays as a result of media converters cannot be excluded.
Ethernet POWERLINK was included in the standards 61158-300, IEC-61158-600 and IEC 61558-500. Different frames can be sent in the asynchronous phase. For example, this makes it possible to use IP-based protocols on higher layers such as UDP, TCP as well as within Ethernet Powerlink networks.
Transmission of safety-critical data
For safety-critical data, the Ethernet POWERLINK can be expanded with the openSAFETY open safety protocol. With this, the safety-critical data are stored by openSAFETY with checksums and are then divided into two frames. The safety functions of the network are provided by the network’s own safety control mechanism. Secure and non-secure members can exist together in a network as well as exchange data that is not essential for the security function. OpenSAFETY is added as a protocol for the application layer. As such, it can be implemented on a high number of industrial Ethernet network topologies. OpenSAFETY has been tested by the TÜV and has been released for use with safety-critical data in accordance with IEC 61508 SIL 3 of the European standard 954-1.
The Data Format of Ethernet POWERLINK
Each packet is divided into a header and the actual data. This is implemented in a normal Ethernet frame, which can and must have a size of 64 bytes up to 1500 bytes. Jumbo frames, i.e. frames larger than 1500 bytes, do not exist and are not permitted in a Powerlink network. 0x88AB was assigned by the IEEE as an EtherType for the Ethernet POWERLINK. The Powerlink header is composed of one bit, which is reserved, seven bits as message type, eight bits as target node number and eight bits as source node number. For deterministic data transmission, collisions on the network must be suppressed.
For this, the data transmissions are controlled by a specific subscriber, the managing node (MN). The other participants, the controlled nodes (CN) may only transmit when they have been specifically requested to do so. A cycle starts with the message’s start of cycle (SoC). The managing node then controls all nodes individually with a poll request (PReq) whereby the controlled node is answered with a poll response (PRes). The answers are sent as multicast. This way, they can be accessed by other Powerlink devices. In this way, lateral traffic between the CNs is fully possible. In order to keep the cycle times as short as possible, not every device has to be controlled in one cycle. The asynchronous phase with the start of asynchronous (SoA) starts after the completion of the cycle. Now, in each case, a CN specified by the MN can transmit non-cyclic data. In this phase, data from a normal, non-deterministic network and from the Powerlink network can be exchanged via special gateways.
Profiles were defined for a number of device classes. These define the structure and the functionality of the directory for the various devices. Greater independence from manufacturers is obtained through the use of devices with specific profiles. At the same time, the Ethernet POWERLINK uses the CANopen profiles. The transformation rules specify the objects that are to be used with this.
Versions of the Ethernet POWERLINK
Currently, there are two different versions of the Ethernet POWERLINK. The first version, the Ethernet 0x3e3f, is a proprietary approach by B&R, which was used at an early stage as the basis for further development. On the other hand, the second version (Ethernet 0x88ab) is the current standard of the EPSG. It has been expanded through the addition of various characteristics. Both systems have many characteristics in common. In spite of this, the first generation is only a temporary solution.