Ethernet Powerlink Introduction
Version 1 of ETHERNET Powerlink was developed in November 2001 by the Austrian control manufacturer Bernecker & Rainer Industrie-Elektronik and was made available to other companies. The EPSG (ETHERNET Powerlink Standardization Group) was founded in November 2002. In November 2003, the specification of the real-time Ethernet protocol Powerlink V2 was adopted, which contains the most important extension of V1, an application layer: a standardized application interface based on the mechanisms defined in CANopen. IXXAT was heavily involved in the standardization work for this, particularly with regard to the CANopen mechanisms.
Fig. 1: EPL cycle
To avoid collisions and to make maximum use of the bandwidth, data exchange between the devices is time-slot-based. One device in the EPL network takes on the function of the "Managing Node" (MN), which controls the communication, defines the clock pulse for synchronization of all nodes, and assigns the right of transmission to the individual devices. The "Controlled Nodes" (CN) only transmit when requested to by the manager. An EPL cycle is divided into four time periods (Fig. 1):
- Start Period: Here the manager transmits a "Start of Cyclic" frame (SoC) as a broadcast message to all controllers. All devices in the EPL network synchronize on the SoC.
- Cyclic Period: Cyclic isochronous data exchange takes place in this time period. According to a preset (configurable) schedule, the manager transmits a "Poll Request" frame (PReq) sequentially to each controller. The addressed controller responds with a "Poll Response" frame (PRes). All nodes interested in these data can receive them, whereby a real producer/consumer communication between the stations is achieved similar to CAN.
- Asynchronous Period: This time interval is available for the asynchronous, non time-critical data exchange. For example, a controller is granted the right to transmit by the manager and it can then transmit an IP-Frame.
- Idle Period: Unused period until the new EPL cycle begins.
Any topologies can be implemented by using hubs. Due to the fact that only one device transmits at a time and that no collisions occur, the number of hubs is no longer restricted to two as it is with Fast Ethernet. It is a distinct advantage from a design standpoint if EPL devices already have an integrated 2-port hub from which line structures can be easily implemented.
The application interface of ETHERNET Powerlink V2 is based on the mechanisms defined in the CANopen communication profile DS301 of CAN in Automation (or EN50325-4). This opens up a wide range of readily available and usable device and application profiles for ETHERNET Powerlink, enables continuity of communication services between CANopen and EPL systems and facilitates migration from CANopen to ETHERNET Powerlink at software level.
Fig. 2: Reference model
The reference model in Figure 2 shows the communication mechanisms and familiar elements from CANopen, such as PDO, SDO, object dictionary and network management. As is also illustrated, the SDO protocol can also be implemented via UDP/IP and therefore using standard IP messages. This enables direct access to the object dictionaries of EPL devices by devices and applications outside the EPL system via special EPL routers.
As a result of its impressive features, the real-time industrial Ethernet protocol "Powerlink" is suitable for implementing applications with hard real-time requirements in the range of µs. However, it is also equally suitable for implementing applications that must guarantee transmission of larger quantities of data within a defined period of time and also require the known flexibility from CANopen. Ethernet Powerlink is a flexible communication protocol that can easily be conformed to the requirements of the application.