Fig. 1: EPL cycle

To avoid collisions and to make maximum use of the bandwidth, data exchange between the devices is time-based. A device in the EPL network takes on the function of the "Managing Node" (MN), which controls the communication, defines the clock pulse for synchronisation 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 state. According to a preset (configurable) schedule, the manager transmits a "Poll Request" frame (PollReq) sequentially to each controller. The addressed controller responds with a "Poll Response" frame (PollRes). In this way, all nodes interested in the data can receive them, whereby cross-traffic between the stations is achieved in a manner similar to CAN.
• Asynchronous Period: the time interval for the asynchronous, non time-critical data exchange. A controller is granted the right to transmit by the manager and it can then transmit an IP-Frame, for example.
• 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 no collisions occur, the number of hubs is no longer restricted to two as it is with Fast Ethernet. It is advantageous 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 elements familiar from CANopen, such as PDO, SDO, object dictionary and network management. As is also illustrated, the SDO protocol is also 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 EPL routers.

Due to its features, ETHERNET Powerlink is suitable for implementing applications with hard real-time requirements. However, it is also just as suitable for implementing applications that do not have such hard real-time requirements, but which must guarantee transmission of larger quantities of data within a defined period of time and also require the known flexibility from CANopen.

With our partner IXXAT Automotion Gmbh we offer the following products supporting engineers developing with ETHERNET Powerlink:

• ETHERNET Powerlink Protocol Stack >>>

• ETHERNET Powerlink Module >>>

• ETHERNET Powerlink PCI Interface Card >>>

• ETHERNET Powerlink Starter Kit >>>

• Free EPL Analysing Tool >>>