Version 1 of ETHERNET
Powerlink was released in November 2001 by the Austrian control manufacturer Bernecker
& Rainer Industrie-Elektronik and was made available to other companies. The EPSG
(ETHERNET Powerlink Standardisation Group) was founded in November 2002. In November 2003,
the specification ETHERNET Powerlink V2 was adopted, which contains the most important
extension of V1, an application layer: a standardised application interface based on the
mechanisms defined in CANopen. IXXAT was heavily involved in the standardisation 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-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. |
