EM58 • HS58 • HM58 POWERLINK
6 POWERLINK interface
6.1 Overview
Ethernet POWERLINK (EPL) is a communication profile for Real-Time Ethernet
(RTE). It extends Ethernet according to the IEEE 802.3 standard with
mechanisms to transfer data with predictable timing and precise
synchronisation. The communication profile meets timing demands typical for
high-performance automation and motion applications. It does not change
basic principles of the Fast Ethernet Standard IEEE 802.3 but extends it towards
RTE. Thus it is possible to leverage and continue to use any standard Ethernet
silicon, infrastructure component or test and measurement equipment like a
network analyser.
EPL was originally designed by B&R GmbH and the first version was released in
2001. Later on, the Ethernet POWERLINK Standardization Group (EPSG) was in
charge of continuing its management and published the first EPSG Draft version
of EPL as an open standard in 2003 fostering for a free distribution of its
specifications as well as for the open-source distribution of the protocol source
code. The current version of this communication standard can actually be
downloaded from the EPSG web site.
EPL is able to provide the real-time capabilities required by critical processes,
control tasks and management functions typical of the industrial scenario. As a
matter of fact, which is compliant with the Industrial Automation Open
Network Alliance (IAONA) real-time class 4 (highest performance)
recommendations, it is able to cope with communication cycles in the order of
hundreds of microseconds, ensuring, at the same time, jitters below 1 µs.
Also, the EPL application layer is based on the popular and settled CANopen
standard (practitioners often refer to EPL as “CANopen over Ethernet”, see below
on page 63). This feature ensures, at high layer of the protocol stack,
compatibility with several other industrial communication devices.
POWERLINK provides mechanisms to achieve the following aims:
1. transmit time-critical data in precise isochronous cycles. Data exchange
is based on a publish/subscribe relationship. Isochronous data
communication can be used for exchanging position data of motion
applications of the automation industry;
2. synchronise networked nodes with high accuracy;
3. transmit less time-critical data asynchronously on request.
Asynchronous data communication can be used to transfer IP-based
protocols like TCP or UDP and higher layer protocols such as HTTP, FTP,
etc.
POWERLINK manages the network traffic in a way that there are dedicated
time-slots for isochronous and asynchronous data. It takes care that always only
one networked device gains access to the network media. Thus transmission of
isochronous and asynchronous data will never interfere and precise
communication timing is guaranteed. The mechanism is called Slot
MAN EM58_HS58_HM58 PL E 1.1.odt
POWERLINK interface
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