wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

39-2 Industrial Communication Systems
EPL
application layer
protocol
Higher layer
protocols
UDP
TCP
IP
EPL data link layer
Legacy Ethernet MAC
Legacy Ethernet physical layer
FIGURE 39.1
Ethernet POWERLINK communication architecture.
EPL defines two types of stations, namely the managing node (MN) and controlled nodes (CNs). The
MN represents, usually, the “controller” of an automation system (i.e., the device that implements the
automation tasks), whereas the CNs are typically field devices such as, for example, sensors/actuators.
Each network contains exactly one MN and up to 240 CNs connected in different topologies, as will be
illustrated in the next section. Several EPL compliant devices are off-the-shelf available from different
vendors which encompass most of the components used in industrial automation systems such as, for
example, programmable logic controllers (PLCs), computer numerical control (CNC) machines, sensors,
actuators, etc. Moreover, since April 2008, the open source software codes of both POWERLINK
MN and CN are fully downloadable [4], allowing for the free implementation of POWERLINK nodes.
Thanks to its performance figures, EPL may be employed in several fields and, in particular, for applications
with very tight timing constraints such as, for example, coordinated motion control systems [9].
The DLL of EPL relies on a time division multiple access (TDMA) technique managed by the MN, which
allows stations to orderly access the physical medium and hence avoiding collisions even if nonswitched
Ethernet configurations are employed. In practice, the network operation is based on a periodic cycle (EPL
cycle) with constant duration. As can be seen in Figure 39.2, the EPL cycle is split into four sections.
The first section, designated start period, is initiated by the MN, which broadcasts the SoC
(start of cycle) frame to synchronize all the CNs. Subsequently, the isochronous period is entered,
Start
period
Isochronous period
Asynchronous
period
Idle
period
MN
MN MN MN MN
MN or
CN #x
SoC
PReq/PRes
SoA
Asnd/Eth
CNs
CN #1
CN #2 CN #n CNs
Ethernet POWERLINK cycle
FIGURE 39.2
Ethernet POWERLINK cycle.
© 2011 by Taylor and Francis Group, LLC