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

Ethernet POWERLINK 39-3
TABLE 39.1
EPL Parameters
Parameter Description
Value
Transmission speed
10–100.Mbps
Maximum number of CNs 240
Duration of the start period
45.μs
Quiet time of MN
1.μs
Elaboration time of each CN
8.μs
where MN and CNs exchange cyclic data. This is accomplished by means of two different frames,
namely PReq (Poll Request) and PRes (Poll Response). PReq is issued by the MN toward a specific
CN and carries the output data. The addressed CN, after an elaboration time, sends as a response
the PRes frame containing the input data. Such a latter frame is not sent exclusively to the MN but,
instead, it is broadcasted on the network so that all CNs receive it. After receiving the PRes frame,
the MN waits for a quiet time, and then it moves on the next CN. It is worth observing that not all
the CNs are necessarily polled at each EPL cycle. Indeed, the standard supports two communication
classes, referred respectively as continuous and multiplexed, which specify the way in which CNs are
addressed. In the continuous class, a CN is polled at every cycle, whereas in the multiplexed class a
CN is polled every n cycles (with n > 1). Both communication classes may be handled contemporaneously
in an EPL cycle, so that it is possible to have a polling scheme in which some CNs that are
queried continuously (i.e., every EPL cycle), whereas some others are polled with a period which is
an integer multiple of the EPL cycle.
At the end of the isochronous period, the MN broadcasts the SoA (start of acyclic) frame informing all
CNs that the asynchronous period is started. In this period, only one asynchronous message may be sent
either by the MN or by one of the CNs. In practice, during the isochronous period, the MN may collect
requests for asynchronous transmissions (from itself and/or the CNs) and then, according to a priority
scheme, it selects the node which is granted to transmit. The selected node, which is informed by the
MN via the SoA frame, may transmit a frame belonging to one of two different categories, namely EPL
AsyncSend frame and legacy Ethernet message. The AsyncSend frame type is defined by the EPL protocol;
it can be used for acyclic process data (typically alarms) and it may be delivered either in unicast or in multicast
mode. Concerning legacy Ethernet messages, they are used for general purpose communication, and
they may be of whatever type although, to this regard, the EPL standard recommends the use of the UDP/
IP protocol suite (rather than the most popular TCP/IP) since UDP introduces a lower overhead.
Finally, the idle period is entered at the end of the asynchronous period. The purpose of this time
interval is to guarantee that there are no message overruns, i.e., no messages cross the EPL cycle boundaries.
Thus, in this period, there is no activity on the network since all stations simply wait for the beginning
of a new cycle. Table 39.1 summarizes the most relevant parameters of the EPL protocol as derived
from the standardization documents.
The correct execution of the EPL cycle requires that all stations connected to the network are compliant
with the EPL protocol. This mode of operation is referred in the standard document as “EPL mode.”
Nevertheless, a specific mode of operation (namely “basic Ethernet mode”) allows for the coexistence
on the same network of EPL stations as well as legacy Ethernet ones. For these configurations, however,
as mentioned in the standard document, the strict determinism of the EPL cycle is no longer ensured.
39.3 Frame Mapping
The EPL frames are encapsulated in standard Ethernet protocol data units as shown in Figure 39.3.
As can be seen, the Ethernet type field is set to the hexadecimal value 88ABh which uniquely identifies
EPL. The first octet of the EPL frame (“message type,” i.e., the first octet in the Ethernet data field) specifies
the type of frame. The codes used in order to specify the different frames are reported in Table 39.2.
© 2011 by Taylor and Francis Group, LLC