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

39-4 Industrial Communication Systems
Number
of octects:
7 1 6 6 2 Up to 1500 > = 0 4
Preamble
Start frame.
delimiter
DEST.
address
SRC.
address
Ethernet
type
Data
PAD
FCS
Standard ethernet protocol data unit
Number
of octects:
EPL
message
type
EPL
source
EPL
destination
EPL data
1 1 1 Up to 1496
Ethernet POWERLINK frame
FIGURE 39.3
Ethernet POWERLINK frame.
TABLE 39.2
Frames
Ethernet Powerlink Frame
Start of cyclic (SoC)
Poll request (PReq)
Poll response (PRes)
Start of asynchronous (SoA)
Asynchronous send (ASnd)
Codes of the Ethernet POWERLINK
Hexadecimal
Code
01 h
03 h
04 h
05 h
06 h
The second and third octets contain, respectively, the source and destination nodes of the message in
the EPL network. The subsequent octets accommodate the data to be exchanged.
39.4 Network Configurations
The connections between the EPL nodes can be realized by means of either traditional Ethernet hubs or
switches. Actually, the EPL standard recommends the use of hubs since they ensure both limited jitter
(about 500.ns) and low path delay (about 40.ns). Nonetheless, switches can also be employed as connection
devices, provided that additional jitter and latency are taken into account during the EPL system
configuration.
The availability of standard Ethernet components allows for the implementation of several different
EPL configurations. Moreover, particular CNs may be equipped with an integrated hub in order
to simplify the implementation of complex network topologies with various hub levels in which
traditional star configurations are combined with linear chains of CNs as shown, for example, in
Figure 39.4.
Since the EPL protocol is purposely developed to avoid frame collisions, the Ethernet standard
constraint of a maximum round trip time of 5.12.μs (at 100.Mbps) has not to be necessarily fulfilled. This
reveals particularly helpful for linear configurations, often deployed in low-level industrial applications
that, due to their structures, may have relevant round trip times.
On the other hand, the tight timing of the EPL protocol introduces further important constraints.
In particular, in order to detect possible transmission errors and/or device failures, the correct operation
of the protocol requires that, after issuing a PReq frame, the MN has to receive the PRes from the
addressed CN within a specified time-out. The default time-out value is 25.μs. However, since it clearly
© 2011 by Taylor and Francis Group, LLC