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

39-6 Industrial Communication Systems
Redundant
managing node
Redundant
managing node
Hub
Hub
Hub
Hub
EPL
link selector
CN
Hub
Hub
EPL
link selector
CN
First link
Second link
FIGURE 39.5
Example of Ethernet POWERLINK medium redundancy.
managing nodes (RMNs) need to be available in the network. In this direction, two new entities have
been defined. The active managing node (AMN) is one (and the unique) of the RMNs that currently
hosts and executes the functionality of the MN. The stand-by managing node (SMN) is one of the RMNs
that is in stand-by state regarding MN functionality. From the network point of view, a SMN behaves
like a CN, with the difference that the SMN is constantly monitoring the network and hosting also the
current status of all participants. In case of any failure of the AMN, one of the RMNs will immediately
takeover the functionality of the AMN. The takeover process will not last more than one single EPL
cycle. The takeover process can introduce jitter, upper bounded by the maximum path delay of the network.
All devices connected to the network shall be capable of tolerating this single jitter of the network.
Since more than one SMN could be available in the network, an election mechanism has to executed
before the beginning of network operation. Such a technique is based on the (unique) priorities of the
different SMNs. The priorities could be either assigned with an engineering tool (and configured within
the object dictionary) or derived from the unique node address of the SMNs.
A further way of providing basic redundancy service is to use a ring structure for the cabling
architecture. A closed ring always offers the possibility of having two independent transmission paths
from every node to every other node in the network. For some branches, this is a cost-effective approach,
as the critical section is the physical connection between the cable and the socket, influenced by such
factors of the rough industrial environment as, for example, temperature, humidity, and vibration. This
feature should not be confused with the full redundant network described above. The ring redundancy
for EPL networks is actually an optional product feature of the MN.
As can be noticed in Figure 39.6, EPL products (MN as well as CNs) that support ring redundancy
need to be equipped with two Ethernet ports. In case of a noninterrupted transmission path, the closed
ring passes the sent frame through the network back to the MN. The MN will recognize this frame as
“recently sent” and simply will filter it. If such a frame is not received within a time-out, then the MN
will conclude that a problem has occurred (typically a cable break). In this case, the MN will send the
PLC
managing node
Sensor
Sensor
Drive Drive Drive
FIGURE 39.6
Example of Ethernet POWERLINK ring redundancy.
© 2011 by Taylor and Francis Group, LLC