EtherNet/IP
12 time-tested principles for architecting EtherNet/IP systems Network Switch High Priority Queue
Low Priority Queue
EtherNet/IP Cyclic I/O Messages EtherNet/IP Acyclic non-control Messages Figure 1 - Priority 0-3 form the lower priority and 4-7 form the higher priority. WHILE NOT OLD, ETHERNET/IP CAN CERTAINLY be labeled “mature.” The specification was first introduced over 25 years ago, and the first EtherNet/IP conformed product left the ODVA test lab a little more than 20 years ago this year. Since then, tens of thousands of words have been written detailing the inner workings of the protocol, and thousands of EtherNet/ IP control systems have been deployed across the globe. The majority of these reliable and wellfunctioning control systems are the product of testing and experimentation: tweaking what’s worked in the past, testing new functionality and experimenting with new infrastructure. Until now, no one has attempted to collect and document the best practices for deploying effective and reliable EtherNet/IP control systems. The EtherNet/IP specification from ODVA is a descriptive, not a prescriptive specification. For example, network designers will find that the specification does not specify an architecture for the underlying Ethernet network. There are numerous types of Ethernet networks that fit within the boundaries of the ODVA specification, but many of them would be inappropriate foundations for EtherNet/IP control traffic exchanges. A new book, The Everyman’s Guide to EtherNet/IP Network Design, fills this gap and details 12 specific guidelines an EtherNet/ IP network designer should use to create practical, optimized and reliable EtherNet/ IP control systems. It is a unique resource
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of proven and time-tested technologies, architectures and recommendations control engineers can use to design and deploy EtherNet/IP control networks on the factory floor. To provide a snapshot of what this book encompasses here are two, highly abbreviated, segments of our recommendations.
Recommendation 3: “Control System Messages Get Priority. Period!”
The most important traffic an EtherNet/IP control system conveys is control signal traffic. Control traffic should preempt any non-control traffic. The impact of even momentary traffic congestion on an EtherNet/IP network could be significant and should be minimized. Non-control traffic, even network management traffic, should defer to control system traffic on an EtherNet/IP control system network. The foundation for an EtherNet/IP network is an Ethernet network, which is also the foundation for most current IT networks. Because they share the same foundation, some facilities try to mix more traditional IT traffic with control signal traffic on an EtherNet/IP network. That creates the possibility of contention and/or congestion on that EtherNet/IP network. Contention (access to the network) is easy to avoid by never using shared network infrastructure devices (Ethernet hubs) and always using full-duplex communication.
Congestion occurs when multiple messages need to be simultaneously transmitted over the same link. There is a brief message transmission delay while one message is temporarily stored in the outgoing message queue of a switch waiting for the transmission of the other message to complete. Congestion is managed using priority. The priority of an Ethernet message is indicated by the value of the priority field in the Ethernet frame. There are eight possible priority values, with 0 indicating the lowest priority and 7 indicating the highest priority. These priorities are mapped to specific message queues in a switch. Two priority queues are sufficient for EtherNet/IP networks: high- and low-priority. EtherNet/IP implicit message traffic should get assigned to the high-priority queue. All other traffic should be assigned to the low-priority queue. No exceptions.
Recommendation 9: “Right Size Your EtherNet/IP Network.”
Right sizing means properly architecting the broadcast domains of both the Ethernet network as a whole and the EtherNet/IP TCP/ IP subnetwork that is superimposed on that Ethernet network. The broadcast domains for each are not necessarily congruent. It is only when devices certified by ODVA to conform to CIP and the EtherNet/IP specifications use a portion of an Ethernet network to exchange CIP communications that
in d u s t r ial et h er ne t b o o k
08.2022
SOURCE: REAL TIME AUTOMATION
Learn what every control engineer needs to know about constructing properly engineered Ethernet/IP control networks. Excerpts from a new book offer a resource and insights into proven technologies, architectures and recommendations for designing and deploying EtherNet/IP control networks on the factory floor.
