Transmission Control Protocol/Internet Protocol (TCP/IP) is to Ethernet as LeBron James is to basketball. You rarely hear about one without the other. That’s because the TCP/IP suite is the key, behind-the-scenes technology that brings the Internet and intranets to life—just as James brings a spark and legions of spectators to every court he graces. As a result, TCP/IP is inextricably linked with Ethernet in the business and consumer worlds.
But what many people don’t understand is that using Ethernet and TCP/IP only guarantees that Ethernet devices can coexist within a network; it does not guarantee that these devices can communicate effectively. For devices to be able to understand and interoperate with each other, a common application-layer protocol is needed.
The TCP/IP architects understood this problem and designed these protocols to work with multiple application layers. To date, there are many used with the TCP/IP suite—such as protocols for file transfer (File Transfer Protocol, FTP), e-mail (Simple Mail Transfer Protocol, SMTP) and World Wide Web (Hyper Text Transfer Protocol, HTTP).
Now that Ethernet has infiltrated the plant floor, networking organizations and automation vendors are promoting several new application-layer protocols that are designed to make Ethernet suitable for the rigors of industrial automation. This includes the Common Industrial Protocol (CIP), which is at the heart of EtherNet/IP.
EtherNet/IP is best defined by deconstructing its name. The “EtherNet” portion is fairly straightforward. It refers to commercial off-the-shelf Ethernet (IEEE 802.3)—not a modified, proprietary version—and the TCP/IP suite.
“IP,” on the other hand, stands for Industrial Protocol and is what distinguishes this network. Unlike many industrial Ethernet options, EtherNet/IP uses an already proven, open protocol at the application layer (i.e., CIP). So EtherNet/IP is CIP deployed on an Ethernet TCP/IP network, just as DeviceNet is CIP deployed on a Controller Area Network (CAN).
In terms of how it works, EtherNet/IP uses TCP/IP to send explicit messages—those in which each packet not only has application data, but also carries the meaning of the data and the service to be performed on the data. With explicit messaging, nodes must interpret each message, execute the requested task and generate responses. These types of messages are used for device configuration and diagnostics, and are highly variable in both size and frequency.
EtherNet/IP also leverages standard User Datagram Protocol/Internet Protocol (UDP/IP, part of the TCP/IP suite) transport services, which provide higher performance and multicast functionality for real-time—also known as implicit—messaging. With implicit messages, the application data field contains only real-time input/output (I/O) data. The meaning of the data is linked to an identifier that is defined at the time the connection is initially established, which reduces the processing time in the node during runtime. Such messages are short, low overhead and provide the required, real-time performance needed for control.
In a plant near you
A traditional application would look something like the following. An information network links a programmable logic controller to an upper-level, Manufacturing Enterprise Solution (MES) system. The controller also is connected to a second network—a dedicated I/O bus—to link all the peripheral devices on the factory floor. The I/O bus is used to modularize the construction and wiring of the system, gather remote data, and manage the speed of production.
Now, with EtherNet/IP, the controller can be linked to a single network for both information and I/O for a flat network architecture. Depending on the application, this setup can offer not only better performance than the traditional application, but additional functionality as well.
This additional functionality includes things such as adding voice and video capability to the network and setting up programming terminals on a virtual local area network. This would allow engineers to connect the terminals on a single network, yet segregate and isolate multiple control systems at the same time.
Dave VanGompel, [email protected], is a
Technology Consultant at Rockwell Automation.
