Ethernet networks are by far the most widely used for business office applications. But more and more these days, Ethernet is expanding its reach to the factory floor. However, not all Ethernet-enabled industrial automation devices "speak" the same Ether-net dialect. What's needed to bridge this gap is an open communications standard. That's the goal of a new industrial networking standard called Ethernet/IP (Industrial Protocol).
A COMMON LANGUAGE
To meet demand for Ethernet-connectable products, manufacturers continue to meld existing protocols with the TCP/IP(Transmission Control Protocol/Internet Protocol). The Modbus/TCP specification and Profibus-over-Ethernet are two examples.
Making Profibus compatible with Ethernet, for example, requires what is called a gateway. Gateways are basically hardware-based translators that convert one protocol to another. But the extra hardware adds cost and complexity and can hurt performance because the conversion process is relatively slow.
In contrast, DeviceNet-to-Ethernet conversion is possible without a gateway and instead uses a router and what is termed TCP/IP encapsulation. Here, DeviceNet data goes in one side of the router and Ethernet TCP/IP comes out the other, bracketed within an Ethernet message. The result is faster transmission, especially important for control applications. Yet other Ethernet devices, such as I/O, output Ethernet directly without conversion or encapsulation.
However, just because all these products can hook to an Ethernet TCP/IP network, doesn't mean they can talk with one another. The reason is, most industrial Ethernet equipment has its own proprietary application layer. This is a problem because TCP/IP only enables two devices to share data but doesn't guarantee they can communicate. It's analogous to one person speaking in a language another doesn't under-stand. For successful communication, both devices must share the same language. That language, and what makes Ethernet/IP unique, is a universal application layer.
The use of a universal application layer over regular Ethernet allows TCP/IP encapsulated Ethernet messages to be understood by devices running the Ethernet/IP protocol. While it may seem that Ethernet/IP is yet another proprietary solution, consider the fact that 350 different vendors already have over 500 Ethernet/IP products in the pipeline with more on the way. Ethernet/IP is considered a major step toward open industrial control and interoperability. Best of all, it leverages existing Ethernet technology.
Through TCP/IP, for example, Ethernet/IP is able to send explicit messages, those in which the data field carries both protocol information and instructions. Here, nodes must interpret each message, execute the requested task, and generate responses. These types of messages configure devices and perform various diagnostics.
For real-time messaging, multicasting, and for sending implicit messages, Ethernet/IP uses the User Datagram Protocol/Internet Protocol (UDP/IP). Implicit messages contain no protocol information in the data field, only real-time I/O data. The meaning of the data is predefined when the connection is established so processing time in the node during runtime is reduced. Because these messages are low overhead and short, they can pass quickly enough to be useful for certain time-critical control applications.
WHY THE LONG WAIT FOR ETHERNET/IP?
Before Ethernet — in any form including Ethernet/IP — became viable for factory automation, several technical hurdles had to be overcome. For one, Ethernet wasn't considered deterministic. In control applications, a single message received later than anticipated can shut down a process, or worse. Variable packet latency (delays) or dropped packets within Ethernet switches are one possible cause cited. However, the determinism issue has mostly been resolved by the use of high-speed switching techniques that allow traffic between two ports on a hub at almost maximum rated or wire speed. Such switches are capable of handling greater throughput without collision delays common to conventional repeater hubs. In addition, Ethernet provides standardized full-duplex operation which gives a single node, in a peer-to-peer connection, full attention and therefore maximum possible bandwidth. Transmissions through these full-duplex products are theoretically collision free.
Another concern is durability. Because Ethernet originated in the office environment, most components aren't industry hardened. Of even greater concern is security. Ethernet/IP is just as vulnerable to security breeches as regular Ethernet.
These concerns aside, industry seems eager to adopt Ethernet/IP. Sharing a common application layer between ControlNet, DeviceNet, and Ethernet/IP will make plug-and-play interoperability possible among complex devices from multiple vendors. Moreover, Ether-net/IP independent test labs are in the works whose charge is to verify that products conform to the new standard.
What this all means is users will be able to link Ethernet/IP-enabled devices such as drives, robot controllers, bar code readers, and weigh scales, without custom hardware and software. Vendors are tooling up to build these products now, so expect the first wave to hit the second half of this year.
BRINGING ETHERNET/IP TO LIFE
Unlike many options available for industrial Ethernet, Ethernet/IP uses an open protocol at the application layer. It is the only standard supported by multiple vendors and three major networking organizations: ControlNet International (CI) www.controlnet.org, the Industrial Ethernet Association (IEA) www.industrialethernet.com, and the Open DeviceNet Vendor Assn. (ODVA) www.odva.org. ODVA will comanage the Ethernet/IP specifications while a special interest group comprised of members from these organizations will manage the technology and enhancements. Also, a common conformance group is developing conformance testware for the protocol which should be available by the third quarter of this year.
CI and ODVA have agreed to make Ethernet/IP free of charge to all product developers and vendors. Along with the Ethernet/IP specifications, a communication stack and sample source code will be distributed via CI, IEA, ODVA, and other Web sites. These organizations will also provide technical support and introductory training for developers and product managers. Ethernet/IP has backing from major industrial automation players including Rockwell Automation, Honeywell, Omron, Cutler-Hammer, Hitachi, Synergetic, and Woodhead/SST.
WHY INDUSTRIAL ETHERNET?
- Low cost and wide acceptance — Ethernet is an established, worldwide standard with support from IEEE and the ISO. In addition, Ethernet has a firm foothold in both industrial and office environments. The large customer base has driven down prices for components. Moreover, IS and IT departments are familiar with Ethernet.
- Baud rate — Recent advances in Ethernet technology include Fast and Gigabit Ethernet. Fast Ethernet (100 Mbit/sec) provides 10 times the bandwidth of traditional Ethernet and Gigabit, 100 times.
- Integration with Internet/intranet — All installed Ethernet networks support one or more communications protocols that run on top of Ethernet and provide sophisticated data transfer and network management functions. Of these, TCP/IP is receiving the most attention due to the World Wide Web and corporate intranets. Many believe that Ethernet employed at all levels in the factory will help integrate and optimize the flow of information from the shop floor to the intra/Internet.
Information for this article comes from Bill Moss, Executive Director, ODVA, Boca Raton, Fla.