GE Fanuc Automation announced that all of its automation devices will support Ethernet. When needed, it will supply a fieldbus on top of an Ethernet network to provide compatibility. Both the Series 90-70 and 90-30 PLCs support 10 Mbps Ethernet, with the larger PLC also offering ports for 10BaseT and 10Base2.

In communications, what are the features that matter most? Based on the growing appeal of Ethernet, they are wide compatibility, installation ease, and relatively low cost. Almost all other capabilities and enhancements available with the latest technology are seen more as a challenge than an aid to communication.

Even as we speak, Ethernet is pervading the enterprise - from executive offices to the plant floor. The list of motion device manufacturers offering an interface to this protocol grows daily, so every device, including motors, actuators, drives, and even bearings, will connect to it at some point.

But Ethernet is not, and may never become, the sole solution for every application. It easily handles byte-size data transmission from mid to high-level control devices. For bit-size data, additional networks or buses are often needed. The choices for this level of connectivity are increasingly narrowing to DeviceNet.

For the other side of drive communications, the signals between drives and motors, the choices are less clearcut. Sercos, Macro, and ±10-V analog still hold sway. However, FireWire is gaining adherents, and USB is mounting a challenge to all of them.

Ethernet

The appeal of standardizing on one network is strong as companies anticipate cost savings in implementation, maintenance, and training. Ethernet’s promise of greater access to operations information, eventually down to individual I/O points from anywhere in the world through the Internet or an intranet, is the engine behind the implementation of this protocol.

However, even though the choice seems settled, obstacles to seamless communication still remain. Ethernet comes in multiple flavors, many of which are incompatible with each other (see the March 2000 issue of PT Design, page 44). And an additional version was introduced in March.

This latest version, EtherNet/IP, is sponsored by the Open DeviceNet Vendors Association (ODVA). It consists of off-the-shelf physical media and components and an “open” application layer. Open in this case means that the protocol will have no trouble communicating with DeviceNet and ControlNet networks. There will also be a routable application layer for connection to FL-Net and FireWire.

Industrial strength

Most applications use Ethernet to conduct program maintenance, send data to and from upper management information systems and manufacturing control systems, and log events and alarms. Some use it for limited control purposes. And a few use it for everyday device as well as controllevel communication.

Select figure to enlarge.

The main obstacle to more device and control-level applications is the lack of compatibility between the various Ethernet versions. Almost every Ethernetbased protocol uses a different top layer, the application layer, which prevents a seamless exchange of data.

EtherNet/IP was developed to solve this problem. The IP in this case stands for Industrial Protocol, not Internet Protocol. Borrowing some of the features of TCP/IP, Ether- Net/IP uses encapsulation to help create a common application layer.

In a typical TCP/IP frame, the data field can contain, or encapsulate, instructions as well as protocol information. Thus, it’s possible to pass the parameters needed to establish compatibility among otherwise incompatible communication systems. Ether-Net/IP uses this feature to allow a DeviceNet node to encase a message along with any DeviceNet protocol specifications, objects, and device profiles, easing the message’s move to the datalink layer. This message format is typically used for device configuration and diagnostics and tends to vary in size and frequency.

For real-time messages, EtherNet/IP relies on User Datagram Protocol/Internet Protocol (UDP/IP). These messages contain only data in the data field. Thus, they have low overhead because there’s no protocol information. In addition, receiving nodes know what type of data to expect in this format, so processing time is further reduced, enabling real-time message.

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