Breaking down the barriers

Users of industrial networks have been drawn to Ethernet, but there have been a number of barriers to its use for real-time plant level control. Now Ethernet/IP is set to break down these barriers, writes Mark Daniels, chairman of ODVA UK

Ethernet is today undoubtedly the first choice for information-layer communications networks. Now, with the recent introduction of Fast and Gigabit Ethernet, as well as more reliable Ethernet switches, industrial users are eager to take Ethernet to the plant floor for real-time control applications.

Ethernet has many benefits, including availability, access to and from the Internet, familiarity and cost. But there are several other aspects to consider when using Ethernet in control applications, such as interoperability, design issues and security. Ethernet/IP, an industrialised standard of Ethernet, addresses many of these concerns

Ethernet has gained popularity because it is readily available and subsequently low in cost in terms of physical media and hardware, manpower and training. Ethernet is also fast. Based on data rate alone, which is only one factor of a network's overall performance, 100Mbps Ethernet has a considerably higher data transfer rate than many traditional automation networks, such as ControlNet and Profibus.

On the downside, Ethernet in an automation environment does not deliver interoperability or interchangeability between competing vendors' devices. It only provides a set of physical media definitions, a scheme for sharing that physical media and a simple frame format and addressing scheme for moving packets of data between devices on a LAN. For that reason, all Ethernet networks support one or more proprietary layer protocols that run on top of it, providing sophisticated data transfer and network management functionality.

Application layer It is the uppermost layer, the application layer, that determines the level of functionality a network supports, which devices may connect to the network and how devices interoperate on the network. In order to recognise the need for Ethernet to be joined by an open application layer, it is vital to understand the OSI seven-layer model representing the components of a network architecture.

Transmission Control Protocol/Internet Protocol (TCP/IP) is the network and transport-layer protocol of the Internet and is commonly linked with Ethernet in the business world. The TCP/IP protocol suite provides a set of services that enable two devices to communicate with each other over an Ethernet LAN. TCP/IP guarantees that messages can be passed between these two devices, but it does not guarantee that the devices will interpret each other's data properly. Without a common application layer, Ethernet applications - with or without TCP/IP - cannot deliver interoperability between multiple devices, and without this, Ethernet alone does not give users a real-time control solution.

Ethernet/IP uses an open protocol at the application layer, and is best described by deconstructing its name. The 'Ethernet' portion of the Ethernet/IP name refers to standard (IEEE-802.3) Ethernet, not a modified, proprietary version. 'IP', on the other hand, stands for 'Industrial Protocol' and this is what distinguishes the network.

Ethernet/IP uses commercial, off-the-shelf technology - such as Ethernet chip sets, cabling and other physical media components like Ethernet transceivers and switches - and an open application layer to deliver interoperable, industrial Ethernet products. The

Ethernet/IP open application layer, called the Control and Information Protocol (CIP), is the same application layer that is used in the existing DeviceNet and ControlNet architectures, and so Ethernet/IP technology is open, widely accepted and backed by proven industry leaders. Using CIP transforms Ethernet TCP/IP (which does not enable product interoperability) into Ethernet/IP (which does enable it). CIP makes interoperability and interchangeability of industrial automation and control devices on Ethernet a reality for automation applications.

By using TCP/IP, Ethernet/IP is able to send explicit messages - those in which the data field carries both protocol information and instructions for service performance. 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.

For real-time messaging, Ethernet/IP also employs the User Datagram Protocol/Internet Protocol (UDP/IP), which can multicast and send implicit messages. With implicit messaging, the data field contains no protocol information, only real-time I/O data. The meaning of the data is predefined at the time the connection is established and processing time in the node is therefore minimised during runtime. Such messages are low overhead, short and provide the required, time-critical performance needed for control.

By using both TCP/IP and UDP/IP protocols to encapsulate networked messages, both real-time I/O and explicit messaging can occur. And by providing Ethernet users with real-time I/O, device-configuration and diagnostic capabilities, along with interoperability and interchangeability, Ethernet/IP becomes the ideal Ethernet standard for automation.

Today, Ethernet is primarily used to conduct program maintenance, send data to and from MIS and MES systems, and log events and alarms. Some users use Ethernet for limited control purposes, such as processor interlocking, but it is rarely used as an everyday device and control-level network. Ethernet/IP is redefining the use of Ethernet on the plant floor by making it a viable option for industrial applications. More than 400 vendors worldwide have developed products using the CIP application layer shared by ControlNet, DeviceNet and now Ethernet/IP (see below). These vendors already have the tools and technology in place to build interoperable Ethernet products.

Developer tools To help accelerate development of interoperable Ethernet/IP products, ODVA and CI have agreed to make a set of developer tools free and downloadable from multiple Web sites, and the ODVA Europe conformance test lab at the University of Warwick in the UK will be expanded to provide Ethernet/IP conformance tests by the middle of 2001.

One of the greatest strengths of Ethernet/IP is that it shares a collective object library with ControlNet and

BOX: In order to recognise the need for Ethernet to be joined by an open application layer, it is vital to understand the OSI seven layer model representing the components of a network architecture

DeviceNet. The common objects and device profiles make it possible for plug-and-play interoperability among complex devices from multiple vendors on all three networks. The object definitions are rigorous and support real-time I/O messaging, configuration and diagnostics over the same network. Users can connect to complex devices like drives, robot controllers, bar code readers and weigh scales without custom software, which results in faster start-ups and better diagnostics.

The primary benefit of Ethernet/IP is that it offers end users every service that is essential in a control network. In addition to essential services, it also provides performance which includes 3ms updates from a remote chassis to a controller, routability to other networks, high-speed processor interlocking, I/O control and several I/O exchange options such as polled, cyclic and change-of-state.

The determinism and bandwidth utilisation of ControlNet are aided by the fact that it employs a producer/consumer communication model, but when looking at network speed, baud rate is only one factor to consider. The network communication model used to exchange data and information between devices actually has the most impact on functionality.

All networks fall into one of two categories: source/destination or producer/consumer. The difference between the two can be compared to telling a room full of people the time of day. With source/destination, one person reads a clock and then proceeds to tell each person in the room, one at a time. In producer/consumer mode, the same person announces the time to everyone at once. In a manufacturing environment, 'identifiers' embedded into each message are used by the devices to determine which messages they should 'consume.' Although the network model does not impact the rate at which data is transmitted, it does affect bandwidth. Because a producer/consumer network transmits a piece of information only once, it uses less bandwidth. And less bandwidth equates to greater efficiency and overall speed. Ethernet/IP is unlike many other Ethernet protocols in that it uses standard TCP/IP and UDP/IP Ethernet protocols to encapsulate networked messages. This enables both real-time I/O (implicit) and information (explicit) messaging. The ability to incorporate implicit and explicit messaging allows:

• Easy programming and configuring of controllers and devices
• Fast and precise I/O data delivery
• Broadcast capabilities for alarms and synchronisation of devices
• Access to all devices from anywhere for troubleshooting or fine tuning.

By providing Ethernet users with real-time I/O, device-configuration and diagnostic capabilities, along with interoperability and interchangeability, Ethernet/IP is positioned as the Ethernet-for-automation solution.

Ethernet has long been the undisputed networking champion for information-layer communications. Now, with the introduction of Ethernet/IP, it may soon be the champion of real-time control-layer communications.

Its strong support among vendors, along with its open application layer based on the superior producer/consumer model, is an irresistible combination for industrial end users who have been looking forward to migrating Ethernet to the plant floor.

 

Ethernet/IP Vendor Support In the spring of 1998, a ControlNet International (CI) Special Interest Group (SIG) developed a way to apply the open, widely accepted CIP application layer shared by ControlNet and DeviceNet over Ethernet. The Ethernet/IP SIG is working to provide the necessary training and installation materials, as well as publishing vendor guidelines on how to make Ethernet/IP products. In addition, the SIG will manage the maintenance and enhancement of the Ethernet/IP specifications, the example code and product development tools. CI, the Industrial Ethernet Association (IEA) and the Open DeviceNet Vendor Association (ODVA) introduced Ethernet/IP in March 2000. One of the ODVA's primary responsibilities is the support and worldwide adoption of Ethernet/IP. ODVA, which currently has more than 300 members, was formed in April 1995 to support worldwide growth of the DeviceNet open communications network designed to connect factory floor devices, such as sensors, push buttons, motor starters and drives. ODVA operates globally from offices in Europe and North America and affiliates in China, Japan, Korea, New Zealand, Australia and the UK. The recent formation of ODVA Europe marked a significant worldwide advancement of DeviceNet and Ethernet/IP. This European organisation, which is led by manufacturers such as Omron, Rockwell Automation, Woodhead SST and HMS, is focusing its efforts on promoting both the DeviceNet and Ethernet/IP technologies and providing local education and training programmes. ODVA recently announced an agreement with the Industrial Automation Open Networking Alliance (IAONA) to co-promote the Ethernet/IP protocol (see Newsdesk page 6). IAONA is a trade group that joins Ethernet-Internet technologists and the leading developers of industrial automation applications to identify, discuss and resolve issues regarding the use of open networking across the entire factory.

• ODVA
  a112@industrialnetworking.co.uk

Sources of further information
www.odva.org
www.devicenet.org.uk
www.iaona-eu.com
www.industrialethernet.com