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August 2001

Quo vadis industrial Ethernet?

by Michael Volz

Different protocols will power tomorrow's automation networks.

No doubt, Ethernet and the TCP/IP protocol are achieving acceptance in industrial automation technology. Major technical advances such as Fast Ethernet, switching, and full duplex communication have turned good old Ethernet into a powerful communication system with a magical attraction to industrial users and manufacturers.

However, there is at least one important step missing before achieving the final breakthrough into this application sector: the specification of a standard application protocol for industrial automation.

Currently, many organizations are working intensively on developing their application protocols for industrial Ethernet. The different approaches such as EtherNet/IP (from the Open DeviceNet Vendors Association, or ODVA), Profinet (from the Profibus User Organization, or PNO), interface for distributed automation (IDA, from the IDA Group), Foundation fieldbus high-speed Ethernet (HSE, from the Fieldbus Foundation), Interbus on Ethernet (from the Interbus club), and Open Modbus (from Modconnect) are incompatible and noninteroperable.

Despite the different approaches to the application layer, or Layer 7, of the Open Systems Interconnect seven-layer model, all concepts have a common core. This common functionality includes the well-established standards for Layer 1–4, such as Ethernet IEEE 802.x data transmission technology (Layer 1), the bus access method (carrier sense multiple access/collision detection, Layer 2), the Internet protocol (IP, Layer 3), and transmission control protocol (TCP) and user datagram protocol (UDP, Layer 4).

In addition, common elements exist in Layer 7 for non-time-critical functions. Here, internationally accepted information technology standards such as hypertext transfer protocol, file transfer protocol, and simple network management protocol are common.

The differences between the current approaches live in the general communication system architecture, the industrial application protocols in Layer 7, the object modeling, and the engineering model for system configuration.

These differences subdivide into encapsulation systems such as EtherNet/IP, Foundation fieldbus HSE, Open Modbus, Interbus, and Ethernet vs. distributed automation concepts such as IDA and Profinet.

Encapsulation technologies

The term encapsulation describes the packaging (or embedding) of a telegram frame into a TCP or UDP container. A typical example of this approach is EtherNet/IP. Rockwell Automation and the ODVA developed this mode using HSE technology from the Fieldbus Foundation and Modbus TCP/IP from Modconnect.

With all of these concepts, a more or less unchanged fieldbus telegram embeds as user data in a TCP/UDP frame transmitting over Ethernet. The advantage of this method is that the benefits of Ethernet as a powerful, scalable communication medium combine with the related existing fieldbus solution without the need to change the overall communication philosophy or the engineering tools.

Another advantage is that completing the specifications did not require long developments. Indeed, the first commercial products are already available and in use. It is easy to provide downward compatibility to the respective fieldbus protocols.

Thus, Ethernet is mainly a new data transmission technology that functions as an alternative to, or in good combination with, the already established conventional fieldbuses such as DeviceNet, ControlNet, Modbus, or Foundation fieldbus H1.

The second category of Ethernet-based industrial network aims to fulfill the communication requirements of new automation concepts with distributed intelligence. In this approach, the application resides on several decentralized controllers that connect via industrial Ethernet.

IDA and Profinet are targeting these applications. While IDA is a complete new approach that includes both fast real-time communication and non-time-critical control functions, Profinet realizes only the non-time-critical control functions via Ethernet and includes a gateway concept to interconnect with the existing Profibus technology for time-critical, real-time communication.

Header indicates significance

The EtherNet/IP uses the control and information protocol (CIP). CIP is the protocol that ControlNet and DeviceNet also leverage. CIP provides a wide range of standard services for access to data and for control of network devices via so-called implicit and explicit messages.

The CIP data packets use encapsulation before they transmit via Ethernet. They assume a telegram header depending on the requested quality of service. The header indicates the significance of the transmitted data to the corresponding service protocol on the receiver side.

The CIP data packets have a special Ethernet header, an IP header, a TCP header, and an encapsulation header. The encapsulation header contains fields with control commands, format and status information, synchronization data, and the like. This allows the CIP data packets to transport via TCP or UDP and ensures that they are recognizable to the receiver.

The disadvantage of encapsulation compared with DeviceNet or ControlNet is that a much lower protocol efficiency is achieved. The Ethernet headers are much larger than the user data and lead to significant protocol overhead. Accordingly, EtherNet/IP is more suitable for sending large data portions—programs, for instance—than analog or discrete I/O data. DeviceNet and ControlNet handle I/O data better.

Use technology modules

Profinet is under development by the Profibus User Organization, with strong support from Siemens. It's a high-level communication system that supports distributed automation. It includes a specification for distributed automation environments based on a vendor-independent object and connection editor and device descriptions in extensible markup language (XML).

Ethernet TCP/IP runs only for non-time-critical communication among the intelligent devices. All real-time data transmits via the standard Profibus-DP technology. Data from the Profibus-DP network integrates into the Profinet system via a proxy.

Profinet uses established information technology standards. It does not define its own industrial application protocol. It uses an object model based on Microsoft's component object model (COM) technology. For all interactions among the distributed objects over the network, it leverages Microsoft's distributed COM wire protocol and TCP and UDP protocols.

In the Profinet concept, machines and plants split into technology modules, each consisting of mechanics, electronics, and application software. The application software develops through proprietary programming tools and downloads locally into the related controllers.

The proprietary software tools must implement the Profinet Componer software interface to export Profinet object definitions in XML. These XML files input into the vendor-independent Profinet connection editor that generates the Profinet components from the XML files.

The connection editor defines the interactions among the Profinet components over the network. Finally, the connection information downloads via Ethernet TCP/IP into the Profinet devices.

PNO started to develop the Profinet specification last year. Today, conceptual specifications are available to PNO members. The first multivendor demonstration was this past April at the Hanover Industrial Fair.

Developing a new approach

IDA is a new approach under development by the IDA Group with strong support from Schneider Automation and Jetter. IDA supplies the infrastructure for modular, distributed, and reusable automation solutions. It is an object-oriented communication system that defines the methods for both real-time communication and management communication among nodes.

The methodology relies on the architecture introduced in the evolving draft IEC 61499, Function Block Standard. The scope of IDA further includes Web-based device management via standard Internet browsers, plug-and-work methods based on XML device descriptions, and synchronization methods to permit clock synchronization of devices as required for axis coordination of drives.

Safety will be another integral part of IDA. IDA defines a safety layer, allowing the combination of safe and nonsafe devices and tools in one application over one Ethernet TCP/IP-based network simultaneously.

The IDA real-time communication draws on the real-time publish/subscribe (RTPS) protocol. The RTPS protocol implements via middleware (Layers 4–7) and is common to all IDA devices. The RTPS protocol and the middleware reside on top of the UDP protocol.

Real-time services in general have the highest priority of all IDA communications. Depending on the application, real-time communication relationships include preconfigured or dynamic, cyclic or on demand, point to point or group oriented, and single source or redundant.

Another important feature of IDA technology is Web-based device management. All field devices have their own built-in Web page, which contains their configuration and operation and diagnostic parameters. Users access this information from any standard Internet browser.

XML-based device descriptions simplify system configuration and support device interchangeability. IDA not only is the missing Layer 7 in industrial Ethernet but also goes further and defines communication features for new automation concepts employing distributed intelligence.

The first IDA specification was recently released as a free white paper.

Device manufacturers' view

Device manufacturers that want to be successful in international markets will have to implement all these different system approaches into their devices. Even if there are common functions in Layers 1–4, the application protocols and system models are totally different.

Implementing the different standards into all the devices will require a huge amount of development resources. A valuable strategy may be to partner with technology suppliers that can offer communication interfaces for all the industrial networks.

Compared with the fiasco surrounding conventional fieldbus systems, users have reason to be optimistic about the future of industrial Ethernet. The fact that all the up-and-coming Ethernet-based industrial networks use the same data transmission technology is a considerable advantage over the large number of fieldbus transmission technologies.

But on top of the unique Ethernet physical layer, there will be several different noninteroperable industrial Ethernet application protocols.

Industrial Ethernet will not replace conventional fieldbuses. It will open new applications and support the migration to distributed intelligence in automation. IT

Figures and Graphics

• OSI seven-layer model: Remodeling in progress
  
• Profinet software interface
  
• Web-based device management
  

Sidebars

• Terminology
  
• The role of IAONA
  

>h2>Behind the Byline

Michael Volz is the managing director of HMS Industrial Networks GmbH in Karlsruhe, Germany. He is also the vice president of IAONA Europe and is actively involved in the development of industrial Ethernet.