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Introduction to DeviceNet

DeviceNet was developed by Rockwell Automation as an open fieldbus standard based on the CAN-protocol. Designed as a powerful protocol for automation technology, it plays a leading role today in the USA and in Asia. More and more systems solutions are also being implemented with DeviceNet in Europe.

The ODVA, being the organization of DeviceNet users, is responsible for the specification and maintenance of the DeviceNet satndard. In addition, the ODVA promotes the worldwide distribution of DeviceNet. The currently available version 2.0 of the standard includes more functions and some corrections.

DeviceNet is an open protocol and every ODVA member can participate in the further development of this standard in the various Special Interest Groups (SIGs). Use of DeviceNet is free of charge. It is only necessary to sign the "Terms of Agreement" in order to be able to use the DeviceNet technology and to receive the vendor ID, a manufacturer's number. The only costs normally incurred are the purchase of the specification. Membership of the ODVA is not necessary.

At the moment over 300 companies are registered members of the ODVA. A total of approx. 700 companies offer DeviceNet products.

DeviceNet is one of three open network standards (DeviceNet, Control-Net and EtherNet/IP), which all use a common application layer (ISO Layer 7), the so-called "Control and Information Protocol" (CIP). This common application layer and open software and hardware interfaces will in future enable a universal connection of automation components from the field level with the Internet. The "Control" part of the CIP defines the exchange I/O data in real time via I/O messages (I/O Messaging or Implicit Messaging). The "Information" part of the CIP defines the exchange of general data for configuration, diagnosis and manangement via explicit messages (Explicit Messaging). These two message types provide optimum communication for industrial controls. CIP thus provides the user with 4 basic functions:

• Uniform control services
• Uniform communication services
• Unifrom distribution of messages
• Common knowledge base

The DeviceNet protocol is designed as a simple, inexpensive yet powerful protocol at the lowest fieldbus level, i.e. for the networking of sensors, actuators and corrensponding controls. The devices that can be connected to DeviceNet range from a simple light barrier to a complex vacuum pump for semiconductor manufacture.

The core function of the DeviceNet protocol is, as with other protocols, data exchange between devices and their corresponding controls. Communication between two devices is based on connection-based communication model, either via a point-to-point or a Multicast connection. This allows the development of Master/Slave systems as well as Multi-Master systems.

The so-called "Predefined Master/Slave Connection Set" was specified for simple DeviceNet slave devices. This subset of the DeviceNet protocol supports Explicit Messages, Polled-I/O, Multicast-Polled-I/O and Bit-Strobed I/O messages from the master to the slave as well as Change-of-State/Cyclic I/O messages from the slave to the master. The "Unconnected Nessage Manager Port" (UCMM) and the dynamic creation of explicit that are Multi-Master-capable and can maintain point-to-point connections with other devices. The Device Heartbeat Message and Device Shutdown Message functions were specified particularly for safety-critical systems. The Offline-Connec-tion-Set simplifies the configuration of off-the-shelf components.

DeviceNet presents communication and application in the object model. Predefined objects facilitate the data exchange of different devices and manufacturers.

Further standardization of benefit to the user was accomplished by creating various device profiles.

Besides Layer 7 (Application Layer), the DeviceNet specification also defines parts of Layer 1 (Transceiver) and Layer 0 (Transmission Media), thus standardizing the physical connection of DeviceNet nodes. Connectors, cable types and cable lengths are specified as well as communication-based displays, operating elements and the cooresponding labeling of the housing.

A DeviceNet network can run up to 64 nodes with baud rates of 125,250 or 500 kBaud. The devices can either be supplies via the DeviceNet-Bus or have their own power supply.

The main field of application of DeviceNet is factory automation. Compared with CANopen, DeviceNet offers virtually the same functions but with emphasis on different priorities. The network management in DeviceNet, for example, is stored decentrally in each node, so that each node monitors the others. CANopen, however, uses a central authority, the NMT-Master. The communication mechanisms under CANopen are simpler, the devices are therefore also less complex. DeviceNet, on the other hand, offers more safety in the use of the protocol but also requires more resources.

IXXAT has been developing CAN-based protocol software for more than 10 years. The first implementation of the DeviceNet Slave Code was carried out in 1996. IXXAT's membership in the ODVA and participation in the Conformance Test Special Interest Group in the ODVA as well as the know-how gained in many projects make IXXAT a highly competent partner for the development of DeviceNet products.

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