DeviceNet is a low-level industrial application layer protocol for industrial automation applications. DeviceNet connects simple industrial devices (sensors and actuators) with higher-level devices such as Programmable Controllers. Built on the standard CAN (Controller Area Network) physical communications standard DeviceNet uses CAN hardware to define an application layer protocol that structures the task of configuring, accessing and controlling industrial automation devices. DeviceNet is very popular in the United States especially with users of Rockwell Automation Programmable Controllers.
NetBurner DeviceNet Product Summary:
All NetBurner products that have a CAN port and support the standard NetBurner development software support DeviceNet. Embedded Gateway and Master and Slave Source File solutions are available from Real Time Automation.
DeviceNet is a CAN open standard created by Allen Bradley in the mid 1990s. To encourage its development and widespread use, management of the specification was transferred to the Open DeviceNet Vendor Association (ODVA). The ODVA is a non-profit organization that develops and markets DeviceNet and promotes its worldwide adoption.
DeviceNet uses abstract object models to describe communication services, data and behavior. A DeviceNet Node is built from a collection of Objects. These objects organize the system behavior and group data using virtual objects much like C++ uses objects and instances to organize data. A node can even have multiple identities. Objects implement services such as Get Attribute and Set Attribute. Required objects are present in all nodes. The Identity and DeviceNet Objects are required objects. The Identity Object contains data such as the Vendor ID, Device Type and Serial Number. The DeviceNet Object contains data describing the DeviceNet CAN connection such as the node address and network baud rate.
DeviceNet is based on the Part A specification and uses the eleven bit identifier in four different ways. These “Message Groups” define the structure of communications between any two communicating end points. Each group maps one or more of the highest bits as priority bits. For example, the value of bits nine and ten of the eleven bit ID Field define the Group number giving Group 1 the highest network priority. The remaining bits of the identifier are mapped between a DeviceNet Message ID field and a Media Access Control Identifier (MAC Id). Message Groups support different number of Message ID’s and MAC ID’s.
DeviceNet uses two types of messaging schemes, Explicit Messaging and I\O Messaging. Explicit Messages configure end points and provide general communications between any two nodes. Explicit Messages use part of the CAN Data Field for message management. I\O Message is used to transfer data. The entire CAN Data Field can contain message data.
One of the most extraordinary features of CAN (and DeviceNet) is bitwise arbitration. Bitwise Arbitration is the process that CAN uses to prioritize messages without losing any network bandwidth. On a CAN network “zero” bits dominate “one” bits. As a device transmits a message it listens to the bits on the network. If a device transmits a one and hears a zero, it knows that a higher priority message is being transmitted and it discontinues transmitting. The node with the higher priority message hears the bits it is transmitting and never knows it conflicted with a lower priority message. The message sequence on the network is preserved.
Data to be transferred using DeviceNet is defined in Electronic Data Sheets (EDS). EDS’s are very useful because host computers being used as masters can read EDS’s and use this information to configure DeviceNet nodes.