A Comprehensive Guide to ODVA DeviceNet Protocol

In continuation of our articles on ODVA CIP and EtherNet/IP protocols, DeviceNet is a widely used industrial communication network that allows devices to communicate seamlessly in a manufacturing environment. It is an industry standard developed by the Open DeviceNet Vendors Association (ODVA) and runs on top of the Controller Area Network (CAN) protocol. In this comprehensive guide, we will delve into the key features of DeviceNet and provide a detailed understanding of the ODVA DeviceNet Protocol.?

Key Features of DeviceNet?

DeviceNet offers several key features that make it a preferred choice in industrial automation. It can support up to 64 nodes per network that should handle most of the process automation use case. Another notable feature of DeviceNet is its real-time communication capability. It provides deterministic data transmission, ensuring that critical data is delivered with minimal delay. This is crucial for applications that require precise synchronization, such as motion control and robotics.?

DeviceNet also supports peer-to-peer and master/slave communication models, allowing devices to communicate directly with each other or through a central controller. This flexibility enables efficient data exchange and control strategies tailored to specific application requirements. This allows for seamless integration of various devices, such as sensors, actuators, drives, and controllers, making it ideal for complex manufacturing environments.?

DeviceNet to OSI Layer mapping?

To understand the internal workings of DeviceNet, it is essential to map its protocol to the Open Systems Interconnection (OSI) model.??

DeviceNet operates at the physical and data link layers of the OSI model. At the physical layer, it utilizes the standard CAN protocol to transmit data over a twisted-pair cable. The data link layer handles message framing, error detection, and retransmission.?

The DeviceNet protocol specifies the transport and network layers while the higher layers of the OSI model, including session, presentation, and application layers and defined by the ODVA CIP (Common Interface Protocol).??

Thus, it is possible to seamlessly integrate DeviceNet with other ODVA based industrial protocols such as EtherNet/IP, ControlNet, CompoNet with changes from the transport layer. ODVA provides a comprehensive framework for data exchange and device management between the sister protocols.?

Understanding the DeviceNet Physical layer?

The physical layer of DeviceNet is responsible for transmitting data between devices. It uses a two-wire, non-polarized, and shielded twisted-pair cable to establish communication. The cable carries both power and data signals (V+, V-, CANH, CANL and Shield), simplifying the wiring process and reducing installation costs.?

DeviceNet supports multiple data rates - 125 kbps, 250 kbps and 500 kbps, allowing for flexibility in different application scenarios. The bus topology is typically linear with distances supported up to 500 m for 125 kilo bauds, 250 m for 250 kilo bauds and 100 m for 500 kilo bauds. DeviceNet devices can either support isolated or non-isolated transceiver. The physical layer can be run over thicker cables for larger distances. The bus must be terminated with a 121-ohm resistance at each end.?

Leveraging CAN-ID for Device Addressing?

As is commonly known, CAN supports an 11-bit format and 29-bit format. DeviceNet uses only the 11-bit mode. Of these only 6 bits are needed for node addressing as there are going to be a maximum of 64 nodes. The rest of the 5 bits are used to group the messages to 4 groups. The mapping to CAN-ID is captured below.?

As CAN assigns higher priority to lower CAN-ID's the Group 1 has highest priority and group-4 the lowest.? Group 1 messages are typically used for real-time IO connections though explicit communications can also be done over that. The higher bit allocation of Message ID enables uniform bus-access priority for all the devices in the network.?

Group 2 messages are reserved for Master/Slave communications and for Network Access State Machine management. Group 3 Messages are used for Unconnected Explicit Messaging Requests as well as Device Heartbeat and Device Shutdown Messages. Group 4 messages are primarily for diagnostic and management purposes.?

DeviceNet has provision for detecting duplicate MAC ID’s and managing them dynamically.?

Explicit Messaging Connections in DeviceNet?

As we have seen earlier, CIP allows for Explicit Messaging between two peers, in a Master-Slave relationship. Devices can initiate a connection with Open Explicit Messaging Connection Request and terminate them with a Close Connection Request. The client device sends a request to the server device, and the server device responds with the requested data. This communication model is widely used for configuration, diagnostics, and device parameter exchange.??

When there is no need for a long-term connection, the Unconnected Message Manager (UCMM) can be used to process the Unconnected Explicit Requests and Responses. UCMM enables devices to initiate communication with any other device on the network. This flexibility allows for dynamic device integration and efficient use of network resources.?

An in-depth look at I/O Connections?

I/O Connections form the backbone of DeviceNet communication, enabling the exchange of input and output data between devices. Devices can be configured as either input or output devices, depending on their role in the system. Input devices provide data to the network, while output devices receive data from the network.?

DeviceNet supports both cyclic and acyclic data exchanges. Cyclic data exchanges occur at regular intervals and are used for real-time control and monitoring. Acyclic data exchanges, on the other hand, are event-driven and are typically used for configuration and diagnostics.?

To establish an IO connection, it is essential to first open an Explicit Messaging Connection with one of the intended end points of the I/O Connection. Then an I/O Connection Object instance must be created with Create Request and configuration of the Connection instance. With this, the originator can start sending the IO data. I/O connections can be either point-to-point or multicast where a single transmission is consumed by many nodes.?

The Predefined Master/Slave Connection Set in DeviceNet?

DeviceNet incorporates a predefined Master/Slave Connection Set (MSCS) that simplifies the configuration process. The MSCS defines a set of parameters that govern the behavior of devices on the network, such as the number of input and output bytes, connection timeouts, and error handling.?

The MSCS allows for plug-and-play integration of devices, as the configuration parameters are predefined and standardized. This streamlines the setup process and reduces network and processing overhead.?

DeviceNet Object Class: A closer look?

As the extension of CIP, DeviceNet employs the Object Class model for device configuration and management. The DeviceNet Object Class defines a set of related attributes that represent the functionality and capabilities of the interface. The attributes provide detailed information about the device, such as its MAC ID, Baud Rate, Bus status, etc.?

The rest of the CIP object library and profiles are applicable for the DeviceNet devices. This Object Class model allows for easy device integration and provides a standardized framework for device management. It enables seamless interoperability between different devices and simplifies the development of software applications that interact with DeviceNet devices.?

Conclusion: Embracing the power of DeviceNet?

In conclusion, DeviceNet is a robust industrial communication network that offers numerous key features, making it a powerful tool in industrial automation. From its scalability and real-time communication capabilities to its support for various communication models and extensive device compatibility, DeviceNet provides a comprehensive solution for seamless integration and efficient data exchange.??

DeviceNet finds applications in a wide range of industries, including automotive, food and beverage, packaging, and pharmaceuticals. It is commonly used for tasks such as motor control, machine monitoring, sensor integration, and overall system control. Additionally, DeviceNet's compatibility with other protocols, such as Ethernet/IP and ControlNet, enables seamless integration with existing architectures.

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A Comprehensive Guide to ODVA DeviceNet Protocol