DeviceNet

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DeviceNet is a communication protocol used in the automation industry to interconnect control devices for data exchange. It uses Controller Area Network as the backbone technology, and defines strong application layer to cover a vast range of device profiles. Typical applications as listed: information exchange, safety devices, and large I/O control networks. [1][2]

Contents

[edit] History

DeviceNet was originally developed by an American company Allen-Bradley (now known as Rockwell Automation). It was adapted from another protocol - ControlNet. ControlNet is a more complex, high performance, and more expensive protocol. The reason for this adaptation is obvious: CAN silicon is much cheaper than the ControlNet ASIC silicon due to the large quantity sold to the automotive industries.[3]

In order to promote the use of DeviceNet worldwide, Rockwell Automation has adopted the "open" concept and decided to share the technology to third party vendors. Hence, it is now managed by the Open DeviceNet Vendors Association (ODVA), an independent organization located in the North America. ODVA maintains specifications of DeviceNet and oversees advances to DeviceNet. In addition, ODVA ensures compliance to DeviceNet standards by providing conformance testing and vendor conformity.[4]

[edit] Technical Snapshot

  1. Defines the Media, Physical, Data-Link, and Application layers of the ISO/OSI 7-layer model (see Open Systems Interconnection)
  2. Incorporates trunkline topology with separate buses for signal and power (Typical configuration: two twisted pairs and a single shield)
  3. Baudrates defined: 125 kbps, 250 kbps, and 500 kbps
  4. Trunk length is inversely proportional to the speed, i.e. 500, 250 and 100 meters respectively
  5. A not-so new flat cable was added to the specification to allow the use of the quick-fix connector
  6. Up to 64 nodes on a single logical network. (Node addresses range from 0 - 63)
  7. Supports master/slave as well as peer-to-peer communication, although majority of the devices work in the master/slave configuration
  8. Allows multiple masters on a single logical network
  9. Network cable can supply device power along same cable as communication cable (Generally smaller devices such as photo-eyes, limit switches, and proximity switches).
  10. Networked devices can be simultaneously controlled and configured
  11. Engineered to withstand noisy environments

References [5][6]

[edit] Architecture

Physical Layer

Nodes are distributed along a DeviceNet network by the means of a trunkline-dropline topology. This topology allows for ease in wiring and access to the network from multiple taps. In addition, nodes can be easily removed and added to the reduce production downtime, increase network flexibility, and decrease troubleshooting time. Since the physical layer is optically isolated from device, communication power and device power can shared the same bus (Further reducing the complexity of the network and components within). [7]

DeviceNet supports 125 kbps, 250 kbps and 500 kbps baud rates. Depending on the chosen cable type, DeviceNet can support communication up to 500 meters (Round thick cable). Typical round cable supports up to 100 meters. While flat style cable supports up to 380 meters at 125 kbps and 75 meters at 500 kbps.[8]

Data Link Layer

DeviceNet uses a differential serial bus (Controller Area Network) as its Data Link Layer. Using CAN as a backbone, DeviceNet requires minimal bandwidth to transmit and package messages. In addition, a smaller processor may be selected in the design of device thanks to data frame format and the ease at which the processor can parse through the data. See below for full format.[9]


CAN Data Frame Format

1 Bit      => Start of Frame
11 Bits    => Identifier
1 Bit      => RTR Bit
6 Bits     => Control Field
0-8 Bytes  => Data Field
15 Bits    => CRC Sequence
1 Bit      => CRC Delimiter
1 Bit      => Acknowledge
1 Bit      => Ack Delimiter
7 Bits     => End of Frame
>2 Bits     => Interframe Space

Reference[10]

Upon transmitting the first packet of data, the "Start of Frame" bit is sent to synchronize all receivers on the network. The CAN identifier (denoted from 0-63) and RTR bit combine to set priority at which the data can be accessed or changed. Lower identifiers have priority over higher identifiers. In addition to transmitting this data to other devices, the device also monitors the data sent. This redundancy validates the data transmitted and eliminates simultaneous transmissions. If a node is transmitting at the same time as another node, the node with the lower 11 bit identifier will continue to transmit while the device with the higher 11 bit identifier will stop.[11]

The following 6 bits contain information for specifying the Control Field. The initial two bits are fixed, while the last four are used to specify length field of the Data Field. The Data Field contains from zero to eight bytes of usable data.[12]

The following data frame is the CRC (Cyclic Redundancy Check) Field. The frame consists of 15 bits to detect frame errors and maintains numerous format delimiters. Due to ease of implementation and immunity to most noisy networks, CAN provides a high level of error checking and fault confinement.[13]

Network

DeviceNet incorporates a connection-based network. A connection must initially be established by either an UCMM (Unconnected Message Manager) or a Group 2 Unconnected Port. From there, Explicit and Implicit messages can be sent and received. Explicit messages are packets of data that general require a response from another device. Typical messages are configurations or non-time sensitive data collection. Implicit messages are packets of data that are time critical and generally communicate real-time data over the network. An Explicit Message Connection has to be used to established first before an Implicit Message Connection is made. Once the connection is made, the CAN identifier routes data to the corresponding node.[14]

[edit] Conformance Test

To declare your product as DeviceNet conformant, a vendor needs to send their product to the ODVA test lab for the certification. ODVA used to have a few other test labs around the world, i.e. UK, Japan, and China. Is has now been consolidated into one that is in the North America.[15]

A full-test version is called the Composite test. It consists of:[16]

  1. Conformance test. Test against the protocol specification.
  2. Interoperability test. Test against devices from various vendors on a single, fully populated, network.

[edit] Conformance Test Procedure

The following procedure shows you how to get your product certified.

  1. Register as vendor with ODVA. You will be given a vendor ID.
  2. Puchase a copy of the DeviceNet specification. A hard and soft copy will be sent to you.
  3. Puchase the conformance test software and corresponding hardware interface card. Note that only selected interface cards from a few vendors can be used.
  4. Develop and test product in-house. You would probably need help from the discussion group, see the External links below.
  5. Submit your product to ODVA test lab for independent verification.
  6. Repeat the above two steps until your product successfully pass the independent test.

Reference[17]

[edit] Sources

Notes

  1. ^ [1] Controller Area Network Solutions FAQ (Frequently Asked Questions).
  2. ^ [2], DeviceNet Technology Overview, URL accessed 2007-02-13.
  3. ^ [3] Controller Area Network Solutions FAQ (Frequently Asked Questions)], What is DeviceNet?, URL accessed 2007-02-13.
  4. ^ [4] Controller Area Network Solutions FAQ (Frequently Asked Questions)], What is DeviceNet?, URL accessed 2007-02-13.
  5. ^ [5] Controller Area Network Solutions FAQ (Frequently Asked Questions)], Basic DeviceNet Concepts?, URL accessed 2007-02-13.
  6. ^ [6], DeviceNet Technology Overview, URL accessed 2007-02-13.
  7. ^ [7], Introduction, URL accessed 2007-02-13.
  8. ^ [8], Physical Layer, URL accessed 2007-02-13.
  9. ^ [9], The Data Link Layer, URL accessed 2007-02-13.
  10. ^ [10], Table: Data Frame Format, URL accessed 2007-02-13.
  11. ^ [11], Introduction & Physical Layer, URL accessed 2007-02-13.
  12. ^ [12], Physical Layer, URL accessed 2007-02-13.
  13. ^ [13], Physical Layer, URL accessed 2007-02-13.
  14. ^ [14], The Network and Transport Layers, URL accessed 2007-02-13.
  15. ^ [15], Conformance Testing, URL accessed 2007-02-13.
  16. ^ [16], Conformance Testing, URL accessed 2007-02-13.
  17. ^ [17], Conformance Testing, URL accessed 2007-02-13.
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