What is DeviceNet?
DeviceNet is a network system used for the task of automation in industry in order to connect different devices. The basis for this is the CAN bus (controller area network) developed by Bosch. Typically, DeviceNet is used for safety equipment, in information exchange, and in large input/output control units. This network system is used mainly in the USA and in Asia. In Europe, Profibus and CANopen have become the leading systems.
DeviceNet – History
DeviceNet was developed by the American enterprise Allen-Bradley, which now belongs to Rockwell Automation. On one hand, this system uses parts from ControlNet, an industrial product from Allen-Bradley, and on the other hand, it offers the advantages of the CAN system. This created a cost-efficient and stable system for traditional protocols based on RS-485. In order to expand the international use of DeviceNet, Rockwell Automation has bought up this “open” technology and has made it available to third-party suppliers. In the meantime, it now is managed by the independent North American Open DeviceNet Vendors Association (ODVA). The ODVA not only manages the specifications, but also the further development and adherance to the standard through conformity tests. Later on, the IDVA decided to combine DeviceNet as Common Industrial Protocol (CIP) with the following technologies:
- EtherNet/IP (Industrial Protocol)
Thanks to the common protocol, there is a high level of integrity between these technologies. This essentially simplifies industrial control as opposed to other systems.
- Definition of the media, physical characteristics, data connections, and applications of the ISO/OSI 7 layer model
- Includes a main line with separate buses for signal and power
- Defines baud rates of 125 kbps, 250 kbps and 500 kbps
- The data cable length is inversely proportional to the speed
- Addition of a conventional flat band cable is possible for quick and easily operated connections
- Support of up to 64 network nodes in a single logical network (0 to 63)
- Supports master/slave as well as peer-to-peer communication, but in most cases the devices are operated in master/slave model
- Enables multiple masters on a single logical network
- For small devices such as photoelectric cells, limit switches and proximity switches, power supply is possible via the network cable
- Networked devices can be configured and controlled at the same time
- Operation in noise-sensitive environments is possible
- Support of four different message types from and to slave devices (clocked, requested, cyclic and COD (status change))
Bit transmission layer / Physical layer:
The nodes of the DeviceNet are available along main lines and branch lines. The topology permits simple wiring and access to the system from several points. Furthermore, network nodes can be removed easily. This reduces the risk of production losses and increases the network flexibility. This opportunity allows for time to be saved while troubleshooting. Since the bit transmission layer is isolated optically from the devices, the communication lines and power supply can be channeled together in one bus. This reduces the complexity of the network and the components.
DeviceNet supports the baud rates of 125 kbps, 250 kbps, and 500 kbps. Depending on the cable type, transmission distances of up to 500 m are possible. Typical round cables support up to 100 m, while flat cables support up to 380 m at 125 kbps or 75 m at 500 kbps
DeviceNet uses a differential, serial bus from the CAN system. As CAN is used as the basis, the system requires only a minimal bandwidth for transmission of data. Furthermore, a smaller processor can be selected, as only a smaller computing capacity is required because of the data format.
The CAN data format consists of the following bits:
1 Bit » Start
11 Bits » Identification
1 Bit » RTR Bit
6 Bits » Control bit
0-8 Bytes » Data field
15 Bits » CRC sequence
1 Bit » CRC delimiter
1 Bit » Confirmation
1 Bit » Ack delimiter
7 Bits » End
> 2 Bits » Space for the next packet
The first start bit serves to synchronize all receivers in the network. The identification bits and the RTR bit ensure the message for the correct receiver. Lower identification numbers have priority over higher ones. In addition to simple data transmission, the device also monitors the process. The transmitted data are checked by this redundancy, and simultaneous data transmissions are prevented. This means that in case of simultaneous data transmission of two network nodes the device with the lower 11-bit identification number has priority for the transmission. The other transmission is stopped.
The next six bits serve to check the data. With this, the last four bits indicate the length of the data field. The length varies between zero and eight bytes. The following 15 bits of the cyclic redundancy check (CRC) field are used for fault tracing. CAN has this good fault tracing system in order to simplify the implementation and the immunity of busy networks
DeviceNet is composed of a connection-based network. A connection first must be led through a UCMM (unconnected message manager) or a G2UP (group 2 unconnected port). From there, explicit and implicit messages can be sent. Explicit messages generally are defined as data packets which expect an answer from another device. Typically, these are settings or data sets without a time limit. In contrast, implicit messages are time-sensitive and are communicated in real-time over the network.
Before an implicit communication can be initiated, an explicit message connection must exist. Once the connection has been established, the CAN identification is used to send the data to the corresponding network node