Device Net - Architecture

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 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 share the same bus (Further reducing the complexity of the network and components within). ( Introduction)

DeviceNet supports 125 kbit/s, 250 kbit/s and 500 kbit/s data 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 kbit/s and 75 meters at 500 kbit/s.( Physical Layer )

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.( The Data Link Layer)

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: Table: Data Frame Format.

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.(Introduction & Physical Layer.)

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.(Physical Layer.)

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.(Physical Layer.)

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.(The Network and Transport Layers.)