Devicenet; Description - WAGO 750-306 Technical Description

106 • DeviceNet

Description

5 DeviceNet

5.1 Description
DeviceNet is a networking concept in the device level based on the serial bus
system CAN (Controller Area Network). It is particularly distinguished by the
problem-free addition and removal of devices, from simple light barriers up to
complex motor controls during operation. DeviceNet is mainly used in indus-
trial automation and for robot controls.
The Data Link Layer, i.e. the physical and data storage layer, is defined in the
CAN specification. The telegram architecture is described. However, there is
no information about the application layer.
This is where DeviceNet comes into play. It describes the defined meaning of
the data transmitted in the application layer.
The Open DeviceNet Vendor Association (abridged: ODVA) is the user or-
ganisation for DeviceNet. In a specification, the ODVA DeviceNet is defined
as a uniform application layer and it lays down technical and functional fea-
tures for device networking.
A maximum of 64 fieldbus nodes can be operated in one DeviceNet network.
The extension of the network depends on the selected baud rate (125 kBaud,
250 kBaud or 500 kBaud).
In contrast to other fieldbus systems, CAN does not address the modules con-
nected to the bus but identifies the messages. Whenever the bus is free, sub-
scribers are allowed to send messages. Each bus subscriber decides on its own
when it wants to send data or instigate other bus subscribers to send data. This
permits a communication without a bus master assembly group.
Bus conflicts are solved in that the messages are assigned a certain priority.
This priority is defined by the CAN identifier, called Connection ID in De-
viceNet. The following rule applies: the smaller the identifier, the higher the
priority.
A general distinction between high priority process messages (I/O Messages)
and low priority management messages (Explicit Messages) is done before.
Messages having a data length of more than 8 bytes can be fragmented.
The communication with DeviceNet occurs always connection-referenced
(connection based). All data and functions of a device are described by means
of an object model. Therefore, for a message exchange directly after switching
on a device, the connections to the desired subscriber have to be established
first and communication objects be created or allocated. Message distribution
is according to the broadcast system, data exchange according to the producer
consumer model.
A transmitting DeviceNet node produces data that is either consumed via a
point-to-point connection (1 to 1) by one receiving node, or via a multicast
connection (1 to n) by several receiving nodes.
Further information
The Open DeviceNet Vendor Association (ODVA) provides further docu-
ments in the Internet under: http://www.odva.org
WAGO-I/O-SYSTEM 750
DeviceNet