Beckhoff EtherCAT EL6752 Documentation page 29

DeviceNet communication
Configuration
The node address is set in the range from 0 to 63 using two decimally coded rotary switches. The data
transfer rate set at the DeviceNet scanner is automatically recognized by the DeviceNet Box (auto baud
rate). "Electronic Data Sheets" (EDS files) for DeviceNet configuration tools are available for download from
the Beckhoff internet site (http://www.beckhoff.de), and on the BECKHOFF product CDs. Special I/O
parameters that are not covered by the DeviceNet standard can be set via the KS2000 software (serial
connection) or via acyclical explicit messages.
Diagnostics
The extensive diagnostic functions of the BECKHOFF DeviceNet devices allow rapid fault localisation. The
diagnostic messages are transmitted over the bus and collated by the master. The status of the network
connection, the device status, the status of the inputs and outputs and of the power supply are displayed by
LEDs.
Data transfer rates
Three data transfer rates from 125 kbaud to 500 kbaud are available for different bus lengths. The effective
utilization of the bus bandwidth allows DeviceNet to achieve short system reaction times at relatively low
data rates.
Topology
DeviceNet is based on a linear topology. The number of devices participating in each network is logically
limited by DeviceNet to 64, but physically the present generation of drivers allows up to 64 nodes in one
network segment. The maximum possible size of the network for any particular data rate is limited by the
signal propagation delay required on the bus medium. For 500 kbaud, for instance, the network may extend
100 m, whereas at 125 kbaud the network may reach up to 500 m. At low data rates the size of the network
can be increased by repeaters, which also allow the construction of tree structures.
Bus access procedures
CAN utilizes the Carrier Sense Multiple Access (CSMA) procedure, i.e. all participating devices have the
same right of access to the bus and may access it as soon as it is free (multi-master bus access). The
exchange of messages is thus not device-oriented but message-oriented. This means that every message is
unambiguously marked with a prioritized identifier. In order to avoid collisions on the bus when messages
are sent by different devices, a bit-wise bus arbitration is carried out at the start of the data transmission. The
bus arbitration assigns bus bandwidth to the messages in the sequence of their priority. At the end of the
arbitration phase only one bus device occupies the bus, collisions are avoided and the bandwidth is optimally
exploited.
Configuration and parameterization
The TwinCAT System Manager allows all the DeviceNet parameters to be set conveniently. An "eds" file
(electronic data sheet) is available on the BECKHOFF website (http://www.beckhoff.de) for the
parameterization of BECKHOFF DeviceNet devices using configuration tools from other manufacturers.
EL6752 Version: 2.1 29