BECKHOFF FC5101 - Manual (Page 10)

Eiserstraße 5 / D-33415 Verl / Telefon 05246/963-0 / Telefax 05246/963-149
10
Topology
CAN is based on a linear topology. The number of devices participating in each network is logically limited by
CANopen to 128, but physically the present generation of drivers allows up to 64 nodes in one network seg-
ment. The maximum possible size of the network for any particular data rate is limited by the signal transit time
required on the bus medium. For 1 Mbaud, for instance, the network may extend 25 m, whereas at 50 kbaud
the network may reach up to 1000 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 utilises 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 prioritised 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 prioritisation phase only one
bus device occupies the bus, collisions are avoided and the bandwidth is optimally exploited.
Configuration and parameterisation
The TwinCAT System Manager allows all the CANopen parameters to be set conveniently. An "eds" file (an
electronic data sheet) is available on the Beckhoff website for the parameterisation of Beckhoff CANopen de-
vices using configuration tools from other manufacturers.
Certification
The Beckhoff CANopen devices have a powerful implementation of the protocol, and are certified by CiA, the
CAN in Automation association.