Balluff BDG F-NH Series, Configuration Manual

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5 General in formation abo ut CAN
5.1 CAN physica l and trans port layer
CAN is a field bus. It operates with the CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) method.
It means that collisions during bus access are avoided by a so called bitwise arbitration. The bits are coded NRZ-
L (Non Return to Zero - Low).
A cyclic redundancy check (CRC) and other safety mechanisms provide a secure transmission. For
synchronisation a mechanism called "bit stuffing" is used. CAN is a multi-master system, i.e. several equal bus
nodes can be connected without a bus master supervising the communication. In principle a CAN bus can be
realized with copper wire or in fibre optic cable.
The common CAN implementation with copper wire operates with differential signals, transmitted via two wires:
CAN
HIGH
, CAN
LOW
. Therefore CAN has a good common mode rejection ratio.
Data is transmitted with bits that can either be dominant or recessive. The dominant (0) always overwrites the
recessive (1).
The topology of a CAN network is a line, which can be extended by stubs. The maximum length of a stub is
limited to 0,5m.
The network always has to be terminated on both ends with 120Ohms each (between CAN
HIGH
und CAN
LOW
).
Other locations or values are not allowed.
The arbitration mentioned before is used to control the bus access of the nodes by prioritization of the CAN-
Identifier of the different messages. Every node monitors the bus. If more than on node wants access on the bus,
the node with the highest priority of the messages ID succeeds and the other nodes retreat until there is "silence"
on the bus (see
Fehler! Verweisquelle konnte nicht gefunden werden. ). Technically the first dominate bit of
the ID send overwrites the corresponding recessive bit of the other IDs. In case that more than one node uses the
same CAN-ID an error occurs only at a collision within the rest of the frame. On principle a CAN-ID should only be
used by a single node!
Figure 5.1: Example of the arbitration
Due to the arbitration there is a ranking of the messages. The message with the lowest ID has the highest priority
and therefore it has almost instant access on the bus. The exception is that an ongoing transmission will not be
interrupted. So time critical messages should be assigned to the high priority CAN-IDs, but even then there is no
determination in the time of transmission (non-deterministic transmission).