Rainbow Electronics CAN интерфейс - Manual (Page 8)

BOSCH
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
Part A - page 6
• The Physical Layer defines how signals are actually transmitted. Within this
specification the physical layer is not defined so as to allow transmission medium
and signal level implementations to be optimized for their application.
• The Transfer Layer represents the kernel of the CAN protocol. It presents
messages received to the object layer and accepts messages to be transmitted
from the object layer. The transfer layer is responsible for bit timing and
synchronization, message framing, arbitration, acknowledgment, error detection and
signalling, and fault confinement.
• The Object Layer is concerned with message filtering as well as status and
message handling.
The scope of this specification is to define the transfer layer and the consequences of
the CAN protocol on the surrounding layers.
Messages
Information on the bus is sent in fixed format messages of different but limited length
(see section 3: Message Transfer). When the bus is free any connected unit may start
to transmit a new message.
Information Routing
In CAN systems a CAN node does not make use of any information about the system
configuration (e.g. station addresses). This has several important consequences.
System Flexibility: Nodes can be added to the CAN network without requiring
any change in the software or hardware of any node and application layer.
Message Routing: The content of a message is named by an IDENTIFIER. The
IDENTIFIER does not indicate the destination of the message, but describes the
meaning of the data, so that all nodes in the network are able to decide by
MESSAGE FILTERING whether the data is to be acted upon by them or not.
Multicast: As a consequence of the concept of MESSAGE FILTERING any
number of nodes can receive and simultaneously act upon the same message.
Data Consistency: Within a CAN network it is guaranteed that a message is
simultaneously accepted either by all nodes or by no node. Thus data
consistency of a system is achieved by the concepts of multicast and by error
handling.
Basic Concepts