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30 02 40, D-70442 Stuttgart Sep.
1991 Part A - page
5 2 BASIC CONCEPTS CAN has the following properties ? prioritization of messages ? guarantee of latency times ? configuration flexibility ? multicast reception with time synchronization ? system wide data consistency ? multimaster ? error detection and signalling ? automatic retransmission of corrupted messages as soon as the bus is idle again ? distinction between temporary errors and permanent failures of nodes and autonomous switching off of defect nodes Layered Structure of a CAN Node Object Layer - Message Filtering - Message and Status Handling Transfer Layer - Fault Confinement - Error Detection and Signalling - Message Validation - Acknowledgment - Arbitration - Message Framing - Transfer Rate and Timing Physical Layer - Signal Level and Bit Representation - Transmission Medium Application Layer Basic Concepts BOSCH ROBERT BOSCH GmbH, Postfach
30 02 40, D-70442 Stuttgart 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 BOSCH ROBERT BOSCH GmbH, Postfach
30 02 40, D-70442 Stuttgart Sep.
1991 Part A - page
7 Bit rate The speed of CAN may be different in different systems. However, in a given system the bitrate is uniform and fixed. Priorities The IDENTIFIER defines a static message priority during bus access. Remote Data Request By sending a REMOTE FRAME a node requiring data may request another node to send the corresponding DATA FRAME. The DATA FRAME and the corresponding REMOTE FRAME are named by the same IDENTIFIER. Multimaster When the bus is free any unit may start to transmit a message. The unit with the message of higher priority to be transmitted gains bus access. Arbitration Whenever the bus is free, any unit may start to transmit a message. If