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BOSCH CAN Specification Version 2.0 1991, Robert Bosch GmbH, Postfach 30 02 40, D-70442 Stuttgart

BOSCH CAN Specification 2.0 Sep. 1991 page 1 Recital introduction of serial communication The acceptance and to more and more applications has led to requirements that the assignment of message identifiers to communication functions be standardized for certain applications. These applications can be realized with CAN more comfortably, if the address range that originally has been defined by 11 identifier bits is enlarged Therefore a second message format (’extended format’) is introduced that provides a larger address range defined by 29 bits. This will relieve the system designer from compromises with respect to defining well-structured naming schemes. Users of CAN who do not need the identifier range offered by the extended format, can rely on the conventional 11 bit identifier range (’standard format’) further on. In this case they can make use of the CAN implementations that are already available on the market, or of new controllers that implement both formats. In order to distinguish standard and extended format the first reserved bit of the CAN message format, as it is defined in CAN Specification 1.2, is used. This is done in such a way that the message format in CAN Specification 1.2 is equivalent to the standard format and therefore is still valid. Furthermore, the extended format has been defined so that messages in standard format and extended format can coexist within the same network. This CAN Specification consists of two parts, with • Part A describing the CAN message format as it is defined in CAN Specification 1.2; • Part B describing both standard and extended message formats. In order to be compatible with this CAN Specification 2.0 it is required that a CAN implementation be compatible with either Part A or Part B. Note CAN implementations that are designed according to part A of this or according to previous CAN Specifications, and CAN implementations that are designed according to part B of this specification can communicate with each other as long as it is not made use of the extended format. ROBERT BOSCH GmbH, Postfach 30 02 40, D-70442 Stuttgart

PART A

BOSCH Contents Sep. 1991 Part A - page 3 1 2 3 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.2 4 5 6 6.1 6.2 7 8 9 INTRODUCTION................................................................................4 BASIC CONCEPTS............................................................................5 MESSAGE TRANSFER .....................................................................10 Frame Types ......................................................................................10 DATA FRAME ....................................................................................10 REMOTE FRAME ..............................................................................15 ERROR FRAME.................................................................................16 OVERLOAD FRAME..........................................................................17 INTERFRAME SPACING...................................................................18 Definition of TRANSMITTER/RECEIVER ..........................................20 MESSAGE VALIDATION ...................................................................21 CODING.............................................................................................22 ERROR HANDLING...........................................................................23 Error Detection ...................................................................................23 Error Signalling...................................................................................23 FAULT CONFINEMENT.....................................................................24 BIT TIMING REQUIREMENTS ..........................................................27 INCREASING CAN OSCILLATOR TOLERANCE..............................31 9.1 Protocol Modifications ........................................................................31 ROBERT BOSCH GmbH, Postfach 30 02 40, D-70442 Stuttgart

BOSCH Introduction Sep. 1991 Part A - page 4 1 INTRODUCTION The Controller Area Network (CAN) is a serial communications protocol which efficiently supports distributed realtime control with a very high level of security. Its domain of application ranges from high speed networks to low cost multiplex wiring. In automotive electronics, engine control units, sensors, anti-skid-systems, etc. are connected using CAN with bitrates up to 1 Mbit/s. At the same time it is cost effective to build into vehicle body electronics, e.g. lamp clusters, electric windows etc. to replace the wiring harness otherwise required. The intention of this specification is to achieve compatibility between any two CAN implementations. Compatibility, however, has different aspects regarding e.g. electrical features and the interpretation of data to be transferred. To achieve design transparency and implementation flexibility CAN has been subdivided into different layers. • • • the (CAN-) object layer the (CAN-) transfer layer the physical layer The object layer and the transfer layer comprise all services and functions of the data link layer defined by the ISO/OSI model. The scope of the object layer includes • • • finding which messages are to be transmitted deciding which messages received by the transfer layer are actually to be used, providing an interface to the application layer related hardware. There is much freedom in defining object handling. The scope of the transfer layer mainly is the transfer protocol, i.e. controlling the framing, performing arbitration, error checking, error signalling and fault confinement. Within the transfer layer it is decided whether the bus is free for starting a new transmission or whether a reception is just starting. Also some general features of the bit timing are regarded as part of the transfer layer. It is in the nature of the transfer layer that there is no freedom for modifications. The scope of the physical layer is the actual transfer of the bits between the different nodes with respect to all electrical properties. Within one network the physical layer, of course, has to be the same for all nodes. There may be, however, much freedom in selecting a physical layer. The scope of this specification is to define the transfer layer and the consequences of the CAN protocol on the surrounding layers. ROBERT BOSCH GmbH, Postfach 30 02 40, D-70442 Stuttgart

BOSCH Basic Concepts 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 Application Layer 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 ROBERT BOSCH GmbH, Postfach 30 02 40, D-70442 Stuttgart

BOSCH Basic Concepts 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. ROBERT BOSCH GmbH, Postfach 30 02 40, D-70442 Stuttgart

BOSCH Basic Concepts 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 2 or more units start transmitting messages at the same time, the bus access conflict is resolved by bitwise arbitration using the IDENTIFIER. The mechanism of arbitration guarantees that neither information nor time is lost. If a DATA FRAME and a REMOTE FRAME with the same IDENTIFIER are initiated at the same time, the DATA FRAME prevails over the REMOTE FRAME. During arbitration every transmitter compares the level of the bit transmitted with the level that is monitored on the bus. If these levels are equal the unit may continue to send. When a ’recessive’ level is sent and a ’dominant’ level is monitored (see Bus Values), the unit has lost arbitration and must withdraw without sending one more bit. Safety In order to achieve the utmost safety of data transfer, powerful measures for error detection, signalling and self-checking are implemented in every CAN node. Error Detection For detecting errors the following measures have been taken: - Monitoring (transmitters compare the bit levels to be transmitted with the bit levels detected on the bus) - Cyclic Redundancy Check - Bit Stuffing - Message Frame Check ROBERT BOSCH GmbH, Postfach 30 02 40, D-70442 Stuttgart

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