BOSCH. CAN Specification. Version , Robert Bosch GmbH, Postfach , D Stuttgart

Transcription

1 CAN Specification Version , Robert Bosch GmbH, Postfach , D Stuttgart

2 CAN Specification 2.0 page 1 Recital The acceptance an introuction of serial communication to more an more applications has le to requirements that the assignment of message ientifiers to communication functions be stanarize for certain applications. These applications can be realize with CAN more comfortably, if the aress range that originally has been efine by 11 ientifier bits is enlarge Therefore a secon message format ( extene format ) is introuce that provies a larger aress range efine by 29 bits. This will relieve the system esigner from compromises with respect to efining well-structure naming schemes. Users of CAN who o not nee the ientifier range offere by the extene format, can rely on the conventional 11 bit ientifier range ( stanar format ) further on. In this case they can make use of the CAN implementations that are alreay available on the market, or of new controllers that implement both formats. In orer to istinguish stanar an extene format the first reserve bit of the CAN message format, as it is efine in CAN Specification 1.2, is use. This is one in such a way that the message format in CAN Specification 1.2 is equivalent to the stanar format an therefore is still vali. Furthermore, the extene format has been efine so that messages in stanar format an extene format can coexist within the same network. This CAN Specification consists of two parts, with Part A escribing the CAN message format as it is efine in CAN Specification 1.2; Part B escribing both stanar an extene message formats. In orer to be compatible with this CAN Specification 2.0 it is require that a CAN implementation be compatible with either Part A or Part B. Note CAN implementations that are esigne accoring to part A of this or accoring to previous CAN Specifications, an CAN implementations that are esigne accoring to part B of this specification can communicate with each other as long as it is not mae use of the extene format.

3 PART A

4 Contents Part A - page 3 1 INTRODUCTION BASIC CONCEPTS MESSAGE TRANSFER Frame Types DATA FRAME REMOTE FRAME ERROR FRAME OVERLOAD FRAME INTERFRAME SPACING Definition of TRANSMITTER/RECEIVER MESSAGE VALIDATION CODING ERROR HANDLING Error Detection Error Signalling FAULT CONFINEMENT BIT TIMING REQUIREMENTS INCREASING CAN OSCILLATOR TOLERANCE Protocol Moifications...31

5 1 INTRODUCTION Introuction Part A - page 4 The Controller Area Network (CAN) is a serial communications protocol which efficiently supports istribute realtime control with a very high level of security. Its omain of application ranges from high spee networks to low cost multiplex wiring. In automotive electronics, engine control units, sensors, anti-ski-systems, etc. are connecte using CAN with bitrates up to 1 Mbit/s. At the same time it is cost effective to buil into vehicle boy electronics, e.g. lamp clusters, electric winows etc. to replace the wiring harness otherwise require. The intention of this specification is to achieve compatibility between any two CAN implementations. Compatibility, however, has ifferent aspects regaring e.g. electrical features an the interpretation of ata to be transferre. To achieve esign transparency an implementation flexibility CAN has been subivie into ifferent layers. the (CAN-) object layer the (CAN-) transfer layer the physical layer The object layer an the transfer layer comprise all services an functions of the ata link layer efine by the ISO/OSI moel. The scope of the object layer inclues fining which messages are to be transmitte eciing which messages receive by the transfer layer are actually to be use, proviing an interface to the application layer relate harware. There is much freeom in efining object hanling. The scope of the transfer layer mainly is the transfer protocol, i.e. controlling the framing, performing arbitration, error checking, error signalling an fault confinement. Within the transfer layer it is ecie 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 regare as part of the transfer layer. It is in the nature of the transfer layer that there is no freeom for moifications. The scope of the physical layer is the actual transfer of the bits between the ifferent noes with respect to all electrical properties. Within one network the physical layer, of course, has to be the same for all noes. There may be, however, much freeom in selecting a physical layer. The scope of this specification is to efine the transfer layer an the consequences of the CAN protocol on the surrouning layers.

6 2 BASIC CONCEPTS CAN has the following properties prioritization of messages guarantee of latency times configuration flexibility Basic Concepts Part A - page 5 multicast reception with time synchronization system wie ata consistency multimaster error etection an signalling automatic retransmission of corrupte messages as soon as the bus is ile again istinction between temporary errors an permanent failures of noes an autonomous switching off of efect noes Layere Structure of a CAN Noe Application Layer Object Layer - Message Filtering - Message an Status Hanling Transfer Layer - Fault Confinement - Error Detection an Signalling - Message Valiation - Acknowlegment - Arbitration - Message Framing - Transfer Rate an Timing Physical Layer - Signal Level an Bit Representation - Transmission Meium

7 Basic Concepts Part A - page 6 The Physical Layer efines how signals are actually transmitte. Within this specification the physical layer is not efine so as to allow transmission meium an signal level implementations to be optimize for their application. The Transfer Layer represents the kernel of the CAN protocol. It presents messages receive to the object layer an accepts messages to be transmitte from the object layer. The transfer layer is responsible for bit timing an synchronization, message framing, arbitration, acknowlegment, error etection an signalling, an fault confinement. The Object Layer is concerne with message filtering as well as status an message hanling. The scope of this specification is to efine the transfer layer an the consequences of the CAN protocol on the surrouning layers. Messages Information on the bus is sent in fixe format messages of ifferent but limite length (see section 3: Message Transfer). When the bus is free any connecte unit may start to transmit a new message. Information Routing In CAN systems a CAN noe oes not make use of any information about the system configuration (e.g. station aresses). This has several important consequences. System Flexibility: Noes can be ae to the CAN network without requiring any change in the software or harware of any noe an application layer. Message Routing: The content of a message is name by an IDENTIFIER. The IDENTIFIER oes not inicate the estination of the message, but escribes the meaning of the ata, so that all noes in the network are able to ecie by MESSAGE FILTERING whether the ata is to be acte upon by them or not. Multicast: As a consequence of the concept of MESSAGE FILTERING any number of noes can receive an simultaneously act upon the same message. Data Consistency: Within a CAN network it is guarantee that a message is simultaneously accepte either by all noes or by no noe. Thus ata consistency of a system is achieve by the concepts of multicast an by error hanling.

8 Basic Concepts Part A - page 7 Bit rate The spee of CAN may be ifferent in ifferent systems. However, in a given system the bitrate is uniform an fixe. Priorities The IDENTIFIER efines a static message priority uring bus access. Remote Data Request By sening a REMOTE FRAME a noe requiring ata may request another noe to sen the corresponing DATA FRAME. The DATA FRAME an the corresponing REMOTE FRAME are name 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 transmitte 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 resolve by bitwise arbitration using the IDENTIFIER. The mechanism of arbitration guarantees that neither information nor time is lost. If a DATA FRAME an a REMOTE FRAME with the same IDENTIFIER are initiate at the same time, the DATA FRAME prevails over the REMOTE FRAME. During arbitration every transmitter compares the level of the bit transmitte with the level that is monitore on the bus. If these levels are equal the unit may continue to sen. When a recessive level is sent an a ominant level is monitore (see Bus Values), the unit has lost arbitration an must withraw without sening one more bit. Safety In orer to achieve the utmost safety of ata transfer, powerful measures for error etection, signalling an self-checking are implemente in every CAN noe. Error Detection For etecting errors the following measures have been taken: - Monitoring (transmitters compare the bit levels to be transmitte with the bit levels etecte on the bus) - Cyclic Reunancy Check - Bit Stuffing - Message Frame Check

9 Basic Concepts Part A - page 8 Performance of Error Detection The error etection mechanisms have the following properties: - all global errors are etecte. - all local errors at transmitters are etecte. - up to 5 ranomly istribute errors in a message are etecte. - burst errors of length less than 15 in a message are etecte. - errors of any o number in a message are etecte. Total resiual error probability for unetecte corrupte messages: less than message error rate * 4.7 * Error Signalling an Recovery Time Corrupte messages are flagge by any noe etecting an error. Such messages are aborte an will be retransmitte automatically. The recovery time from etecting an error until the start of the next message is at most 29 bit times, if there is no further error. Fault Confinement CAN noes are able to istinguish short isturbances from permanent failures. Defective noes are switche off. Connections The CAN serial communication link is a bus to which a number of units may be connecte. This number has no theoretical limit. Practically the total number of units will be limite by elay times an/or electrical loas on the bus line. Single Channel The bus consists of a single channel that carries bits. From this ata resynchronization information can be erive. The way in which this channel is implemente is not fixe in this specification. E.g. single wire (plus groun), two ifferential wires, optical fibres, etc. Bus values The bus can have one of two complementary logical values: ominant or recessive. During simultaneous transmission of ominant an recessive bits, the resulting bus value will be ominant. For example, in case of a wire-and implementation of the bus, the ominant level woul be represente by a logical 0 an the recessive level by a logical 1. Physical states (e.g. electrical voltage, light) that represent the logical levels are not given in this specification.

10 Basic Concepts Part A - page 9 Acknowlegment All receivers check the consistency of the message being receive an will acknowlege a consistent message an flag an inconsistent message. Sleep Moe / Wake-up To reuce the system s power consumption, a CAN-evice may be set into sleep moe without any internal activity an with isconnecte bus rivers. The sleep moe is finishe with a wake-up by any bus activity or by internal conitions of the system. On wake-up, the internal activity is restarte, although the transfer layer will be waiting for the system s oscillator to stabilize an it will then wait until it has synchronize itself to the bus activity (by checking for eleven consecutive recessive bits), before the bus rivers are set to "on-bus" again. In orer to wake up other noes of the system, which are in sleep-moe, a special wake-up message with the eicate, lowest possible IDENTIFIER (rrr rrr rrrr; r = recessive = ominant ) may be use.

11 Message Transfer Part A - page 10 3 MESSAGE TRANSFER 3.1 Frame Types Message transfer is manifeste an controlle by four ifferent frame types: A DATA FRAME carries ata from a transmitter to the receivers. A REMOTE FRAME is transmitte by a bus unit to request the transmission of the DATA FRAME with the same IDENTIFIER. An ERROR FRAME is transmitte by any unit on etecting a bus error. An OVERLOAD FRAME is use to provie for an extra elay between the preceing an the succeeing DATA or REMOTE FRAMEs. DATA FRAMEs an REMOTE FRAMEs are separate from preceing frames by an INTERFRAME SPACE DATA FRAME A DATA FRAME is compose of seven ifferent bit fiels: START OF FRAME, ARBITRATION FIELD, CONTROL FIELD, DATA FIELD, CRC FIELD, ACK FIELD, END OF FRAME. The DATA FIELD can be of length zero. Interframe Space DATA FRAME Interframe Space Start of Frame or Overloa Frame Arbitration Fiel Control Fiel Data Fiel CRC Fiel ACK Fiel En of Frame

12 Data Frame Part A - page 11 START OF FRAME marks the beginning of DATA FRAMES an REMOTE FRAMEs. It consists of a single ominant bit. A station is only allowe to start transmission when the bus is ile (see BUS IDLE). All stations have to synchronize to the leaing ege cause by START OF FRAME (see HARD SYNCHRONIZATION ) of the station starting transmission first. ARBITRATION FIELD The ARBITRATION FIELD consists of the IDENTIFIER an the RTR-BIT. Interframe Space Start of Frame ARBITRATION FIELD Control Fiel Ientifier RTR Bit IDENTIFIER The IDENTIFIER s length is 11 bits. These bits are transmitte in the orer from ID-10 to ID-0. The least significant bit is ID-0. The 7 most significant bits (ID-10 - ID-4) must not be all recessive. RTR BIT Remote Transmission Request BIT In DATA FRAMEs the RTR BIT has to be ominant. Within a REMOTE FRAME the RTR BIT has to be recessive. CONTROL FIELD The CONTROL FIELD consists of six bits. It inclues the DATA LENGTH CODE an two bits reserve for future expansion. The reserve bits have to be sent ominant. Receivers accept ominant an recessive bits in all combinations. DATA LENGTH CODE The number of bytes in the DATA FIELD is inicate by the DATA LENGTH CODE. This DATA LENGTH CODE is 4 bits wie an is transmitte within the CONTROL FIELD.

13 Data Frame Part A - page 12 Arbitration Fiel CONTROL FIELD Data Fiel r1 r0 DLC3 DLC2 DLC1 DLC0 or CRC Fiel reserve bits Data Length Coe Coing of the number of ata bytes by the DATA LENGTH CODE abbreviations: ominant r recessive Number of Data Bytes Data Length Coe DLC3 DLC2 DLC1 DLC0 0 1 r 2 r 3 r r 4 r 5 r r 6 r r 7 r r r 8 r DATA FRAME: amissible numbers of ata bytes: {0,1,...,7,8}. Other values may not be use.

14 Data Frame Part A - page 13 DATA FIELD The DATA FIELD consists of the ata to be transferre within a DATA FRAME. It can contain from 0 to 8 bytes, which each contain 8 bits which are transferre MSB first. CRC FIELD contains the CRC SEQUENCE followe by a CRC DELIMITER. Data or Control Fiel CRC FIELD Ack Fiel CRC Sequence CRC Delimiter CRC SEQUENCE The frame check sequence is erive from a cyclic reunancy coe best suite for frames with bit counts less than 127 bits (BCH Coe). In orer to carry out the CRC calculation the polynomial to be ivie is efine as the polynomial, the coefficients of which are given by the estuffe bit stream consisting of START OF FRAME, ARBITRATION FIELD, CONTROL FIELD, DATA FIELD (if present) an, for the 15 lowest coefficients, by 0. This polynomial is ivie (the coefficients are calculate moulo-2) by the generator-polynomial: X 15 + X 14 + X 10 + X 8 + X 7 + X 4 + X The remainer of this polynomial ivision is the CRC SEQUENCE transmitte over the bus. In orer to implement this function, a 15 bit shift register CRC_RG(14:0) can be use. If NXTBIT enotes the next bit of the bit stream, given by the estuffe bit sequence from START OF FRAME until the en of the DATA FIELD, the CRC SEQUENCE is calculate as follows: CRC_RG = 0; REPEAT CRCNXT = NXTBIT EXOR CRC_RG(14); CRC_RG(14:1) = CRC_RG(13:0); CRC_RG(0) = 0; // initialize shift register // shift left by // 1 position

15 Data Frame Part A - page 14 IF CRCNXT THEN CRC_RG(14:0) = CRC_RG(14:0) EXOR (4599hex); ENDIF UNTIL (CRC SEQUENCE starts or there is an ERROR conition) After the transmission / reception of the last bit of the DATA FIELD, CRC_RG contains the CRC sequence. CRC DELIMITER The CRC SEQUENCE is followe by the CRC DELIMITER which consists of a single recessive bit. ACK FIELD The ACK FIELD is two bits long an contains the ACK SLOT an the ACK DELIMITER. In the ACK FIELD the transmitting station sens two recessive bits. A RECEIVER which has receive a vali message correctly, reports this to the TRANSMITTER by sening a ominant bit uring the ACK SLOT (it sens ACK ). CRC Fiel ACK FIELD En of Frame ACK Slot ACK Delimiter ACK SLOT All stations having receive the matching CRC SEQUENCE report this within the ACK SLOT by superscribing the recessive bit of the TRANSMITTER by a ominant bit. ACK DELIMITER The ACK DELIMITER is the secon bit of the ACK FIELD an has to be a recessive bit. As a consequence, the ACK SLOT is surroune by two recessive bits (CRC DELIMITER, ACK DELIMITER). END OF FRAME Each DATA FRAME an REMOTE FRAME is elimite by a flag sequence consisting of seven recessive bits.

16 Remote Frame Part A - page REMOTE FRAME A station acting as a RECEIVER for certain ata can initiate the transmission of the respective ata by its source noe by sening a REMOTE FRAME. A REMOTE FRAME is compose of six ifferent bit fiels: START OF FRAME, ARBITRATION FIELD, CONTROL FIELD, CRC FIELD, ACK FIELD, END OF FRAME. Contrary to DATA FRAMEs, the RTR bit of REMOTE FRAMEs is recessive. There is no DATA FIELD, inepenent of the values of the DATA LENGTH CODE which may be signe any value within the amissible range The value is the DATA LENGTH CODE of the corresponing DATA FRAME. Inter Frame Space REMOTE FRAME Inter Frame Space or Overloa Frame Start of Frame Arbitration Fiel Control Fiel CRC Fiel ACK Fiel En of Frame The polarity of the RTR bit inicates whether a transmitte frame is a DATA FRAME (RTR bit ominant ) or a REMOTE FRAME (RTR bit recessive ).

17 Error Frame Part A - page ERROR FRAME The ERROR FRAME consists of two ifferent fiels. The first fiel is given by the superposition of ERROR FLAGs contribute from ifferent stations. The following secon fiel is the ERROR DELIMITER. Data Frame Error Flag ERROR FRAME Interframe Space or Overloa Frame superposition of Error Flags Error Delimiter In orer to terminate an ERROR FRAME correctly, an error passive noe may nee the bus to be bus ile for at least 3 bit times (if there is a local error at an error passive receiver). Therefore the bus shoul not be loae to 100%. ERROR FLAG There are 2 forms of an ERROR FLAG: an ACTIVE ERROR FLAG an a PASSIVE ERROR FLAG. 1. The ACTIVE ERROR FLAG consists of six consecutive ominant bits. 2. The PASSIVE ERROR FLAG consists of six consecutive recessive bits unless it is overwritten by ominant bits from other noes. An error active station etecting an error conition signals this by transmission of an ACTIVE ERROR FLAG. The ERROR FLAG s form violates the law of bit stuffing (see CODING) applie to all fiels from START OF FRAME to CRC DELIMITER or estroys the fixe form ACK FIELD or END OF FRAME fiel. As a consequence, all other stations etect an error conition an on their part start transmission of an ERROR FLAG. So the sequence of ominant bits which actually can be monitore on the bus results from a superposition of ifferent ERROR FLAGs transmitte by iniviual stations. The total length of this sequence varies between a minimum of six an a maximum of twelve bits. An error passive station etecting an error conition tries to signal this by transmission of a PASSIVE ERROR FLAG. The error passive station waits for six consecutive bits

18 Overloa Frame Part A - page 17 of equal polarity, beginning at the start of the PASSIVE ERROR FLAG. The PASSIVE ERROR FLAG is complete when these 6 equal bits have been etecte. ERROR DELIMITER The ERROR DELIMITER consists of eight recessive bits. After transmission of an ERROR FLAG each station sens recessive bits an monitors the bus until it etects a recessive bit. Afterwars it starts transmitting seven more recessive bits OVERLOAD FRAME The OVERLOAD FRAME contains the two bit fiels OVERLOAD FLAG an OVERLOAD DELIMITER. There are two kins of OVERLOAD conitions, which both lea to the transmission of an OVERLOAD FLAG: 1. The internal conitions of a receiver, which requires a elay of the next DATA FRAME or REMOTE FRAME. 2. Detection of a ominant bit uring INTERMISSION. The start of an OVERLOAD FRAME ue to OVERLOAD conition 1 is only allowe to be starte at the first bit time of an expecte INTERMISSION, whereas OVERLOAD FRAMEs ue to OVERLOAD conition 2 start one bit after etecting the ominant bit. En of Frame or Error Delimiter or Overloa Delimiter Overloa Flag OVERLOAD FRAME Inter Frame Space or Overloa Frame superposition of Overloa Flags Overloa Delimiter At most two OVERLOAD FRAMEs may be generate to elay the next DATA or REMOTE FRAME.

19 Overloa Frame Part A - page 18 OVERLOAD FLAG consists of six ominant bits. The overall form correspons to that of the ACTIVE ERROR FLAG. The OVERLOAD FLAG s form estroys the fixe form of the INTERMISSION fiel. As a consequence, all other stations also etect an OVERLOAD conition an on their part start transmission of an OVERLOAD FLAG. (In case that there is a ominant bit etecte uring the 3r bit of INTERMISSION locally at some noe, the other noes will not interpret the OVERLOAD FLAG correctly, but interpret the first of these six ominant bits as START OF FRAME. The sixth ominant bit violates the rule of bit stuffing causing an error conition). OVERLOAD DELIMITER consists of eight recessive bits. The OVERLOAD DELIMITER is of the same form as the ERROR DELIMITER. After transmission of an OVERLOAD FLAG the station monitors the bus until it etects a transition from a ominant to a recessive bit. At this point of time every bus station has finishe sening its OVERLOAD FLAG an all stations start transmission of seven more recessive bits in coincience INTERFRAME SPACING DATA FRAMEs an REMOTE FRAMEs are separate from preceing frames whatever type they are (DATA FRAME, REMOTE FRAME, ERROR FRAME, OVERLOAD FRAME) by a bit fiel calle INTERFRAME SPACE. In contrast, OVERLOAD FRAMEs an ERROR FRAMEs are not precee by an INTERFRAME SPACE an multiple OVERLOAD FRAMEs are not separate by an INTERFRAME SPACE. INTERFRAME SPACE contains the bit fiels INTERMISSION an BUS IDLE an, for error passive stations, which have been TRANSMITTER of the previous message, SUSPEND TRANSMISSION.

20 Interframe Space Part A - page 19 For stations which are not error passive or have been RECEIVER of the previous message: Frame INTERFRAME SPACE Frame Intermission Bus Ile For error passive stations which have been TRANSMITTER of the previous message: Frame INTERFRAME SPACE Frame Intermission Bus Ile Suspen Transmission INTERMISSION consists of three recessive bits. During INTERMISSION no station is allowe to start transmission of a DATA FRAME or REMOTE FRAME. The only action to be taken is signalling an OVERLOAD conition. BUS IDLE The perio of BUS IDLE may be of arbitrary length. The bus is recognize to be free an any station having something to transmit can access the bus. A message, which is pening for transmission uring the transmission of another message, is starte in the first bit following INTERMISSION. The etection of a ominant bit on the bus is interprete as a START OF FRAME. SUSPEND TRANSMISSION After an error passive station has transmitte a message, it sens eight recessive bits following INTERMISSION, before starting to transmit a further message or recognizing the bus to be ile. If meanwhile a transmission (cause by another station) starts, the station will become receiver of this message.

21 Transmitter / Receiver Part A - page Definition of TRANSMITTER / RECEIVER TRANSMITTER A unit originating a message is calle TRANSMITTER of that message. The unit stays TRANSMITTER until the bus is ile or the unit loses ARBITRATION. RECEIVER A unit is calle RECEIVER of a message, if it is not TRANSMITTER of that message an the bus is not ile.

22 Message Valiation Part A - page 21 4 MESSAGE VALIDATION The point of time at which a message is taken to be vali, is ifferent for the transmitter an the receivers of the message. Transmitter: The message is vali for the transmitter, if there is no error until the en of END OF FRAME. If a message is corrupte, retransmission will follow automatically an accoring to prioritization. In orer to be able to compete for bus access with other messages, retransmission has to start as soon as the bus is ile. Receivers: The message is vali for the receivers, if there is no error until the last but one bit of END OF FRAME.

23 5 CODING BIT STREAM CODING Coing Part A - page 22 The frame segments START OF FRAME, ARBITRATION FIELD, CONTROL FIELD, DATA FIELD an CRC SEQUENCE are coe by the metho of bit stuffing. Whenever a transmitter etects five consecutive bits of ientical value in the bit stream to be transmitte it automatically inserts a complementary bit in the actual transmitte bit stream. The remaining bit fiels of the DATA FRAME or REMOTE FRAME (CRC DELIMITER, ACK FIELD, an END OF FRAME) are of fixe form an not stuffe. The ERROR FRAME an the OVERLOAD FRAME are of fixe form as well an not coe by the metho of bit stuffing. The bit stream in a message is coe accoring to the Non-Return-to-Zero (NRZ) metho. This means that uring the total bit time the generate bit level is either ominant or recessive.

24 6 ERROR HANDLING 6.1 Error Detection Error Hanling Part A - page 23 There are 5 ifferent error types (which are not mutually exclusive): BIT ERROR A unit that is sening a bit on the bus also monitors the bus. A BIT ERROR has to be etecte at that bit time, when the bit value that is monitore is ifferent from the bit value that is sent. An exception is the sening of a recessive bit uring the stuffe bit stream of the ARBITRATION FIELD or uring the ACK SLOT. Then no BIT ERROR occurs when a ominant bit is monitore. A TRANSMITTER sening a PASSIVE ERROR FLAG an etecting a ominant bit oes not interpret this as a BIT ERROR. STUFF ERROR A STUFF ERROR has to be etecte at the bit time of the 6th consecutive equal bit level in a message fiel that shoul be coe by the metho of bit stuffing. CRC ERROR The CRC sequence consists of the result of the CRC calculation by the transmitter. The receivers calculate the CRC in the same way as the transmitter. A CRC ERROR has to be etecte, if the calculate result is not the same as that receive in the CRC sequence. FORM ERROR A FORM ERROR has to be etecte when a fixe-form bit fiel contains one or more illegal bits. ACKNOWLEDGMENT ERROR An ACKNOWLEDGMENT ERROR has to be etecte by a transmitter whenever it oes not monitor a ominant bit uring the ACK SLOT. 6.2 Error Signalling A station etecting an error conition signals this by transmitting an ERROR FLAG. For an error active noe it is an ACTIVE ERROR FLAG, for an error passive noe it is a PASSIVE ERROR FLAG. Whenever a BIT ERROR, a STUFF ERROR, a FORM ERROR or an ACKNOWLEDGMENT ERROR is etecte by any station, transmission of an ERROR FLAG is starte at the respective station at the next bit. Whenever a CRC ERROR is etecte, transmission of an ERROR FLAG starts at the bit following the ACK DELIMITER, unless an ERROR FLAG for another conition has alreay been starte.

25 Fault Confinement Part A - page 24 7 FAULT CONFINEMENT With respect to fault confinement a unit may be in one of three states: error active error passive bus off An error active unit can normally take part in bus communication an sens an ACTIVE ERROR FLAG when an error has been etecte. An error passive unit must not sen an ACTIVE ERROR FLAG. It takes part in bus communication but when an error has been etecte only a PASSIVE ERROR FLAG is sent. Also after a transmission, an error passive unit will wait before initiating a further transmission. (See SUSPEND TRANSMISSION) A bus off unit is not allowe to have any influence on the bus. (E.g. output rivers switche off.) For fault confinement two counts are implemente in every bus unit: 1) TRANSMIT ERROR COUNT 2) RECEIVE ERROR COUNT These counts are moifie accoring to the following rules: (note that more than one rule may apply uring a given message transfer) 1. When a RECEIVER etects an error, the RECEIVE ERROR COUNT will be increase by 1, except when the etecte error was a BIT ERROR uring the sening of an ACTIVE ERROR FLAG or an OVERLOAD FLAG. 2. When a RECEIVER etects a ominant bit as the first bit after sening an ERROR FLAG the RECEIVE ERROR COUNT will be increase by When a TRANSMITTER sens an ERROR FLAG the TRANSMIT ERROR COUNT is increase by 8. Exception 1: If the TRANSMITTER is error passive an etects an ACKNOWLEDGMENT

26 Fault Confinement Part A - page 25 ERROR because of not etecting a ominant ACK an oes not etect a ominant bit while sening its PASSIVE ERROR FLAG. Exception 2: If the TRANSMITTER sens an ERROR FLAG because a STUFF ERROR occurre uring ARBITRATION whereby the STUFFBIT is locate before the RTR bit, an shoul have been recessive, an has been sent as recessive but monitore as ominant. In exceptions 1 an 2 the TRANSMIT ERROR COUNT is not change. 4. If an TRANSMITTER etects a BIT ERROR while sening an ACTIVE ERROR FLAG or an OVERLOAD FLAG the TRANSMIT ERROR COUNT is increase by If an RECEIVER etects a BIT ERROR while sening an ACTIVE ERROR FLAG or an OVERLOAD FLAG the RECEIVE ERROR COUNT is increase by Any noe tolerates up to 7 consecutive ominant bits after sening an ACTIVE ERROR FLAG, PASSIVE ERROR FLAG or OVERLOAD FLAG. After etecting the 14th consecutive ominant bit (in case of an ACTIVE ERROR FLAG or an OVERLOAD FLAG) or after etecting the 8th consecutive ominant bit following a PASSIVE ERROR FLAG, an after each sequence of aitional eight consecutive ominant bits every TRANSMITTER increases its TRANSMIT ERROR COUNT by 8 an every RECEIVER increases its RECEIVE ERROR COUNT by After the successful transmission of a message (getting ACK an no error until END OF FRAME is finishe) the TRANSMIT ERROR COUNT is ecrease by 1 unless it was alreay After the successful reception of a message (reception without error up to the ACK SLOT an the successful sening of the ACK bit), the RECEIVE ERROR COUNT is ecrease by 1, if it was between 1 an 127. If the RECEIVE ERROR COUNT was 0, it stays 0, an if it was greater than 127, then it will be set to a value between 119 an A noe is error passive when the TRANSMIT ERROR COUNT equals or excees 128, or when the RECEIVE ERROR COUNT equals or excees 128. An error conition letting a noe become error passive causes the noe to sen an ACTIVE ERROR FLAG.