Rainbow Electronics CAN User Manual
BOSCH
CAN Specification
Version 2.0
1991, Robert Bosch GmbH, Postfach 50, D-7000 Stuttgart 1
The document as a whole may be copied and distributed without
restrictions. However, the usage of it in parts or as a whole in other
documents needs the consent of Robert Bosch GmbH. Robert Bosch
GmbH retains the right to make changes to this document without
notice and does not accept any liability for errors.
Imported into Framemaker 4 by:
Chuck Powers, Motorola MCTG Multiplex Applications, April 5,1995.
Sep. 1991
BOSCH
The acceptance and introduction of serial communication 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.
CAN Specification 2.0
Recital
page 1
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 300240, D-7000 Stuttgart 30
PART A
Sep. 1991
BOSCH
1 INTRODUCTION................................................................................4
2 BASIC CONCEPTS............................................................................5
3 MESSAGE TRANSFER.....................................................................10
3.1 Frame Types......................................................................................10
3.1.1 DATA FRAME....................................................................................10
3.1.2 REMOTE FRAME ..............................................................................15
3.1.3 ERROR FRAME.................................................................................16
Contents
Part A - page 3
3.1.4 OVERLOAD FRAME..........................................................................17
3.1.5 INTERFRAME SPACING...................................................................18
3.2 Definition of TRANSMITTER/RECEIVER ..........................................20
4 MESSAGE VALIDATION...................................................................21
5 CODING.............................................................................................22
6 ERROR HANDLING...........................................................................23
6.1 Error Detection...................................................................................23
6.2 Error Signalling...................................................................................23
7 FAULT CONFINEMENT.....................................................................24
8 BIT TIMING REQUIREMENTS..........................................................27
9 INCREASING CAN OSCILLATOR TOLERANCE..............................31
9.1 Protocol Modifications........................................................................31
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
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.
Introduction
Part A - page 4
• 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 50, D-7000 Stuttgart 1
BOSCH
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
Basic Concepts
Sep. 1991
Part A - page 5
• 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 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
• 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.
Basic Concepts
Part A - page 6
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:
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.
As a consequence of the concept of MESSAGE FILTERING any
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
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.
Basic Concepts
Part A - page 7
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 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
Performance of Error Detection
The error detection mechanisms have the following properties:
- all global errors are detected.
- all local errors at transmitters are detected.
- up to 5 randomly distributed errors in a message are detected.
- burst errors of length less than 15 in a message are detected.
- errors of any odd number in a message are detected.
Total residual error probability for undetected corrupted messages: less than
message error rate * 4.7 * 10
Error Signalling and Recovery Time
Corrupted messages are flagged by any node detecting an error. Such messages are
aborted and will be retransmitted automatically. The recovery time from detecting an
error until the start of the next message is at most 29 bit times, if there is no further
error.
Basic Concepts
-11
Part A - page 8
.
Fault Confinement
CAN nodes are able to distinguish short disturbances from permanent failures.
Defective nodes are switched off.
Connections
The CAN serial communication link is a bus to which a number of units may be
connected. This number has no theoretical limit. Practically the total number of units
will be limited by delay times and/or electrical loads on the bus line.
Single Channel
The bus consists of a single channel that carries bits. From this data resynchronization
information can be derived. The way in which this channel is implemented is not fixed
in this specification. E.g. single wire (plus ground), two differential wires, optical fibres,
etc.
Bus values
The bus can have one of two complementary logical values: ’dominant’ or ’recessive’.
During simultaneous transmission of ’dominant’ and ’recessive’ bits, the resulting bus
value will be ’dominant’. For example, in case of a wired-AND implementation of the
bus, the ’dominant’ level would be represented by a logical ’0’ and 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.
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
Acknowledgment
All receivers check the consistency of the message being received and will
acknowledge a consistent message and flag an inconsistent message.
Sleep Mode / Wake-up
To reduce the system’s power consumption, a CAN-device may be set into sleep mode
without any internal activity and with disconnected bus drivers. The sleep mode is
finished with a wake-up by any bus activity or by internal conditions of the system. On
wake-up, the internal activity is restarted, although the transfer layer will be waiting for
the system’s oscillator to stabilize and it will then wait until it has synchronized itself to
the bus activity (by checking for eleven consecutive ’recessive’ bits), before the bus
drivers are set to "on-bus" again.
In order to wake up other nodes of the system, which are in sleep-mode, a special
wake-up message with the dedicated, lowest possible IDENTIFIER (rrr rrrd rrrr; r =
’recessive’ d = ’dominant’) may be used.
Basic Concepts
Part A - page 9
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
Part A - page 10
BOSCH
Message Transfer
3 MESSAGE TRANSFER
3.1 Frame Types
Message transfer is manifested and controlled by four different frame types:
A DATA FRAME carries data from a transmitter to the receivers.
A REMOTE FRAME is transmitted by a bus unit to request the transmission of the
DATA FRAME with the same IDENTIFIER.
An ERROR FRAME is transmitted by any unit on detecting a bus error.
An OVERLOAD FRAME is used to provide for an extra delay between the preceding
and the succeeding DATA or REMOTE FRAMEs.
DATA FRAMEs and REMOTE FRAMEs are separated from preceding frames by an
INTERFRAME SPACE.
3.1.1 DATA FRAME
A DATA FRAME is composed of seven different bit fields:
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
Start of Frame
Arbitration Field
DATA FRAME
Control Field
Data Field
CRC Field
Interframe
Space
or
Overload
Frame
ACK Field
End of Frame
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
START OF FRAME
marks the beginning of DATA FRAMES and REMOTE FRAMEs. It consists of a single
’dominant’ bit.
A station is only allowed to start transmission when the bus is idle (see BUS IDLE). All
stations have to synchronize to the leading edge caused by START OF FRAME (see
’HARD SYNCHRONIZATION’) of the station starting transmission first.
ARBITRATION FIELD
The ARBITRATION FIELD consists of the IDENTIFIER and the RTR-BIT.
Data Frame
Part A - page 11
Interframe
Space
IDENTIFIER
The IDENTIFIER’s length is 11 bits. These bits are transmitted in the order 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 ’dominant’. Within a REMOTE FRAME the
RTR BIT has to be ’recessive’.
Start
of Frame
ARBITRATION FIELD
Identifier
Control
Field
RTR Bit
CONTROL FIELD
The CONTROL FIELD consists of six bits. It includes the DATA LENGTH CODE and
two bits reserved for future expansion. The reserved bits have to be sent ’dominant’.
Receivers accept ’dominant’ and ’recessive’ bits in all combinations.
DATA LENGTH CODE
The number of bytes in the DATA FIELD is indicated by the DATA LENGTH CODE.
This DATA LENGTH CODE is 4 bits wide and is transmitted within the CONTROL
FIELD.
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
BOSCH
Data Frame
Sep. 1991
Part A - page 12
Arbitration
Field
r1 r0 DLC3 DLC2 DLC1 DLC0
bits
CONTROL FIELD
Data Length Codereserved
Coding of the number of data bytes by the DATA LENGTH CODE
abbreviations: d ’dominant’
r ’recessive’
Number of Data
Bytes
0
DLC3 DLC2 DLC1 DLC0
d
Data Length Code
d
d
d
Data
Field
or
CRC
Field
1
2
3
4
5
6
7
8
d
d
d
d
d
d
d
r
d
d
d
r
r
r
r
d
d
r
r
d
d
r
r
d
r
d
r
d
r
d
r
d
DATA FRAME: admissible numbers of data bytes: {0,1,....,7,8}.
Other values may not be used.
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
DATA FIELD
The DATA FIELD consists of the data to be transferred within a DATA FRAME. It can
contain from 0 to 8 bytes, which each contain 8 bits which are transferred MSB first.
CRC FIELD
contains the CRC SEQUENCE followed by a CRC DELIMITER.
Data Frame
Part A - page 13
Data
or
Control
Field
CRC FIELD
CRC Delimiter
CRC Sequence
Ack
Field
CRC SEQUENCE
The frame check sequence is derived from a cyclic redundancy code best suited for
frames with bit counts less than 127 bits (BCH Code).
In order to carry out the CRC calculation the polynomial to be divided is defined as the
polynomial, the coefficients of which are given by the destuffed bit stream consisting of
START OF FRAME, ARBITRATION FIELD, CONTROL FIELD, DATA FIELD (if
present) and, for the 15 lowest coefficients, by 0. This polynomial is divided (the
coefficients are calculated modulo-2) by the generator-polynomial:
X
15
+ X
14
+ X
10
+ X
8
+ X
7
+ X
4
+ X
3
+ 1.
The remainder of this polynomial division is the CRC SEQUENCE transmitted over the
bus. In order to implement this function, a 15 bit shift register CRC_RG(14:0) can be
used. If NXTBIT denotes the next bit of the bit stream, given by the destuffed bit
sequence from START OF FRAME until the end of the DATA FIELD, the CRC
SEQUENCE is calculated as follows:
CRC_RG = 0; // initialize shift register
REPEAT
CRCNXT = NXTBIT EXOR CRC_RG(14);
CRC_RG(14:1) = CRC_RG(13:0); // shift left by
CRC_RG(0) = 0; // 1 position
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
IF CRCNXT THEN
CRC_RG(14:0) = CRC_RG(14:0) EXOR (4599hex);
ENDIF
UNTIL (CRC SEQUENCE starts or there is an ERROR condition)
After the transmission / reception of the last bit of the DATA FIELD, CRC_RG contains
the CRC sequence.
CRC DELIMITER
The CRC SEQUENCE is followed by the CRC DELIMITER which consists of a single
’recessive’ bit.
ACK FIELD
The ACK FIELD is two bits long and contains the ACK SLOT and the ACK DELIMITER.
In the ACK FIELD the transmitting station sends two ’recessive’ bits.
A RECEIVER which has received a valid message correctly, reports this to the
TRANSMITTER by sending a ’dominant’ bit during the ACK SLOT (it sends ’ACK’).
Data Frame
Part A - page 14
CRC
Field
ACK SLOT
All stations having received the matching CRC SEQUENCE report this within the ACK
SLOT by superscribing the ’recessive’ bit of the TRANSMITTER by a ’dominant’ bit.
ACK DELIMITER
The ACK DELIMITER is the second bit of the ACK FIELD and has to be a ’recessive’
bit. As a consequence, the ACK SLOT is surrounded by two ’recessive’ bits (CRC
DELIMITER, ACK DELIMITER).
END OF FRAME
Each DATA FRAME and REMOTE FRAME is delimited by a flag sequence consisting
of seven ’recessive’ bits.
ACK FIELD
ACK Delimiter
ACK Slot
End of
Frame
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
3.1.2 REMOTE FRAME
A station acting as a RECEIVER for certain data can initiate the transmission of the
respective data by its source node by sending a REMOTE FRAME.
A REMOTE FRAME is composed of six different bit fields:
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, independent of the values of the DATA LENGTH CODE which may
be signed any value within the admissible range 0...8. The value is the DATA LENGTH
CODE of the corresponding DATA FRAME.
Remote Frame
Part A - page 15
Inter
Frame
Space
Start of Frame
Arbitration Field
Control Field
REMOTE FRAME
CRC Field
Inter
Frame
Space
or
Overload
Frame
ACK Field
End of Frame
The polarity of the RTR bit indicates whether a transmitted frame is a DATA FRAME
(RTR bit ’dominant’) or a REMOTE FRAME (RTR bit ’recessive’).
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
3.1.3 ERROR FRAME
The ERROR FRAME consists of two different fields. The first field is given by the
superposition of ERROR FLAGs contributed from different stations. The following
second field is the ERROR DELIMITER.
Error Frame
Part A - page 16
Data
Frame
Error Flag
In order to terminate an ERROR FRAME correctly, an ’error passive’ node may need
the bus to be ’bus idle’ for at least 3 bit times (if there is a local error at an ’error
passive’ receiver). Therefore the bus should not be loaded to 100%.
ERROR FLAG
There are 2 forms of an ERROR FLAG: an ACTIVE ERROR FLAG and a PASSIVE
ERROR FLAG.
1. The ACTIVE ERROR FLAG consists of six consecutive ’dominant’ bits.
ERROR FRAME
superposition of
Error Flags
Error Delimiter
Interframe
Space or
Overload
Frame
2. The PASSIVE ERROR FLAG consists of six consecutive ’recessive’ bits unless it
is overwritten by ’dominant’ bits from other nodes.
An ’error active’ station detecting an error condition signals this by transmission of an
ACTIVE ERROR FLAG. The ERROR FLAG’s form violates the law of bit stuffing (see
CODING) applied to all fields from START OF FRAME to CRC DELIMITER or destroys
the fixed form ACK FIELD or END OF FRAME field. As a consequence, all other
stations detect an error condition and on their part start transmission of an ERROR
FLAG. So the sequence of ’dominant’ bits which actually can be monitored on the bus
results from a superposition of different ERROR FLAGs transmitted by individual
stations. The total length of this sequence varies between a minimum of six and a
maximum of twelve bits.
An ’error passive’ station detecting an error condition tries to signal this by transmission
of a PASSIVE ERROR FLAG. The ’error passive’ station waits for six consecutive bits
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
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 detected.
ERROR DELIMITER
The ERROR DELIMITER consists of eight ’recessive’ bits.
After transmission of an ERROR FLAG each station sends ’recessive’ bits and
monitors the bus until it detects a ’recessive’ bit. Afterwards it starts transmitting seven
more ’recessive’ bits.
3.1.4 OVERLOAD FRAME
The OVERLOAD FRAME contains the two bit fields OVERLOAD FLAG and
OVERLOAD DELIMITER.
There are two kinds of OVERLOAD conditions, which both lead to the transmission of
an OVERLOAD FLAG:
Overload Frame
Part A - page 17
1. The internal conditions of a receiver, which requires a delay of the next DATA
FRAME or REMOTE FRAME.
2. Detection of a ’dominant’ bit during INTERMISSION.
The start of an OVERLOAD FRAME due to OVERLOAD condition 1 is only allowed to
be started at the first bit time of an expected INTERMISSION, whereas OVERLOAD
FRAMEs due to OVERLOAD condition 2 start one bit after detecting the ’dominant’ bit.
End of Frame or
Error Delimiter or
Overload Delimiter
Overload
OVERLOAD FRAME
Flag
superposition of
Overload Flags
Inter
Frame
Space or
Overload
Frame
Overload Delimiter
At most two OVERLOAD FRAMEs may be generated to delay the next DATA or
REMOTE FRAME.
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
OVERLOAD FLAG
consists of six ’dominant’ bits. The overall form corresponds to that of the ACTIVE
ERROR FLAG.
The OVERLOAD FLAG’s form destroys the fixed form of the INTERMISSION field. As
a consequence, all other stations also detect an OVERLOAD condition and on their
part start transmission of an OVERLOAD FLAG. (In case that there is a ’dominant’ bit
detected during the 3rd bit of INTERMISSION locally at some node, the other nodes
will not interpret the OVERLOAD FLAG correctly, but interpret the first of these six
’dominant’ bits as START OF FRAME. The sixth ’dominant’ bit violates the rule of bit
stuffing causing an error condition).
OVERLOAD DELIMITER
consists of eight ’recessive’ bits.
Overload Frame
Part A - page 18
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 detects a
transition from a ’dominant’ to a ’recessive’ bit. At this point of time every bus station
has finished sending its OVERLOAD FLAG and all stations start transmission of seven
more ’recessive’ bits in coincidence.
3.1.5 INTERFRAME SPACING
DATA FRAMEs and REMOTE FRAMEs are separated from preceding frames
whatever type they are (DATA FRAME, REMOTE FRAME, ERROR FRAME,
OVERLOAD FRAME) by a bit field called INTERFRAME SPACE. In contrast,
OVERLOAD FRAMEs and ERROR FRAMEs are not preceded by an INTERFRAME
SPACE and multiple OVERLOAD FRAMEs are not separated by an INTERFRAME
SPACE.
INTERFRAME SPACE
contains the bit fields INTERMISSION and BUS IDLE and, for ’error passive’ stations,
which have been TRANSMITTER of the previous message, SUSPEND
TRANSMISSION.
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
For stations which are not ’error passive’ or have been RECEIVER of the previous
message:
Interframe Space
Part A - page 19
Frame
For ’error passive’ stations which have been TRANSMITTER of the previous message:
Frame
INTERFRAME SPACE
Intermission
INTERFRAME SPACE
Intermission
Frame
Bus Idle
Frame
Bus Idle
Suspend Transmission
INTERMISSION
consists of three ’recessive’ bits.
During INTERMISSION no station is allowed to start transmission of a DATA FRAME
or REMOTE FRAME. The only action to be taken is signalling an OVERLOAD
condition.
BUS IDLE
The period of BUS IDLE may be of arbitrary length. The bus is recognized to be free
and any station having something to transmit can access the bus. A message, which is
pending for transmission during the transmission of another message, is started in the
first bit following INTERMISSION.
The detection of a ’dominant’ bit on the bus is interpreted as a START OF FRAME.
SUSPEND TRANSMISSION
After an ’error passive’ station has transmitted a message, it sends eight ’recessive’
bits following INTERMISSION, before starting to transmit a further message or
recognizing the bus to be idle. If meanwhile a transmission (caused by another station)
starts, the station will become receiver of this message.
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
BOSCH
3.2 Definition of TRANSMITTER / RECEIVER
TRANSMITTER
A unit originating a message is called “TRANSMITTER” of that message. The unit stays
TRANSMITTER until the bus is idle or the unit loses ARBITRATION.
RECEIVER
A unit is called “RECEIVER” of a message, if it is not TRANSMITTER of that message
and the bus is not idle.
Transmitter / Receiver
Part A - page 20
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Loading...