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FRAME; the sixth dominant bit violates the rule of bit stuffing causing an error
condition.
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 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.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Interframe Space
BOSCH
Part B - page 53
3.2.5 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.
For stations which are not error passive or have been RECEIVER of the previous
message:
INTERFRAME SPACE
Frame Frame
Intermission Bus Idle
For error passive stations which have been TRANSMITTER of the previous message:
INTERFRAME SPACE
Frame Frame
Bus Idle
Suspend Transmission
Intermission
INTERMISSION
consists of three recessive bits.
During INTERMISSION the only action to be taken is signalling an OVERLOAD
condition and no station is allowed to actively start transmission of a DATA FRAME or
REMOTE FRAME.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Interframe Space
BOSCH
Part B - page 54
Note:
If a CAN node has a message waiting for transmission and it samples a dominant
bit at the third bit of INTERMISSION, it will interpret this as a START OF FRAME
bit, and, with the next bit, start transmitting its message with the first bit of its
IDENTIFIER without first transmitting a START OF FRAME bit and without
becoming receiver.
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 300240, D-7000 Stuttgart 30
Sep. 1991
Conformance
BOSCH
Part B - page 55
3.3 Conformance with regard to Frame Format
The Standard Format is equivalent to the Data/Remote Frame Format as it is described
in the CAN Specification 1.2. In contrast the Extended Format is a new feature of the
CAN protocol. In order to allow the design of relatively simple controllers, the
implementation of the Extended Format to its full extend is not required (e.g. send
messages or accept data from messages in Extended Format), whereas the Standard
Format must be supported without restriction.
New controllers are considered to be in conformance with this CAN Specification, if
they have at least the following properties with respect to the Frame Formats defined in
3.1 and 3.2:
- Every new controller supports the Standard Format;
- Every new controller can receive messages of the Extended Format. This requires
that Extended Frames are not destroyed just because of their format. It is, however,
not required that the Extended Format must be supported by new controllers.
3.4 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.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Message Filtering
BOSCH
Part B - page 56
4 MESSAGE FILTERING
Message filtering is based upon the whole Identifier. Optional mask registers that allow
any Identifier bit to be set don t care for message filtering, may be used to select
groups of Identifiers to be mapped into the attached receive buffers.
If mask registers are implemented every bit of the mask registers must be
programmable, i.e. they can be enabled or disabled for message filtering. The length of
the mask register can comprise the whole IDENTIFIER or only part of it.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Message Validation
BOSCH
Part B - page 57
5 MESSAGE VALIDATION
The point of time at which a message is taken to be valid, is different for the transmitter
and the receivers of the message.
Transmitter:
The message is valid for the transmitter, if there is no error until the end of END OF
FRAME. If a message is corrupted, retransmission will follow automatically and
according to prioritization. In order to be able to compete for bus access with other
messages, retransmission has to start as soon as the bus is idle.
Receivers:
The message is valid for the receivers, if there is no error until the last but one bit of
END OF FRAME. The value of the last bit of END OF FRAME is treated as don t care ,
a dominant value does not lead to a FORM ERROR (cf. section 7.1).
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Coding
BOSCH
Part B - page 58
6 CODING
BIT STREAM CODING
The frame segments START OF FRAME, ARBITRATION FIELD, CONTROL FIELD,
DATA FIELD and CRC SEQUENCE are coded by the method of bit stuffing. Whenever
a transmitter detects five consecutive bits of identical value in the bit stream to be
transmitted it automatically inserts a complementary bit in the actual transmitted bit
stream.
The remaining bit fields of the DATA FRAME or REMOTE FRAME (CRC DELIMITER,
ACK FIELD, and END OF FRAME) are of fixed form and not stuffed. The ERROR
FRAME and the OVERLOAD FRAME are of fixed form as well and not coded by the
method of bit stuffing.
The bit stream in a message is coded according to the Non-Return-to-Zero (NRZ)
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