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dominant or recessive .
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Error Handling
BOSCH
Part B - page 59
7 ERROR HANDLING
7.1 Error Detection
There are 5 different error types (which are not mutually exclusive):
" BIT ERROR
A unit that is sending a bit on the bus also monitors the bus. A BIT ERROR has to
be detected at that bit time, when the bit value that is monitored is different from the
bit value that is sent. An exception is the sending of a recessive bit during the
stuffed bit stream of the ARBITRATION FIELD or during the ACK SLOT. Then no
BIT ERROR occurs when a dominant bit is monitored. A TRANSMITTER sending
a PASSIVE ERROR FLAG and detecting a dominant bit does not interpret this as
a BIT ERROR.
" STUFF ERROR
A STUFF ERROR has to be detected at the bit time of the 6th consecutive equal bit
level in a message field that should be coded by the method 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 detected, if the calculated result is not the same as that received
in the CRC sequence.
" FORM ERROR
A FORM ERROR has to be detected when a fixed-form bit field contains one or
more illegal bits. (Note, that for a Receiver a dominant bit during the last bit of END
OR FRAME is not treated as FORM ERROR).
" ACKNOWLEDGMENT ERROR
An ACKNOWLEDGMENT ERROR has to be detected by a transmitter whenever it
does not monitor a dominant bit during the ACK SLOT.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Error Handling
BOSCH
Part B - page 60
7.2 Error Signalling
A station detecting an error condition signals this by transmitting an ERROR FLAG. For
an error active node it is an ACTIVE ERROR FLAG, for an error passive node it is a
PASSIVE ERROR FLAG. Whenever a BIT ERROR, a STUFF ERROR, a FORM
ERROR or an ACKNOWLEDGMENT ERROR is detected by any station, transmission
of an ERROR FLAG is started at the respective station at the next bit.
Whenever a CRC ERROR is detected, transmission of an ERROR FLAG starts at the
bit following the ACK DELIMITER, unless an ERROR FLAG for another condition has
already been started.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Fault Confinement
BOSCH
Part B - page 61
8 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 and sends an
ACTIVE ERROR FLAG when an error has been detected.
An error passive unit must not send an ACTIVE ERROR FLAG. It takes part in bus
communication, but when an error has been detected 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 allowed to have any influence on the bus. (E.g. output drivers
switched off.)
For fault confinement two counts are implemented in every bus unit:
1) TRANSMIT ERROR COUNT
2) RECEIVE ERROR COUNT
These counts are modified according to the following rules:
(note that more than one rule may apply during a given message transfer)
1. When a RECEIVER detects an error, the RECEIVE ERROR COUNT will be
increased by 1, except when the detected error was a BIT ERROR during the
sending of an ACTIVE ERROR FLAG or an OVERLOAD FLAG.
2. When a RECEIVER detects a dominant bit as the first bit after sending an ERROR
FLAG the RECEIVE ERROR COUNT will be increased by 8.
3. When a TRANSMITTER sends an ERROR FLAG the TRANSMIT ERROR COUNT
is increased by 8.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Fault Confinement
BOSCH
Part B - page 62
Exception 1:
If the TRANSMITTER is error passive and detects an ACKNOWLEDGMENT
ERROR because of not detecting a dominant ACK and does not detect a
dominant bit while sending its PASSIVE ERROR FLAG.
Exception 2:
If the TRANSMITTER sends an ERROR FLAG because a STUFF ERROR occurred
during ARBITRATION, and should have been recessive , and has been sent as
recessive but monitored as dominant .
In exceptions 1 and 2 the TRANSMIT ERROR COUNT is not changed.
4. If an TRANSMITTER detects a BIT ERROR while sending an ACTIVE ERROR
FLAG or an OVERLOAD FLAG the TRANSMIT ERROR COUNT is increased by 8.
5. If an RECEIVER detects a BIT ERROR while sending an ACTIVE ERROR FLAG or
an OVERLOAD FLAG the RECEIVE ERROR COUNT is increased by 8.
6. Any node tolerates up to 7 consecutive dominant bits after sending an ACTIVE
ERROR FLAG, PASSIVE ERROR FLAG or OVERLOAD FLAG. After detecting the
14th consecutive dominant bit (in case of an ACTIVE ERROR FLAG or an
OVERLOAD FLAG) or after detecting the 8th consecutive dominant bit following a
PASSIVE ERROR FLAG, and after each sequence of additional eight consecutive
dominant bits every TRANSMITTER increases its TRANSMIT ERROR COUNT by
8 and every RECEIVER increases its RECEIVE ERROR COUNT by 8.
7. After the successful transmission of a message (getting ACK and no error until END
OF FRAME is finished) the TRANSMIT ERROR COUNT is decreased by 1 unless it
was already 0.
8. After the successful reception of a message (reception without error up to the ACK
SLOT and the successful sending of the ACK bit), the RECEIVE ERROR COUNT is
decreased by 1, if it was between 1 and 127. If the RECEIVE ERROR COUNT was
0, it stays 0, and if it was greater than 127, then it will be set to a value between 119
and 127.
9. A node is error passive when the TRANSMIT ERROR COUNT equals or exceeds
128, or when the RECEIVE ERROR COUNT equals or exceeds 128. An error
condition letting a node become error passive causes the node to send an ACTIVE
ERROR FLAG.
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Fault Confinement
BOSCH
Part B - page 63
10.A node is bus off when the TRANSMIT ERROR COUNT is greater than or equal to
256.
11.An error passive node becomes error active again when both the TRANSMIT
ERROR COUNT and the RECEIVE ERROR COUNT are less than or equal to 127.
12.An node which is bus off is permitted to become error active (no longer bus off )
with its error counters both set to 0 after 128 occurrence of 11 consecutive
recessive bits have been monitored on the bus.
Note:
An error count value greater than about 96 indicates a heavily disturbed bus. It may be
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