Rainbow Electronics CAN интерфейс - Manual (Page 25)

BOSCH
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
Part A - page 23
6 ERROR HANDLING
6.1 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.
• ACKNOWLEDGMENT ERROR
An ACKNOWLEDGMENT ERROR has to be detected by a transmitter whenever it
does not monitor a ’dominant’ bit during the ACK SLOT.
6.2 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.
Error Handling