Hardware Reference
In-Depth Information
There are five types of errors. These errors are not mutually exclusive.
A node that is sending a bit on the bus also monitors the bus. When the bit value moni-
tored is different from the bit value being sent, the node interprets the situation as an error.
There are two exceptions to this rule.
•
A node that sends a recessive bit during the stuffed bitstream of the arbitration
field or during the ACK slot detects a dominant bit.
•
A transmitter that sends a passive-error flag detects a dominant bit.
A stuff error is detected whenever six consecutive dominant or six consecutive recessive
levels occur in a message field.
The CRC sequence consists of the result of the CRC calculation by the transmitter. The
receiver calculates the CRC in the same way as the transmitter. A CRC error is detected if the
calculated result is not the same as that received in the CRC sequence.
A form error is detected when a fixed-form bit field contains one or more illegal bits. For a
receiver, a dominant bit during the last bit of the end-of-frame field is not treated as a form error.
An acknowledgement error is detected whenever the transmitter does not monitor a domi-
nant bit in the ACK slot.
A node that detects an error condition signals the error by transmitting an error flag.
An error-active node will transmit an
active-error
flag; an error-passive node will transmit a
passive-error
flag. Whenever a node detects a bit error, a stuff error, a form error, or an acknowl-
edgement error, it will start transmission of an error flag at the next bit time. Whenever a CRC
error is detected, transmission of an error flag will start at the bit following the ACK delimiter,
unless an error flag for another error condition has already been started.
A node in error may be in one of three states: error active, error passive, or bus off. An error-
active node can normally take part in bus communication and sends an active-error flag when
an error has been detected. An error-passive node 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.
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