CAN bus - Computer Busses - page 326

21.3.1 Bus values
The CAN bus defines two levels for bits on the bus, these are:

• Dominant level - all nodes on the bus connect to the common bus. A dominant bit will
always overrule a recessive bit. In a wired-AND implementation of the bus, a dominant
level is represented by a logical 0 and the recessive level by a logical 1.
• Recessive level - a recessive level on the bus is always overruled by a dominant level. If
all levels are at a recessive level, the resulting level will also be recessive. In a wired-OR
connection a recessive level will be a logical 0 (as a single logical 1 level will make the
output also a logical 1).

Message bit streams uses a non-return-to zero (NRZ) technique. The frame segments start of
frame, arbitration field, control field, data field and CRC sequence are coded using bit stuff-
ing. When the transmitter detects five consecutive bits of identical values in the bit stream, it
automatically adds an extra complementary bit in the actually transmitted bit stream. When
the receiver detects this is automatically deletes the inserted bit.
In data frames and remote frames, the CRC delimiter, ACK field and end of frame are not
stuffed. Error frames and overload frames are not bit stuffed.
21.3.2 Error detection

The main error detection methods are:
• Monitoring bit levels - transmitters check the transmitted level with the level on the bus.
• Cyclic redundancy check.
• Bit stuffing.
• Message frame check.

The error detection scheme detects the following:

• All global errors.
• All local errors at transmitters.
• Up to five randomly distributed errors in a message.
• Burst errors of length less that 15 in a message.

10 -11 .

The resultant probability of undeleted errors is less than 4.7 ×

21.4 Message transfer

Message transfer involves four different frame types:

• Data frame - contains data from a transmitter to a number of receivers.
• Remote frame - transmitted by a unit to request the transmission of the data frame with
the same identifier.