Rainbow Electronics CAN интерфейс - Manual (Page 41)

BOSCH
ROBERT BOSCH GmbH, Postfach 300240, D-7000 Stuttgart 30
Sep. 1991
Part B - page 39
Remote Data Request
By sending a REMOTE FRAME a node requiring data may request another node to
send the corresponding DATA FRAME. The DATA FRAME and the corresponding
REMOTE FRAME are named by the same IDENTIFIER.
Multimaster
When the bus is free any unit may start to transmit a message. The unit with the
message of higher priority to be transmitted gains bus access.
Arbitration
Whenever the bus is free, any unit may start to transmit a message. If 2 or more units
start transmitting messages at the same time, the bus access conflict is resolved by
bitwise arbitration using the IDENTIFIER. The mechanism of arbitration guarantees that
neither information nor time is lost. If a DATA FRAME and a REMOTE FRAME with the
same IDENTIFIER are initiated at the same time, the DATA FRAME prevails over the
REMOTE FRAME. During arbitration every transmitter compares the level of the bit
transmitted with the level that is monitored on the bus. If these levels are equal the unit
may continue to send. When a ’recessive’ level is sent and a ’dominant’ level is
monitored (see Bus Values), the unit has lost arbitration and must withdraw without
sending one more bit.
Safety
In order to achieve the utmost safety of data transfer, powerful measures for error
detection, signalling and self-checking are implemented in every CAN node.
Error Detection
For detecting errors the following measures have been taken:
- Monitoring (transmitters compare the bit levels to be transmitted with the bit
levels detected on the bus)
- Cyclic Redundancy Check
- Bit Stuffing
- Message Frame Check
Performance of Error Detection
The error detection mechanisms have the following properties:
- all global errors are detected.
- all local errors at transmitters are detected.
- up to 5 randomly distributed errors in a message are detected.
- burst errors of length less than 15 in a message are detected.
- errors of any odd number in a message are detected.
Basic Concepts