Baumuller b maXX 1000, Руководство по параметрам

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Basic of CAN
Parameter manual b maXX
®
1000
Document no. 5.07004.03 Baumüller Nürnberg GmbH
170
of 208
B.3
B.3 Basic of CAN
The CAN field bus is implemented using a line structure. A three-wire cable provides the physical basis of data
transfer with the connections CAN_High, CAN_Low and CAN_Ground. CAN employs transmissions balanced
to ground for the suppression of common mode interference. For this reason, the difference signals are eval-
uated.
Network
CAN is a multi-master network. Every participant can have access to and be active on the bus with equal pri-
ority. CAN uses object-oriented addressing, i. e. the transferred message is identified by an identifier defined
network-wide. The identifier represents the encoded name of the message.
Bus access
Bus access takes place using the CSMA/CA method (Carrier Sense Multiple Access / Collision Avoidance).
Because after detecting the necessary bus quiescence every participant has the right to begin transmitting a
message, collisions can occur. This is avoided by means of bit-by-bit arbitration of the messages to be sent.
During this, two bus levels are differentiated, a dominant level, logical bit value 0 and a recessive level, logical
bit value 1. In the worst case, all participants wishing to transmit simultaneously begin sending their messages
onto the bus. If a recessive bit of a participant is overwritten by a dominant bit of another, then the “recessive”
node withdraws from the bus and, after detecting bus quiescence, attempts once more to transmit its message.
Consequently, it is guaranteed that the most important and highest priority message (with the lowest identifier)
will be transmitted in a collision-free manner and without delay. For this reason it is of course necessary that
every identifier is permitted to be placed onto the CAN bus only once.
Identifier
In the CAN specification CAN 2.0A 2032 (CiA), different identifiers are available. Each participant can transmit
in a requested manner (multi-master capability). A transmitter sends its message to all CAN nodes (broadcast)
and each then decides on the basis of the identifier whether they will continue processing the message or not.
Error
Up to 8 bytes of user data can be transmitted within a CAN data message frame. For error or overload signal-
ing, a CAN node can send error or overload message frames. This occurs on Layer 2 of the OSI/ISO reference
model, the Data Link layer, therefore independently of the application. Due to high quality error detection and
handling on Layer 2, a Hamming distance (a measure of error detection) of HD=6 is achieved, i. e. a maximum
of 5 simultaneously-occurring bit errors within a message frame will be reliably detected as an error.