CAN Bus Workshop_Version 03__06-2008_EN.pdf - 第33页
1 - 7 S tudent Guide CAN BUS W orkshop Edition 0 6 /2008 2 Comm unication and Control 7 2.2. 1 Gene ral st ructu re The CAN B us is a dece ntral mul ti-mas ter bu s. The d ata are tran smitte d via the differenti al vo l…
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Student Guide CAN BUS Workshop
2 Communication and Control Edition 06/2008
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2.2 CAN Bus
The development of CAN began when more and more electronic devices were implemented into
modern motor vehicles. Examples of such devices include engine management systems, active
suspension, ABS, gear control, lighting control, air conditioning, airbags and central locking. All
this means more safety and more comfort for the driver.
Fig. 2.2 - 1 communication via cable connection
To improve the behavior of the vehicle even further, it was necessary for the different control sys-
tems (and their sensors) to exchange information. This was usually done by discrete interconnec-
tion of the different systems (i.e. point to point wiring). The requirement for information exchange
has then grown to such an extent that a cable network with a length of up to several miles and
many connectors was required. This produced growing problems concerning material cost, pro-
duction time and reliability.
The solution to this problem was the connection of the control systems via a serial bus system.
This bus had to fulfill some special requirements due to its usage in a vehicle. With the use of CAN,
point-to-point wiring is replaced by one serial bus connecting all control systems. This is accom-
plished by adding some CAN-specific hardware to each control unit that provides the ’rules’ or pro-
tocol for transmitting- and receiving information via the bus.
Fig. 2.2 - 2 Communication via CAN bus on example car controlling
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Student Guide CAN BUS Workshop
Edition 06/2008 2 Communication and Control
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2.2.1 General structure
The CAN Bus is a decentral multi-master bus. The data are transmitted via the differential voltage
of the two CAN_High and CAN_Low lines, which are each fitted with a terminating resistance of
120 Ohm.
Fig. 2.2 - 3 CAN Bus structure
Fig. 2.2 - 4 CAN Bus -Controller and Microcontroller
Legend
– Microcontroller: Exchanges data with the CAN controller
– CAN controller: Adds the data frame, establishes the connection and manages errors.
– Transmitter/receiver: Adjusts the level (driver levels)
Each bus node has a CAN controller, which can transmit and receive data if the bus is free.
This CAN controller communicates with a microcontroller. The microcontroller steers and controls
the relevant CAN bus nodes.
A CAN Bus node can only transmit if the bus is free i.e. if there is no communication taking place
with other nodes. Access to the CAN BUS is fixed in the CAN protocol (identifier). This results in
differing priorities among the individual CAN bus nodes.
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Student Guide CAN BUS Workshop
2 Communication and Control Edition 06/2008
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2.2.1.1 CAN Bus protocol
Fig. 2.2 - 5 CAN-bus protocol
Start: This bit indicates the beginning of a telegram and is a dominant bit. After this bit is set, no
other user of the CAN bus is able to send.
Address field (11 bit identifier): The 11 bit address identifier value determines the bus access.
The lower value has the highest priority.
Control field: The 4 lowest bits in the 6 bit field show the data length of the following data field in
bytes (DLC: Data Length Code.
Data field: Contains the information actually required and can be from 0 byte to 8 byte. The trans-
fer of a byte begins with the most significant bit (the bit with the highest value).
Data control field CRC: Consists of a 15 bit check sequence (CRC sequence + CRC delimiter =
CRC Field - Cyclic Redundancy Check) and a recessive delimiter bit. The redundant information
in the control sequence allows the receiver to check whether the message received has been fal-
sified by interference.
End: Each data telegram is terminated by a sequence of 7 recessive bits.
2.2.1.2 11 Bit Identifier
Fig. 2.2 - 6 11 bit identifier
The CAN bus system is using the 11 Bit identifier for addressing the different CAN objects
An 11 Bit identifier (address) identifies the type, priority, source and /or target of the message.
This identifier also controls the bus access (arbitration).
start
address
(11 bit identifier)
control
information.
data (0-8 bytes user information) CRC
end