00196044-05 - sg x und x4i fse_en.pdf - 第101页
Communication and Control General Structure CAN Bus S tudent Guide (FSE) SIPL ACE X Series and X4I Edition 01/2009 EN Communication and Control 101 4.3.1 General Structure See also: J 4.3.2 CAN Bus Protocol [ J 102] J …
Communication and Control
CAN Bus Checking the Network Addresses
Student Guide (FSE) SIPLACE X Series and X4I
Communication and Control Edition 01/2009 EN
100
4.3 CAN Bus
The development of CAN began when more and more electronic devices were implemented into modern
motor vehicles. All this means more safety and more comfort for the driver. Examples of such devices
include engine management systems, active control systems such as ABS, gear control, lighting control,
ventilation, air conditioning and central locking.
4-8: Communication via cable connection
To improve the behavior of the vehicle even further, it was necessary for the different control systems
(and their sensors) to exchange information. This was usually done by discrete interconnection 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, production 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 Each module is given a CAN Bus connection. This is accomplished
by adding some CAN-specific hardware to each control unit that provides the ’rules’ or protocol for
transmitting- and receiving information via the bus.
4-9: Communication via CAN bus
Communication and Control
General Structure CAN Bus
Student Guide (FSE) SIPLACE X Series and X4I
Edition 01/2009 EN Communication and Control
101
4.3.1 General Structure
See also:
J
4.3.2 CAN Bus Protocol [
J
102]
J
4.3.2.2 CAN Bus Arbitration [
J
103]
4-10: CAN Bus
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.
4-11: 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.
Communication and Control
CAN Bus CAN Bus Protocol
Student Guide (FSE) SIPLACE X Series and X4I
Communication and Control Edition 01/2009 EN
102
4.3.2 CAN Bus Protocol
4-12: 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 transfer 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 falsified by
interference.
End
Each data telegram is terminated by a sequence of 7 recessive bits.
4.3.2.1 11 Bit Identifier
4-13: 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).