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Communication and Control CAN Bus CAN Bus Protocol S tudent Guide (FSE) SI PL ACE 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…

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:

4.3.2 CAN Bus Protocol [

J

102]

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).

Communication and Control

CAN Bus Protocol CAN Bus

Student Guide (FSE) SIPLACE X Series and X4I

Edition 01/2009 EN Communication and Control

103

4.3.2.2 CAN Bus Arbitration

Arbitration (arbitration means decision)

In CAN networks, there is no addressing of subscribers or stations in the conventional sense, but

instead, prioritized messages are transmitted. Whenever the bus is free, any unit may start to transmit a

message. In general, a subscriber can only occupy the bus if this is free. The bus subscriber can detect

the bus occupation state by analyzing a certain time period within which the bus must be inactive.

When multiple nodes begin to send a message at the same time, a selection phase (arbitration phase)

is used to decide which node may remain on the bus.

Bus access conflicts are resolved by including a message arbitration field (as a default the 11 bit

identifier is used).

The basis of bit-wise arbitration is the differentiation of 2 physical bus levels, a dominant one (low) and

a recessive bit (high).

A free bus is always on the recessive level. A 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, the unit has lost arbitration and must

withdraw without sending one more bit. At the end of arbitration, the only subscriber left on the bus is the

one whose message has the lowest identifier value (logical zero is a dominant level). The lower the

identifier value is, the higher the priority of a message is.

When the bus is free any unit may start to transmit a message. The message sent by this subscriber is

not destroyed here i.e. it is a loss-free arbitration.

4-14: Flow chart bus arbitration