CAN COMMUNICATION SYSTEM: System Description: Description

CAN (Controller Area Network) is a serial communication line for real time application. It is an on-vehicle multiplex communication line with high data communication speed and excellent error detection ability. Many electronic control units are equipped onto a vehicle, and each control unit shares information and links with other control units during operation (not independent). In CAN communication, control units are connected with 2 communication lines (CAN-H line, CAN-L line) allowing a high rate of information transmission with less wiring. Each control unit transmits/receives data but selectively reads required data only.

CAN COMMUNICATION SIGNAL GENERATION

Termination circuits (resistors) are connected across the CAN communication system. When transmitting a CAN communication signal, each control unit passes a current to the CAN-H line and the current returns to the CAN-L line.

Fig 1: CAN Communication Signal Generation Diagram
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The current flows separately into the termination circuits connected across the CAN communication system and the termination circuits drop voltage to generate a potential difference between the CAN-H line and the CAN-L line.
NOTE: A signal with no current passage is called "Recessive" and one with current passage is called "Dominant".

Fig 2: CAN-H Line And CAN-L Line Voltage Graph
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The system produces digital signals for signal communications, by using the potential difference.

Fig 3: CAN Communication Signal Pattern
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THE CONSTRUCTION OF CAN COMMUNICATION SIGNAL (MESSAGE)

Fig 4: CAN Communication Signal Construction Diagram

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No.	Message name	Description
1	Start of frame (1 bit)	Start of message.
2	Arbitration of field (11 bit)	Priorities of message-sending are shown when there is a possibility that multiple messages are sent at the same time.
3	Control field (6 bit)	Signal quantity in data field is shown.
4	Data field (0-64 bit)	Actual signal is shown.
5	CRC field (16 bit)	The transmitting control unit calculates sending data in advance and writes the calculated value in a message. The receiving control unit calculates received data and judges that the data reception is normal when the calculated value is the same as the value written in the sent data.
6	ACK field (2 bit)	The completion of normal reception is sent to the transmitting control unit.
7	End of frame (7 bit)	End of message.

CAN Communication Line

The CAN communication line is a twisted pair wire consisting of strands of CAN-L (1) and CAN-L (2) and has noise immunity.

Fig 5: Identifying CAN-L Twisted Wire

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NOTE: The CAN communication system has the characteristics of noise-resistant because this system produces digital signals by using the potential difference between the CAN-H line and the CAN-L line and has the twisted pair wire structure.

Since the CAN-H line and the CAN-L line are always adjacent to each other, the same degree of noise occurs, respectively, when a noise (1) occurs. Although the noise changes the voltage, the potential difference (2) between the CAN-H line and the CAN-L line is insensitive to noise. Therefore, noise-resistant signals can be obtained.

Fig 6: Noise Resistance Signal Graph

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CAN Signal Communications

Each control unit of the CAN communication system transmits signals through the CAN communication control circuit included in the control unit and receives only necessary signals from each control unit to perform various kinds of control.

Example: Transmitted signals

Fig 7: CAN Communication Function Diagram (Transmitted Signals)
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Example: Received signals

Fig 8: CAN Communication Function Diagram (Received Signals)
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NOTE: The above signal names and signal communications are provided for reference purposes. For CAN communications signals of this vehicle, refer to "CAN COMMUNICATION SYSTEM: CAN COMMUNICATION SIGNAL CHART ".