Basic concepts and structural principles of CAN data bus

The data bus is a channel for data communication between nodes and can be regarded as an information highway. It provides an effective means of information transmission in the vehicle electrical system.

The CAN data bus has the characteristic of multiplex transmission, which allows multiple pieces of information to be transmitted simultaneously on the same channel or line. This parallel transmission method improves the efficiency of communication.

The principle of serial communication:

The CAN data bus uses serial communication to send or receive data bit by bit on the same data line. This serial communication method helps reduce the complexity of the wiring harness and improve the reliability of data transmission.

Synchronous communication is a communication method that transmits data continuously, in which multiple characters of data can be transmitted in one communication, which is called frame information. The CAN data bus uses synchronous communication to effectively support continuous data transmission.

The CAN data bus uses a half-duplex format, which means that both parties in the communication have a transmitter and a receiver. Both parties can send and receive, but not at the same time. This method helps avoid conflicts and ensures smooth communication.

Cyclic redundancy check is a commonly used check method in CAN data bus. Its basic principle is to regard a data block as a long binary number, then divide it by a specific number, and attach the remainder as a check code to the data block and send it together. The receiving end uses the same operation to check whether the transmission is wrong, thereby improving the reliability of data transmission.

The structural principle of CAN data bus It consists of two main parts: hardware and software. The hardware part consists of CAN transceiver, data transmission terminal and data transmission line.

The software part is the core of the communication system and is responsible for defining the communication rules, namely the communication protocol, including communication speed, format, priority, etc.

In the CAN communication system, all control units have the same rights and can occupy the bus to send and receive.

In order to ensure the order of communication, the software part has arbitration function and error disconnection function. The arbitration mechanism ensures the smooth transmission of high-priority data, while the error disconnection function allows the node to actively withdraw from the bus after a communication error occurs to prevent it from affecting other nodes.

To reduce interference, CAN transmission lines use twisted pair cables, with the CAN-H line in yellow and the CAN-L line in green.

A car usually uses 2~3 CAN communication systems, one of which is used for the drive system high-speed CAN with a rate of 500kb/s, which is suitable for control units with high real-time requirements, such as engines and motors;

The other is a low-speed CAN for the body system with a rate of 100 kb/s. It is used for body control, such as signal collection and feedback of lights, doors, windows, etc. It is characterized by many signals but low real-time requirements and low implementation costs.

Both CAN communication systems use the voltage difference between two wires to transmit signals.

During the maintenance of the CAN data bus, it is necessary to ensure that there are no untwisted wire segments larger than 50 mm, and the distance between two adjacent repair points must be at least 100 mm to avoid interference. Such maintenance requirements help to ensure the stability and reliability of the CAN data bus.

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