What is the data transmission process of CAN bus?

CAN bus (Controller Area Network) is a communication protocol used in automobile and industrial control systems. It has high reliability, real-time and flexibility, so it has been widely used in many fields. This article will introduce the data transmission process of CAN bus in detail, including its basic principles, communication mechanism, data frame structure, error handling and arbitration mechanism.

1. Basic principles of CAN bus

CAN bus is a communication protocol based on time division, which uses master-slave mode for data transmission. In the CAN bus system, there are one or more master devices (such as microcontrollers) and multiple slave devices (such as sensors, actuators, etc.). The master device is responsible for initiating communication requests, and the slave device responds to these requests and sends data. The data transmission process of CAN bus can be divided into the following steps:

1.1 Bus Initialization
When the CAN bus system starts, all devices need to be initialized. The initialization process includes setting the parameters of the CAN controller, such as baud rate, bit timing, etc. After the initialization is completed, the CAN bus enters the listening state and waits for data transmission requests.

1.2 Data Request
When the master device needs to obtain data from the slave device, it sends a data request to the CAN bus. The data request includes information such as the address of the target slave device and the requested data type.

1.3 Data Response
After receiving a data request, the slave device will prepare the data according to the content of the request, encapsulate it into a data frame, and then send it to the CAN bus.

1.4 Data Reception
After receiving the data frame, the master device will parse it and obtain the required data. If the data in the data frame is correct, the master device will send a confirmation signal to the slave device, indicating that the data has been successfully received.

1.5 Data transmission ends
After data transmission is completed, the CAN bus enters the idle state and waits for the next data transmission request.

1. CAN bus communication mechanism

2.1 Communication Mode
CAN bus supports multiple communication modes, including single-master mode, multi-master mode and broadcast mode. In single-master mode, only one master device can initiate a communication request; in multi-master mode, multiple master devices can initiate communication requests at the same time; in broadcast mode, the master device sends data to all slave devices, and the slave device does not need to respond after receiving the data.

2.2 Communication Rate
The communication rate of the CAN bus can be adjusted according to actual needs. Common communication rates include 1Mbps, 500kbps, 250kbps, etc. The higher the communication rate, the faster the data transmission speed, but the signal transmission distance will be shortened accordingly.

2.3 Communication distance
The communication distance of the CAN bus is affected by signal attenuation and noise interference. Under ideal conditions, the communication distance of the CAN bus can reach more than 10 kilometers. In practical applications, the communication distance is usually between tens of meters and hundreds of meters.

1. CAN bus data frame structure

The data frame of the CAN bus consists of multiple fields, including the frame start, arbitration field, control field, data field, CRC field and frame end, etc. The following is a detailed introduction to the functions and structures of these fields:

3.1 Frame Start
The frame start is a single bit that indicates the beginning of a data frame. When the frame start bit appears on the CAN bus, all devices enter the receiving state.

3.2 Arbitration Field
The arbitration field is used to determine the priority of a data frame. On the CAN bus, the priority of a data frame is determined by the identifier in the arbitration field. The smaller the identifier, the higher the priority. When multiple data frames are sent at the same time, the data frame with a higher priority will overwrite the data frame with a lower priority.

3.3 Control Field
The control field includes the data length and the remote transmission request (RTR) flag. The data length indicates the number of bytes of data in the data field, and the RTR flag is used to indicate whether the data frame is a remote frame or a data frame.

3.4 Data Field
The data field is the part of the data frame that actually transmits data. The length of the data field is determined by the data length field in the control field. The data field can contain 0 to 8 bytes of data.

3.5 CRC Field
The CRC field is used to detect whether errors occur during the transmission of data frames. The CRC field contains a 16-bit cyclic redundancy check code (CRC) to verify the integrity of the data frame.

3.6 End of Frame
The end of frame is a single bit used to indicate the end of a data frame. When the end of frame bit appears on the CAN bus, all devices will exit the receiving state.

1. CAN bus error handling

During the CAN bus communication process, various errors may occur, such as bit errors, frame errors, CRC errors, etc. In order to ensure the reliability of data transmission, the CAN bus adopts a variety of error handling mechanisms:

4.1 Error Detection
During the transmission of data frames, the CAN bus will detect errors in real time. Once an error is found, the device will immediately stop data transmission and send an error flag.

4.2 Error Notification
When a device detects an error, it sends an error notification to other devices so that other devices know that something went wrong during communication.

4.3 Error Count
CAN bus devices have error counters to record the number of errors that occur during the communication process. When the error counter reaches a certain threshold, the device is considered to be in an error state and automatically exits the CAN bus communication.