1, CAN bus structure
The physical layer of the CAN bus is the drive circuit that connects the ECU(Electronic Control Unit- electronic control unit, also known as \"driving computer\
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In theory, the number of nodes on the CAN bus is almost unlimited, up to 2000, in fact, limited by electrical characteristics,CAN only connect to more than 100 nodes.
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CAN's data Link layer is its core content, in which logical Link Control (LLC) completes functions such as filtering, overload notification and management recovery, and Medium Access Control (Medium Access Control). The MAC sub-layer performs functions such as data packaging\/unpacking, frame coding, media access management, error detection, error signaling, response, string and conversion. These functions revolve around the information frame transmission process.
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2, the advantages of CAN bus
①CAN work in multi-master mode. Any node on the network can actively send information to other nodes on the network at any time, regardless of the master and slave.
② In terms of packet identifiers, nodes on CAN are divided into different priorities to meet different real-time requirements. Data with higher priorities can be transmitted within a maximum of 134μs.
③CAN adopts non-destructive bus arbitration technology. When multiple nodes send messages to the bus at the same time, the node with the lower priority will actively quit the transmission, and the node with the highest priority will continue to transmit data without being affected, thus greatly saving the bus conflict arbitration time. Especially in the case of heavy network load, there will be no network breakdown (Ethernet may be).
④CAN nodes only need to filter the identifier of the message to achieve point-to-point, point-to-multipoint and global broadcast to transmit and receive data.
⑤ The direct communication distance of CAN can reach up to 10km(speed below 5kbps) and the communication rate can reach up to 1Mbps(the maximum communication distance at this time is 40m).
The number of nodes on CAN depends on the bus driver circuit and can currently reach 110. The number of standard frame message identifiers (11 bits) and extended frame message identifiers (29 bits) is almost unlimited.
⑦ The message uses short frame structure, short transmission time, low probability of interference, to ensure that the data error rate is very low.
Each frame of CAN information has CRC check and other error detection measures, with excellent error detection effect.
⑨CAN communication media can be twisted pair, coaxial cable or optical fiber, the choice is flexible.
⑩CAN nodes can automatically shut down the output function in the case of serious errors, so that the operation of other nodes on the bus is not affected.
The main hardware structure of CAN bus system includes: CAN controller, CAN interface chip and special communication cable.
The function of CAN controller is to configure bus parameters, generate communication frames according to CAN protocol, analyze the number of receiving, and run the can bus control mechanism such as arbitration, response and retransmission.
The function of Can interface chip is to convert controller serial data and bus balanced differential signal.
The special communication cable is a kind of twisted-pair cable with shielding. Its function is to complete the link and transmission of data receiving and sending.