The Design Of Car Body Network Based On CAN Bus

With the development of electronic technology, more and more electronicequipments are fixed into vehicles to make driving more comfortable. But thewiring harness which joins two different electronic equipments together alsoincreases amazingly at the same time. It leads to many hidden troubles, suchas harder to maintain and less reliability. Secondly, the communication modeof those equipments is point to point mode, which means messages betweentwo points can?ˉt be shared by other points and they have to be sent againwhenever another point needs them. Fortunately, we can solve all of theseproblems mentioned above easily by using CAN (Controller Area Network)technology into car body network.CAN is a serial communication protocol which was declared by Boschcompany. Compared with other kinds of field buses, CAN bus has low priceand good performance. It saves lots of wirings because all of the CAN nodesare linked to the same bus. One message on CAN bus needn?ˉt be sent severaltimes because it is transferred serially and so it is shared by all nodes.Messages of CAN have short frame, whose data field is composed of 8 bytesat most, as a result, every message is transmitted quickly on bus. Nodes in buscompete with each other for bus occuping privilege when they are going tosend message to others. CAN bus uses message coding instead of addresscoding, and its baud rate could reach 1Mbps. It also has advantages in errordetection and self recovery, which evidently ensure the reliabile datatransmission.researching and some of them have worked out products of CAN. We caninfer easily that CAN bus would have bright future in the next few years.Car body network is usually composed of many nodes, such as chairs,meter, lights or automatic windows. In this designment, there are 5 nodes: oneis the central meter, which is linked to many light switches;the other fournodes are fixed separately in the four corners of vehicle, which are linked tomany lights (such as hight beam, passing lamp, turn light, emergency light,etc.). The meter node is responsible for checking the changes of switches?ˉstate;while the other four are responsible for controlling the lights?ˉ states.When a driver turns on any light switch, the meter node will find out aswitch-state-change at once and it will send a message to CAN bus accordingto the switch and the state, then the other four will receive the message if theyneed and consequently control the corresponding light.Among all the elements used in node circuit, the main chips are:AT89C51, SJA1000 and 82C250. Their characters and functions are shortlydescribed in this paper. The circuit diagrams are also provided in detail.Besides the existing physical layer protocol data link layer protocol, theapplication layer protocol should be defined according to the special purposein this designment. Eight different message IDs are used because there areeight kinds of light should be controlled. Those IDs present the meaning ofmessages, and they also denote the static priority of messages. One byte datais used in every message frame to denote the specified operation type to bedone. When several nodes are sending messages to bus at the same time, theone whose message has high priority will get the access to use bus, and theothers have to wait until the bus is free again.Filter configuration is also necessary in application layer definition. If anode receives every message transferred on bus, its CPU will be very busy. Byconfiguring the ACR and AMR for every node, they could pick out themessage they need and give up useless messages and then alleviate the burdenof CPU.Aside from the hardware circuit and the protocol definition, the softwarealso plays a big part in this designment. The software is programmed inMCS-51 ASM language because ASM has high efficiency and fast runningspeed and could operate hardware directly. The whole program is divided intoseveral small parts according to their basic functions, so that the code is easierto be read and is much more robust.This designment testifies the feasibility of using CAN bus to car bodynetwork. The error disposal part could be added in the future work, and wewill put emphasis upon the research of communication between different busnetwork, such as LIN or MOST.