CAN Bus Technology, Working and Operation of CAN System - Coursework Example

Report by ashok kaul CAN Bus Technology CONTENTS Introduction and Automotive controls history............................page CAN Bus Technologydescription................................................Page 1,2 CAN Bus Standards Organization................................................Page 2 CAN Bus Specifications- standards Page 2,3 Working and Operation of CAN System Page 4-7 2.0A Message Format Page 7 LAN and WAN Protocols Page 8,9 Comparative features of CAN and Ethernet Page 9,10 Sources for reference Page 10,11 Conclusion Page 11,12 Page 1 Automotive controls history Since early 1940s, there has been continuous effort to improve the vehicular technology for giving better and comforts. This in turn necessitated the gradual replacement of mechanical parts by the more modern electrical and electronic components. While the better driving performance was achieved by such replacements, which also meant lesser maintenance, the visible hazard apparently was the huge amount of wiring required for this purpose. Each functional component on the vehicle required lot of wiring to be connected to the other component or to the power source. This caused many problems in the after sales repairs as each of the models had a different set of wiring, which meant qualified and trained staff to work out the repairs or even the general maintenance. The intermingling of the wires was also causing problems. Hence, a need was felt to have the vehicle control system that will eliminate largely the number of connectors and the resultant wiring problem. This need gave birth to the invention of CAN bus technology. CAN Bus technology description Mr. Robert Bosch introduced the CAN bus system for the first time at a SAE congress in 1986 at Detroit. This resulted in the vehicle wiring getting reduced up to 2Kms. Thus, the overall weight of the vehicle was reduced up to 50 Kgs. In a vehicle like BMW 850 coupe Page 2 Controller Area Network serial bus system, called as CAN bus technology is designed to support multi master access, while handling short messages up to 8 bytes. While this means that collusions get resolved by priority, the system offers high degree of reliability also, by having 15 bit CRC for every message. Since itd introduction at Society of Automotive Engineers (SAE), there have been many further innovations in the CAN technology. While Intel developed CAN chip 82526 and Philips also developed semiconductor 82C200 as a CAN controller, there are about 20 chip manufacturers catering to this technology now. CAN technology being purely a data link layer implementation program, it became quite popular for use in various capital equipment like textile machinery, apart from its original use in passenger cars and other transport vehicles. However, this further gave rise to the need of having standards for these layer implementations and data exchange programs. CAN Bus Standards organizations An international users and manufacturers’ group called CAN in automation (CIA) International organization for standardization (ISO) and other relevant standards. CAN Bus Specifications—Standards Page 3 ISO/DIS 11898-1 ---Deals with Data link layer and physical signals of CAN system, for vehicles. ISO / DIS 11898-2 for high speed medium access unit for road vehicles with CAN technology ISO / DIS 11898 -3 ,for low speed fault tolerant medium dependant interface, for transport vehicles using CAN technology. ISO / DIS 11898-4 for time trigged communication in road vehicles using CAN system. Apart from the above, standardization like EN50325 have enabled application layers now to be used in different kinds of machine controls with the use of components like DeviceNet and CANopen. Both these are addressing different applications. While CANopen is useful for almost all kinds of machine controls, DeviceNet is used for maximizing the factory automation. All the software and devices as well as the components required for the CAN Bus technology have to satisfy and perform as per one of these standards, depending on the application that CAN technology is delivering. Needless to mention here, the optimum benefit of this technology has been derived by the automotive sector particularly the luxury car segment. We shall discuss the benefits of this system for the car users, later in this report. The benefits of the CAN technology are huge and varied. While using this system, many features can be provided in a car, through a single or dual Page 4 hook up, thus reducing the number of connectors and the resulting wiring required. While this increases installers efficiency, this also means more product reliability by having very few points of failure. Working and Operation of CAN System CAN principal Identification of a message is unique as it is the content of the message like revolutions per minute, X-Ray Dossage, etc. which is labeled by an identifier, throughout the network.. Therefore, any all data messages transmitted from any node on a CAN bus do not contain the address of the sender or the receiver. Only the relevant messages are processed and the unique identifier determines the priority of the message based on the numerical value of the identifier. Lower numerical value will mean higher priority. To ensure that no message is lost, there is a non-destructive arbitration technique guarantees transfer of messages as per their priority. The Bus This is usually a two pair twisted wire structure, which may have shielded or unshielded wires. It can also be a pair of flat wire but it is prone to produce noise and be more vulnerable to external sources of noise. Bit Encoding On a differential two wire bus CAN uses Non-Return to Zero (NRZ) encoding system for data communication. While it ensures higher resilience to Page 5 external noise sources, their use of NRZ means compact messages with minimum number of transitions. Robust Operation CAN system is designed to work under any harsh environment and al transmission errors are detected by its error checking mechanism. The above-referred standard ISO 11898 lays down that the CAN system should continue working 9May be with a reduced signal to noise ratio, even if there is a short connection of either wire to the power or earth, or any of the two wires of the bus is broken. Flexibility of Network The system configuration is more flexible due to the above-explained nature of the message being content oriented. Without making any changes to the hardware or software, new nodes can be added to the network. These nodes are purely receivers that need existing transmitted data only. There is no need for each controller to have its own sensor as the measurements needed by various controllers can be transmitted via the system. CAN Bus and working of Arbitration When a technology is developed, it has to take care of the parameters that may change in any application. For a vehicle on the road the engine revolution per minute can change more rapidly, while and the coolant temperature may change slowly. Hence, the more rapidly changing parameters need to be transmitted more frequently and thus given a higher priority. Page 6 For this purpose CAN uses a method called “Carrier Sense, Multiple Access and Collision Detect” (CSMA/CD) to enable it to deliver the optimum use of the available capacity. This is done while CAN determines the priority of the messages by this method with the greater capability of providing collision resolution. The method also ensures the priority be given that is directly related to the higher rapidly changing parameters. Non-Destructive Bitwise Arbitration and its Benefits While the priority of the CAN message is determined by the numerical value of its identifier, as explained above, the numerical value of each message identifier is given when the system design begins to take shape. As the identifier with the lowest numerical value has the highest priority, the potential conflicts are resolved by bitwise distribution. This has to be in according to the wired-and-mechanism. There is also a destructive bus allocation system, which is called “Ethernet”, ,but the benefits of non-destructive bit-wise arbitration in providing bus allocation on the basis of need and the efficiency of the system cannot be obtained under any other allocation system. CAN will never collapse. The speed-limiting factor is only the maximum capacity of the bus. Message Format of CAN Message frames are used to transmit and receive message in a CAN system. These frames are responsible to carry the data from one node to one or more receiving nodes. While the standard CAN protocol (version 2,0A), also Page 7 known as base frame format, supports messages with 11 bit identifiers, extended frame format , also known as Extended CAN Protocol(version 2.0 B)supports 11 bit as well as 29 bit identifiers. While version 2.0B controllers can send and receive messages in both the formats, it is the version 2.0A that is used in most of the cases. Hence, we will discuss here this format in detail, as most of the time standard format messages are sent and received through this version.. 2.0A Message Format Various bit fields make a standard 2.0A message format. These are explained briefly below. Start of message frame indicator called Start of Frame (SOF) field. A message identifier and remote transmission request (RTR) bit are in an arbitration field. Two reserve bits ( r0 and r1) and a four bit data length code are contained in a control field containing six bits. A Data field containing bytes ranging from Zero to Eight. A fifteen-bit cyclic redundancy check code is in the CRC field. Two bits are in the ACKnowledge field. The first bit is a slot bit and the second is the recessive delimiter bit. Seven recessive bits are in the End of the Frame field. Page 8 LAN and WAN protocols These are the various media access methods, used mainly in Ethernet/IEEE 802.3 applications. LAN is a communication network that covers small geographic area and it also has high speed transmission capabilities. Hence, its use in Ethernet is very important, as it provides shared access to devices and other applications. It also provides exchange of files between the connected users and the electronic mail communication among them. LAN protocols function between the physical layer and data link layer, these two layers being at the lowest level of OSI reference model. There are mainly three LAN transmission methods, classified as Unicast, Multicast and Broadcast type of transmission. .In all types of transmissions a single packet is sent to one or more nodes. Hubs, Repeaters, LAN Extenders and Bridges, Routers etc are the devices used in LAN technology. Wide Area Network communication system, called WAN is used to cover a large area For this purpose the system uses the facilities available with the common utilities and carriers like telephone cables etc.The function of WAN network is covered generally in lower three layers of the OSI reference model. These layers are the physical layer, the data link layer and the network layer. The application of LAN and WAN network protocols is mainly advantageous for the Ethernet system, which requires high-speed transmission of messages at a frequent rate. However, in the CAN bus technology the Page 9 message being limited to the 8 byte structure, the use of LAN and Wan is not found much beneficial. Ethernet communication and computer networking system has been applicable to the automotive controls also. .However, we find that the modern CAN bus technology has an edge over the Ethernet, when it comes to the conflict resolution and data transmission. Here the priority of transmission is related to the frequency of change in various parameters of the applicable system, like the RPM of a car or the change in the engine temperature being reflected by the change in the coolant characteristics etc.. We will now discuss the various comparative features of Controller Area Network and Ethernet. Ethernet Contoller Area Network Ethernet cannot guarantee a system to determine the non-destructive priority arbitration Can technology uses a non-destructive priority arbitration system while tackling the data transmission While TCP/IP protocol is used in Ethernet, it can transfer data of large amounts at a high sped rate Can bus technology is eligible to transmit data in smaller amounts only, as the message,size is limited to 8 bytes. Ethernet can provide more powerful and effective communication among the advanced as well as complex situations using TCP/IP protocols. CAN bus is a technology which provides solutions on real time based application, as it is mainly used in an embedded system, for providing fast communication among the microcontrollers of the application. Ethernet while using the TCP/IP connection protocols has the possibility of connection through LAN or WAN and further worldwide through the internet the Thus Ethernet has far more information carrying capacity than CAN system While CAN technology is designed for the real time parameters, it has an edge over the Ethernet when it comes to reaction time with regard to error detection and repair etc. Sources: CISCO Internet working Technology handbook (http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-WAN.html) Operation and working of CAN (http://www.mjschofield.com/canworks.htm) CANopen.us (http://www.canopen.us/history.htm) CAN Bus new installation technology (http://canbuskit.com/what.php) Following books are recommended to gain further knowledge on the subject. Page 11 Controller Area Network by Konrad Etschberger, 2001 CAN System Engineering: From Theory to Practical Applications by Wolfhard Lawrenz, 1997 Intelligent Vehicle Technologies By Ljubo Vlacic, Michel Parent, available for limited preview at Google books Conclusion While the automotive industry has been doing research and development in the field of automotive controls, The need for having the electrical and electronic controls, with minimum amount of wiring has always been there. However the growth of computerized solutions for all application, customized and tailred for a certain function has revolutionized the product innovations in a far better and positive direction. This technology has brought the CAN Bus system, innovated mainly for the automotive industry in the year 1986, by BMW and Mr. Bosche. Thereafter, CAN bus technology proved a boon for the industry and apart from reducing the wiring hassles , almost completely, the technology brought newer comforts with fever manual controls for the cars and other vehicles on the road. Page 12 The benefits of CAN Bus technology were extended to other industry like machine and capital equipment manufacturers, who applied the CAN Bus technology to optimize the production and other benefits. Use of this technology for textile machinery manufacture is worth mentioning here as the CAN Bus controls find multiple use in this industry. Although CAN Bus technology does not work in a wider area like the Ethernet that works with protocols like TCP or IP, the customized solution for a particular application is best delivered through the CAN Bus technology only. Read More