Report on CANbus Technologies - Case Study Example

Report on CAN bus Technologies Historical background to CAN bus - why was it developed Operation of practical LAN and WAN protocols and systems commonly used in networks and inter networks Operation of CAN Data transfer mode - Asynchronous Line coding scheme Data transfer mode - Asynchronous Transmission made smooth through Arbitration [3] Frame Structure and frame types Edge over Ethernet: The Application Bibliography Historical background to CAN bus - why was it developed As the industry in general and automobile in particular faced rapid growth in Europe and US, the need to communicate and control the performance in the form of signals between micro-controllers (8-bit or 16-bit) with the devices at the vehicles, generated the idea of developing a system of devices that assured affordable and reliable message delivery without jamming the channel used for data transfer in the shortest time. In February 1986 Robert Bosch introduced the CAN (Controller Area Network) serial bus system at the SAE congress in Detroit, to handle short messages in a network of Multi-master access. The requests for sending the packets (Data and other identification bits) and linking up between the ends were to be controlled by an arbitrating system to avoid collisions by assigning priorities. In the wake of which Intel delivered the first CAN chip, the 82526. Shortly thereafter, Philips Semiconductors introduced the 82C200 CAN controller On the academic front where larger universities and research labs obtained more computers during the late 1960s, experiments were started to meet the demand of setting up communication links between these computers so that the data could be shared swiftly with least interruptions and without other undesirable delays. Hence the development of Ethernet at XEROX PARC 1973-75[4], and its subsequent deployment followed by the seminal paper - "Ethernet Distributed Packet Switching For Local Computer Networks" in 1976 by Metcalfe and Boggs. By early 80s, the flux of Dos based computers in the Industry, where resources such as Disk space and Laser printers were dear, triggered the need to share them along with the data over the channel that could be easily adjusted. In other words these were the early attempts made to provide a solution to meet the demand of sharing resources and smart delivery system and transferring data over affordable physical wiring. Of which only Novel Netware could provide a relatively feasible solution with an operating system that could put 40 computers in a network sharing data and the resources over the same wiring network. By 1992, when many vendors used their technologies, only compatible to their own equipments, to produce solutions by setting up communication links between two or more devices, no one could convince the other to form an open system that was compatible in general, the need to found a user's group to standardize the different solutions forced Holger Zeltwanger to bring together users and manufacturers to establish the 'CAN in Automation' (CiA) international users and manufacturers association. Since no standard protocols existed to transfer or receive data at either end of the communication channel provided by various vendors, the foremost job of CiA was to set up the specifications of the CAN Application Layer (CAL). And by 1993, Bosch led a European consortium to design a test project with communication protocols- a step towards setting up a compatible system for internal working of productions cells: the CANopen. The system aimed at providing a framework for programmable systems, devices to suit the systems, interfacing between various components and the application profiles. This facility enabled the industry to exploit it in the printing devices, medicals devices and many more. By early 1990s efforts were made to develop a communication profile to address the layers that dealt with the applications at communicating ends. 'DeviceNet' and 'Smart Distributed System' (SDS) were developed. These were the higher layers and the focus of these communication profiles was to develop the protocols to synchronize and harmonize the signals transferred between the applications run at communicating peers.. Although these were quite similar in the lower communication layers, not beyond architecture, they had certain protocols that made them special. Allen-Bradely who developed DeviceNet, called the group of vendors to use it in their products and made the architecture open in 1994. This made the DeviceNet popular and later it became the top bus system for the devices used in the factory automation. The open architecture of Open DeviceNet Vendor Association (ODVA) helped it to alleviate its status to Standard, henceforth making it easier for other vendors to develop devices using its protocols and expanding the connectivity to the products of other vendors. The SDS however could not follow the success of its peer and became oblivious in the throng of new developments. Therefore both theoretically and practically CAN bus technology gained commercial foothold with DeviceNet and CANopen, two standardized (EN 50325) application layers. Operation of practical LAN and WAN protocols and systems commonly used in networks and inter networks CAN bus technology aims at providing the OSI model Data Link Layer [1] to address the higher layer which mainly postulates methodologies by which data can be transferred between network entities. Further more the general problems that might occur in the physical layer are also detected and corrected. In reality it was meant to serve one-to-one or one-to-many communicating terminals-a model readily acceptable for fast WAN and LAN communication. in ISO 11898-1 (2003). This standard describes mainly the data link layer - composed of the Logical Link Control (LLC) sublayer and the Media Access Control (MAC) sublayer - and some aspects of the physical layer of the OSI Reference Model. The Data Link layer Operation of CAN By using Balanced (differential) 2-wire interface running over either a Shielded Twisted Pair (STP), Un-shielded Twisted Pair (UTP), or Ribbon cable a CAN bus technology was designed originally to link different devices such as actuators, sensors, etc, connected at nodes (up to 127 in CANOpen protocol) to set up a control network in a vehicle to control and adjust the overall performance of a certain unit such as a cruise control. Each of these devices connected to the nodes send signals in serial sequence to the host processor, Intel 82527, for example, that used CAN Technology. The signals are not interchanged between the devices but are sent the host processor, which refers to the logical circuitry of the CAN processor to reach a solution. The solution is directly converted into electrical pulses through Modulation and are sent real time to the device through the wires. The rate with which the signals are sent can vary from 1 Mbits to 10 Kbits per second. The kind of modulation used is Baseband. It can be seen in the following (NRZ) Non-Return-Zero [2] by line coding scheme, how the digital signals are mapped over fixed length scale, representing continuous electrical pulses (wave cycles). The signals can have either a low voltage or high voltage to assign the 0 or 1 binary states. The On state is represented by 1- the horizontal line on top and the off state 0 is represented by the horizontal line at the bottom for example. Line coding scheme An example of coding a binary signal using rectangular pulse amplitude modulation with polar non-return-to-zero code By using differential-signaling, variation in the voltage produces two signals for binary messaging through the Bus, representing 0 and 1. When the messages are to be transmitted over long ranges of physical channels or microwave media, these signal (baseband) can be further put through reshaping to reduce the size of the wave length and then modulated again to change the bandwidth, creating RF signal that are transmitted through microwave. The common types of line encoding are unipolar, polar, bipolar and Manchester encoding. Data transfer mode - Asynchronous The timing adopted for signal transmission in the CAN controller is asynchronous which has a unique way of controlling the electrical voltage being sent through wire to generate fluctuation for representing the different signals for the data it transfers through. It uses another wire to send fluctuation in voltage to represent the start/end of a bit/byte. The two forms of signal transferring through the bus are: Sending and Receiving. For sending the message the signals are stored by the host processor into the Can controller, which is then transmitted into the bus serially. While the can controller after receiving the signals one bit at a time, sends an interrupt at which the host processor fetches the completed message. Transmission made smooth through Arbitration [3] When two devices connected at different nodes, send signals to the host controller on the network for an adjustment/ change in a certain operation, (on CANOpen system up to 127nodes are present: a situation highly likely for a collision) CAN saves the transmission by a smart arbitration. It assigns the priority, a single bit (binary) value, either 0 or 1based on the protocol, to the node for preference to save collision. The 1 state represents the 'recessive' and the 0 state represents 'dominant' value. Hence when the tie occurs between the two, the node with a dominant or 0 value wins and gets to transmit the signals through the wire, while the one with a recessive or 1 value is put on hold or backs off. The system distinguishes between the two priorities by creating a voltage across the transmission line. The signal whose presence produces the voltage is the dominant bit and the signal that doesn't produce any voltage is the recessive bit. Such are the collisions that can be asserted on the bus, where as all the other collisions are not asserted on the bus. Such a scheme is called Carrier Sense Multiple Access/Bitwise Arbitration (CSMA/BA) and is used when differential bus is used. Data discrimination or data prioritization means the Internet Service Provider (ISP) assigns a priority level to data frames. Frame Structure and frame types The signals are sent on the network in frames. A new signal in transmission can be detected when it receives the [SOF] start of frame field. It is followed by Arbitration field, Control field, Data field, CRC field, ACK field. The last bit in the train is End of File [2]. [SOF] end of frame The first bit in the train is [SOF]: Start of frame. Arbitration field Arbitration field consists of single bit; Dominant 0 or Recessive 1. Control field Control field consists of a single bit with control signal. Data field Data field consists of the linear array of bytes (from 0 up to 8 bytes) Data to be transmitted. CRC field. CRC field carries the bit that identifies whether the cycle has been repeated or not: a check to ensure the safety of information being sent. ACK field. Acknowledgement field carries the single bit usually with a recessive 1 sent by the transmitter. [EOF] end of frame The last bit in the train is [EOF] end of frame. To ensure a successful transmission CAN adopts five error detection mechanisms; 3 at the message level and 2 at the bit level. In case of any error a flag is marked which can be recognized as error. Edge over Ethernet: 1. Can bus uses on an arbitration technology which assigns priorities to Arbitration bit, based on which a frame is transmitted without disturbing the flow of transmission, where as Ethernet does not commit any such arbitration scheme. Since two or more stations send data simultaneously the data collision is inevitable and all the transmission becomes unreadable. A back-off algorithm is introduced to save the transmission which leaves the collided signal corrupted and sends it back to where it came from before it is ready to resubmit: thus producing a time delay in transmission and data loss. 2. Canbus has a greater flexibility edge over Ethernet in that it can be used in control processors needed to control devices in cars, machines such are dish washers, coffee makers, lifts, forks, airplanes etc. in addition to its use in Local and Wide area Networking. Where as Ethernet has limited application beyond Local and Wide area Networking. The new areas where CAN technology has possible potential are being investigated. However larger compatibility and easy connectivity are the core issues which are discussed in Report on Broadband Connectivity Competition Policy published by Federal Trade Commission (FTC) [4]. It suggests that the keeping in view of the interest of the consumers the broadband providers should look into the development/ improvement of the existing facilities including data prioritization, exclusive deals, and vertical integration into online content and applications without doing major changes in the current regulations. [5] The Application All less critical functions in an automobile such as window operations (doing up or down, Rear view mirror adjustments & seat adjustments can be done by employing low speed CANbus, where as more important functions such as engine management or brake control need high speed Canbus. Conclusion CANbus therefore offers one of a highly reliable technology to link up Local networking needs as well as Wide area Networks. Whether the small adjustment of air sack in the automatic blood pressure check-up machine is needed or a critical air pressure in the cabin of a DC-10 is to be controlled, CANbus has proven its efficiency. Many other applications are possible [Engine Sensors, Anti-Skid Systems]. Bibliography: [1]Collision detect [2] End of File . [3] Transmission made smooth through Arbitration http://en.wikipedia.org/wiki/Data_discrimination#cite_note-0 [4] Federal Trade Commission (FTC) [5] Current regulations http://en.wikipedia.org/wiki/Data_discrimination#cite_note-1 Read More