Vehicle-Bus Interface with GMLAN for Data Collection

1 Vehicle-Bus Interface with GMLAN for data CollectionDavid C. PheanisJeffrey A. TenneyComputer Science and EngineeringWestern MicrosystemsArizona State University938 E. Ranch RoadTempe, Arizona 85287-5406 Tempe, Arizona a few years, nearly every vehicle that Gen-eral Motors (GM) manufactures will have a microcon-troller network known asGMLAN. A fewGMvehiclesalready have theGMLAN network, and more vehicleswithGMLAN will appear each theBosch-designedCAN(Controller Area Network) proto-col, and it links all of a vehicle s various processors ornodesto form an in- vehicle data aNon-Return-to-Zero protocol,NRZ-5, with bit stuffing,and the network implements Carrier Sense MultipleAccess with Collision Resolution (CSMA/CR).

2 GMLAN actually includes two linkedCANbuses: a high-speeddual-wireCANfor high-speed nodes and a lower-speedsingle-wireCANfor other primary purpose of theGMLAN network is toimprove reliability while simultaneously lowering costby reducing the numbers of wires, connections, andspecial-purpose circuits in a vehicle . TheGMLANnet-work also promotes synergism among the vehicle nodesto provide vehicle features that would simply not befeasible without the have developed a Vehicle-Bus Interface (VBI)that allows us to collect test data directly from a ve-hicle s nodes via theGMLAN network with aGMStan-dard Thermal Instrument (STI).

3 TheVBIfacilitatesvehicle testing by greatly reducing the need for exter-nal sensors and IntroductionMicrocontrollers are changing the automotive in-dustry. Microcontrollers now control vehicle compo-nents such as the engine, the antilock braking system,the transmission, the instrument panel, and the enter-tainment center, to name just a few. A microcontrollerdoes more than just make its own component workcorrectly it also participates in one of possibly sev-eral networks in the vehicle . Such a network, usuallycalled avehicle bus, allows the sharing of informationamong components and also reduces the numbers ofwires and sensors required in the just recently, automotive testing by vehiclemanufacturers has required the expensive and time-consuming installation of myriad sensors and trans-ducers to collect data from the vehicle .

4 In the late1990 sGMstarted using the Class-2 Bus [1] to net-work the nodes onGMvehicles, and engineers atGM sDesert Proving Ground developed a Vehicle-Bus In-terface (VBI) that allowed them to collect test data di-rectly from the vehicle nodes via the low-speed Class-2 Bus, which runs at approximately KBS or 10,400bits per second. Now we can interact with the muchfasterGMLAN network at 500 KBS to collect a wealthof data from sensors and controllers that already ex-ist on the vehicle . This paper introduces a vehicle -BusInterface (VBI) that allowsGM s Standard Thermal In-strument (STI) to collect data by interacting with thenodes on theGMLAN network that is already on someGMvehicles and will soon be on CAN ProtocolBosch pioneered theCANserial data -link protocolin the early 1980 s and subsequently licensed a num-ber of semiconductor vendors to develop and producecontroller chips for today include not just automotive networks butalso medical systems, home appliances, trains, ships,airplanes, factories, and even vending only the data -link layer of the networkprotocol, and it leaves all other layers open.

5 For exam-ple,CANdoes not specify any of the layers above thedata-link layer, nor does it specify the physical layer,which is below the data -link , how-ever, specify that the physical layer must support anactive( , dominant) state for a zero bit and apassive( , recessive) state for a one bit. Many groups havedeveloped differing physical layers that , for example, has developedGMLAN, which definestwo different physical layers along with the higher-levellayers to provide two thatCANspecifies the operation of everyCANcontroller chip, but the controller chip is separate fromthe transceiver chip, which interfaces with the physicallayer and is not part of theCANspecification.

6 Eachdifferent implementation ofCANis therefore indepen-dent with regard to its physical layer, and anyCANcontroller chip can support any physicalCANimple-mentation if we merely pair it with the appropriatetransceiver not specify the physical layer, de-signers are free to choose the bit rate as well as thephysical signals and the bus media ( , single-wire ordual-wire copper, fiber, etc.). A designer might choosea slower bit rate to be able to have a longer physicalbus length, or a designer might reduce the bus lengthto achieve a faster bit a Non-Return-to-Zero protocol,NRZ-5, with bit stuffing. The idea behind bit stuffing is toprovide a guaranteed edge on the signal so the receivercan resynchronize with the transmitter before minorclock discrepancies between the two nodes can causea problem.

7 WithNRZ-5the transmitter transmits atmost five consecutive bits with the same value. Afterfive bits with the same value (zero or one), the trans-mitter inserts astuff bitwith the opposite state. Thenext bit of the frame then follows the stuff bit, so thestuff bit is a wasted bit in the sense that it consumesbandwidth without transmitting information. How-ever, the stuff bit guarantees an edge on the transmis-sion signal after a maximum of five bit times, so thereceiver can resynchronize with the sender on a one-to-zero edge after a maximum of only ten bit stuff bit allows higher clock rates in spite of thefact that a frame can be as long as 108 first eleven bits after the start bit in a stan-dardCANframe contain a frame identifier.

8 The frameidentifier and the following bit serve as an arbitra-tion field. Frames with lower identifier values havehigher priority because of the fact that zero bits (notCAN) specifies a particularidentifier value for each type ofGMLAN frame. Forexample, one particular identifier might indicate thatthe frame contains engine RPM and throttle positionwhile another identifier might indicate a frame withengine node that transmits on aCANbus also listenson the bus at the same time. If a node receives an ac-tive state when the node is transmitting a passive statein the arbitration field, that node loses arbitration andimmediately ceases its transmission without affectingthe dominant transmission that is on the bus.

9 Eventu-ally, one node wins arbitration as all others drop out,and the winning node s message is the only messagethat appears on the bus. This mechanism implementsCarrier Sense Multiple Access with Collision Resolu-tion (CSMA/CR). The appropriate message drives thebus, and we resolve collisions without damaging thatmessage on the bus. Thus there is no need to retrans-mit the dominant message following a standardCANframe is a variable-length framethat contains from zero to as many as eight data byteswith eight bits each. A four-bit data -length field spec-ifies the number of data bytes that follow in the a higher protocol levelGMLAN(notCAN) definesthe concept of amessage.

10 A message can be a single-frame message, or a message can span multiple frameswith the frame identifier and various data fields in-dicating that the message is a multi-frame frame in a multi-frame message has the sameframe node sCANcontroller automatically appendsa 15-bitCRC(Cyclical Redundancy Check) to eachframe that it transmits, and the controller validatestheCRCwhen receiving a message. Since each nodereceives all messages, including the messages that ittransmits itself, a node s controller automatically re-transmits any message that doesn t transmit properlythe first time. Also, at least one recipient for eachframe must send an acknowledgement bit after vali-dating theCRCof the received frame.