A Closer Look At Vehicle Data Communications A Closer

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A Closer Look at Vehicle Data CommunicationsA Closer Lookat Vehicle DataCommunicationsby Steve Garrettmembers.atra.comThrough the years we’ve cometo depend on the information we can collect with a scan tool. Andthrough those years, thedata communicationsystems or data buseshave become morecomplex. With OBDII, many of us hopedthat the data bus wouldbe standardized and thatall our problems wouldbe solved. No such luck.Today’s vehicles use avariety of data transmission protocols. This forces scan tool manufacturers to constantly adapt their softwareto those changes. This can be quite achallenge, even if they’re working witha single brand of vehicle. Let’s look atGM as an example: GM has used allof these data communication protocolsover the years: 22U A RT(UniversalAsynchronous Receive andTransmit)E&C (Entertainment andComfort)SBI (Simple Bus Interface)SPI(SerialPeripheralInterface)LIN (Local InterconnectNetwork)Keyword 81,82 and 2000Class 2CAN(ControllerAreaNetwork) or GMLAN (GeneralMotors Local Area Network)Figure 1With the average GM vehiclecontaining more than a mile of wire,dozens of computers, hundreds of connectors, and thousands of terminals,the need for a standardized diagnosticsystem became imperative. A new system was introduced for the 2010 modelyear on the Chevrolet Camaro, CadillacSRX, GMC Terrain, Buick Allure andLaCrosse. The new platform, known asthe Global Diagnostic System or GDS,will be introduced in all GM vehicles in2010/2011 models.The new diagnostic system stilluses several communication protocolsfrom previous models, but it will alsoGEARS April 2010

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A Closer Look at Vehicle Data CommunicationsFigure 2Figure 3Figure 4Figure 5vary greatly when it comes to diagnostic processes and equipment. To understand the typical Global Architecture(Global A) we’ll need to look at thedata bus and its operation. The standardized, 16-pin DLC connector thatyou connect to your scan tool will havethese pin assignments (figure 1):Pin 1 — Low Speed GM LANPin 3 — Mid Speed GM LAN; terminalPin 4 — Scan Tool GroundPin 5 — Common GroundPin 6 — High Speed GM LAN; terminalPin 11 — Mid Speed GM LAN; –terminalPin 12 — Expansion Bus HighSpeed GM LANPin 13 — Expansion Bus HighSpeed GM LANPin 14 — High Speed GM LAN;– terminalPin 16 — Scan Tool Power B The primary function of the vehicle data bus is still communicationbetween the different control modules.GM has used several different data bus24configurations, including:Ring Network — Connects eachmodule through a serial data line.Star Network — Serial data linesfrom all the modules are connected at asingle point; typically a splice pack.Ring/Star Hybrid — A combinationof the two common bus configurations. Let’s take a look at some of thebasics for a GM data bus system:Power Moding — Power modingwas introduced on GM vehicles manyyears ago to reduce electrical loads,wiring, and improve module responsetime during wakeup. Years ago, GMmodules required a wakeup signal forthe module to come on line. This wastypically accomplished using an inputfrom the ignition switch.In today’s vehicles, wakeup isaccomplished by an assigned PowerMode Master module (PMM). ThePMM is typically the BCM or DIM.The PMM receives the input fromthe ignition switch and then relaysthat information to the other modulesthrough serial data on the data bus.Messages to the modules include: Off/A Sleep — Modules needto power down.Off Awake — Modules needto wake up and get ready forwork.RAP (Retained AccessoryPower) — Modules that areused after the key is turnedoff but before the door isopened should stay awake fora set period of time; all othersshould sleep.Accessory — Modules thatare used when the key is in theaccessory position should stayawake.Run — All modules should beawake.Crank — Modules that areused for cranking the engineshould wake up while all others should continue to sleep.Gate Way Module — Since 1986,many GM vehicles have used a mastermodule to control data transmissionon the bus. With a CAN/GM LANnetwork, the BCM typically acts as thegateway or master control module. TheGEARS April 2010

gateway module translates and prioritizes the messages sent on the bus. Inother words, the BCM acts as the interpreter and traffic cop for the system.CAN/GM LAN — The CAN databus isn’t new, but it’s becoming morecommon throughout the industry. GMhas its own version of the CAN system known as the GM LAN. The GMLAN supports three types of bus configurations: low speed (33.3 kilobits persecond), mid speed (95.2 kilobits persecond) and high speed (500 kilobitsper second) data transmission.The high speed bus (figure 2) consists of a twisted wire pair that transmitsa voltage signal that switches high andlow to create the data signal. The highspeed GM LAN bus is terminated witha pair of 120 ohm resistors. One line isdriven high (3.5 volts) while the otherdata line is pulled low (1.5 volts) in aspecific sequence to create the signal.With a CAN-type system, eachmessage is assigned a unique identifier to classify the type message andits priority. Each module on the busprocesses only the messages related tothe identifiers stored in the module’sprogrammed listing. If more than onemodule is trying to communicate at thesame time, priority is assigned to themessage that has the greatest importance.The mid speed GM LAN bus (figure 3) is very similar in operation to thehigh speed circuit, with one exception:the controllers attached to it. The midspeed GM LAN uses the same type ofmessage transmission protocols that thehigh speed system does. The mid speedbus consists of a twisted wire pair thattransmits a voltage signal that switcheshigh and low to create the data signal.The mid speed GM LAN bus is terminated with resistors.Network (LIN) is used to reduce the costof data bus communications. The LINis a single-wire network that togglesbetween system voltage and ground toproduce series of voltage pulses.Multiple LIN circuits are used inGM vehicles. A LIN circuit doesn’tconnect directly to the DLC; it communicates with various sensors, actuatorsand modules on its own circuit.An example of a LIN circuit wouldbe the power windows on a late modelGM (figure 5). The window switchcommands the driver or passengerdoor control module to open or closethe window. The module performs thecommanded operation while communicating with other modules regarding theaction it performed.GM Expansion Bus — This buscircuit can be referred to by many different names, but the function is thesame. The expansion bus is designed toreduce the congestion on the high speedbus. By adding a bus that is designedand functions the same as the highspeed bus reduces the amount of trafficon the primary high speed bus. This islike adding parallel freeway to reducetraffic congestion in a large city.With the global architecture, lots ofthings have changed. For decades tech-The low speed GM LAN is a singlewire circuit that replaced the class 2data circuits on many GM vehicles(Figure 4). The GM low speed LAN isbidirectional and operates at three timesthe speed of a class 2 communicationcircuit. The circuit toggles from 0-5volts. To wake up the modules on thebus, the voltage level is pulsed to 10-12volts. The modules are connected inparallel on the low speed bus.LIN — The Local InterconnectGEARS April 201025

A Closer Look at Vehicle Data Communicationsnicians have used the Tech 2 scan toolto help diagnose vehicles. With GlobalA vehicles, the Tech 2 will no longerfunction. GM now uses a PC-basedprogram known as GDS or GlobalDiagnostic System (figure 6) to interface with the vehicle. This means youneed to ask your scan tool manufacturerif your scan tool will be compatiblewith this new system. Lots of otherareas have also changed, including: Swapping computers for testpurposes can cause major problems.While this isn’t new with some GMvehicle platforms, it’s an issue withall Global A applications. Swappingan ECM, BCM, EBCM, SDM, ECC,IPC as well as many other modules canprevent the engine from starting or settrouble codes. In some instances youwon’t be able to clear these codes.The modules are all coded withspecific ID information codes whenthey’re programmed at the assemblyplant. So modules that have nothingto do with starting the engine canprevent it from starting. In addition,other symptoms may occur such as theIPC odometer reading “----,” the radiodisplays “Locked,” or “service the theftsystem” is displayed on the DIC.Not only do the Global A vehiclesuse controller security codes to preventvehicle theft, they also use a new typeof coding known an Environmental ID.This type of coding is designed to prevent someone from stealing a vehicleby swapping modules and keys. TheImmobilizer Master (BCM) identifiesitself to the other modules. If it hasthe correct ID, the other modules willrespond with a specific EnvironmentalID code. The BCM will look at all themodules’ Environmental IDs and, ifthey’re correct, it’ll allow the vehicleto operate. Reprogramming is sequential.This means that several controllers areupdated at the same time in a specificsequence to prevent software conflicts.Typical programming now requires alot of data to be transferred (typically10-40 MB). Programming will takemuch longer than in the past (up to anhour).In addition, to protect the modulesfrom unauthorized programming (nonGM), the assembly plant uses a processknown as Seed and Key. The assemblyplant installs a specific Seed valueduring plant programming. When themodule is connected to reprogrammingequipment, that equipment must provide a specific Key value or the modulewon’t accept the program. Many modules require a specific setup procedure to relearn component and system values. Many modules require a security code configuration procedure forthe module to operate properly. If theconfiguration codes aren’t installed,some modules will lock out immediately, while others will initially operate and then lock out after several keycycles or after battery power has beenremoved. This could lead to a customercomplaint because the engine won’tstart or other conditions after you performed a repair.Well as you can see times are stillchanging until next time, remember:“In the middle of difficulty lies opportunity.”Figure 626GEARS April 2010

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