If you think multiplex is a build-ing with many movie theatresand a single entrance, you’reright. In fact, no matter how youuse the term, it refers to multi-ple elements. Normally, howev-

er, we think of it as simultaneous trans-mission of many things—such as bits ofinformation—on a single wire orthrough-the-air “channel.”

If you’re a hands-on kind of guywho’s far more interested in hearty bitesthan bytes, you’re not alone. It’s easy toget turned off by so many computeresereferences to data buses, networks, pro-tocols, gateways and assorted acronyms.

However, multiplexing in motor vehi-cles is what makes possible today’s elec-tronic controls and features. It elimi-nates an enormous number of wiresand connections, saving weight andspace and improving reliability.

Unless you learn the ins and outs ofmultiplexing, you won’t know what todo when your OBD II scan tool refusesto work on an OBD II vehicle. Or evenif the tool seems to be working, youwon’t know why you can’t find a prob-lem that should have been detected.You should also know that OBD II isbeing phased out in favor of somethingcalled CAN (Controller Area Network),which will mean either a new scan toolor a major upgrade to the one you have.If you’re buying a new scan tool, thinkof not only what it does now, but what itwill require to work with CAN diagnos-tics, reprogramming, etc.

Going further, a new world of add-onautomotive electronics is coming nextyear, and you’ll want to get your share ofthat installation and service business.

Just as the wireless phone is every-where today, the “wireless repair shop”

also is coming—not in a few years, butperhaps in a matter of months. The ca-pability of remote diagnosis already is inplace on millions of vehicles, so youshould not doubt that a vehicle cantransmit what it needs as it breaksdown, or is being driven in a limp-inmode to the shop. Will it be your shop?

Learning the TerminologyDevelopments in electronics go intoconsumer use almost with the speed oflight. But put aside your qualms. Herewe’ll use terms that you understand(even if it makes the computer guruscringe). Let’s begin with the most com-mon terminology, then see how thingsfit together.

Multiplex—many pieces of infor-mation going through the same chan-nel or wire at the same time. Soundsimpossible, and in a sense it is. Actual-

24 July 2001

NEGOTIATING

Multiplexing is here to stay. This primer will help youbetter understand the on-board information

superhighways of today’s and tomorrow’s vehicles.

ly, the data goes through in a sequence,but so fast that it seems simultaneous.A tenth of a second is pretty fast whenyou’re reading a watch, but to even acomparatively “slow” computer, that’s along, long time. Many individual dataitems can be transmitted if that frac-tion of a second is divided into seg-ments—one for each item. That’scalled time division multiplexing.

If there’s a way to separate thestreams of data through the atmos-phere, such as radio or cell phone wavesof different frequencies, there actuallycan be simultaneous flow of differentdata streams. As wireless multiplexingcomes to today’s and tomorrow’s vehi-cles, you can expect simultaneous datatransmissions based on frequency, waveamplitude or other methods. But in anautomotive system with one or twowires, time division is the way it’s done.

Module—an electronic device thatcan be a simple temperature or pres-sure sensor, or a sophisticated comput-er (microprocessor). Sensors are ana-log devices, and produce voltage read-ings related to temperature or pres-sure. These voltages are converted todigital signals at the entrance to thecomputer, a digital device. Some sim-pler modules in certain types of com-puter multiplexing systems are callednodes.

Data bus—the so-called informa-tion superhighway along which datatravels between modules. If data canbe both transmitted and received bythe modules, the data bus is bidirec-tional. In a vehicle, the informationsuperhighway actually is a wire, orperhaps two wires—not for an extratraffic lane, but sort of like a shoulderthat holds the speed limit signs and

traffic lights. In case the data lanebreaks, this “shoulder” might be usedin some data buses to carry “traffic.”Or traffic actually might run in reverseto permit data travel along the intactsection of a one- or two-wire data bus.

The two wires of a data bus aretwisted to reduce electrical interfer-ence. Each vehicle maker has beendesigning its own data bus. If they’reexclusive designs, they’re called pro-prietary data buses. If they’re de-signed to some international standard,they’re not supposed to be proprietarybut, in reality, could be, as you’ll see.Now that you’ve come this far, you cango back a step and think of modules asentrances/exits on the superhighway.

Network—more than one data bustied together to share information, orthe data bus and its modules all con-sidered as one system. The new Lexus

25July 2001

THE

BY PAULWEISSLERMAZE

LS 430 has 29 computer modules inseveral data buses, which exchange in-formation with each other. Becausethe modules and data buses are physi-cally so close to each other in a vehi-cle, it’s often called a LAN (local areanetwork). A Motorola-proposed low-cost, low-speed network for intelligentbody accessories is being called a LIN(local interconnect network).

Architecture—the layout of the in-formation highway, with all its en-trances and exits labeled for what in-formation can get on and leave. If “po-lice” (special-function electronic chips)are to be used to regulate traffic, therewill be stations for them, perhaps ateach module’s entrance/exit. The archi-tecture covers the number of wires—usually one or two. Where two wiresare used, the difference in voltage be-tween them may be the basis for thedata transmission. When one wire is

used, the voltage reading is referencedagainst electrical ground.

Other important characteristics ofany data bus/network architecture in-clude:

•The number of modules that canwork together.

•Expandability. Can a new modulebe added without a major rework?

•The types of information ex-changed.

•Data transmission speed.•“Robustness,” or fault tolerance—

resistance to significant breakdownfrom a failure, and how consistentand accurate the information ex-change will be.

•Cost—the bottom line.The architecture is designed to work

with certain protocols.Protocol—the so-called rules of the

road, including the way the “roadsigns” are written. When the presiden-

tial limousine is on a highway, it has ab-solute priority over any other vehicle.After the President’s car and those ofother VIPs, police cars, fire trucks, am-bulances, etc., get priority, but only ifthey’re performing a critical function,not merely cruising or returning totheir base.

The details of a data bus protocol arenot simple, but here’s a simple example:When Module A determines that theengine is close to overheating, it getspriority over less important information,such as that from Module B, whichwants to report the latest change, say, inbarometric pressure. Standard inclu-sions in a protocol usually are wake-upcall (a signal to a module that’s “sleep-ing” to conserve current) and hand-shake (an exchange of signals betweenmodules to acknowledge compatibilityand readiness to perform).

There are practically as many totally

26 July 2001

NEGOTIATING THE MULTIPLEXING MAZE

This simplified comparison of conventional wiring vs. multiplexing almost makes it seem that the conventional wiring isa lot less complex. However, notice there’s far less wiring to each ECU in the multiplex system than with conventionalwiring. A lot more feature content is possible, as well, as the ECUs can trigger dashboard warning lights, do burned-out lamp substitutions, etc., and execute multiple commands through just the single wire (data bus) connecting them.

Sou

rce:

Toy

ota

Mot

or C

orp

.

Conventional Wiring System

Multiplex Communication System

different protocols as the human mindcan devise:

•In a simple protocol, all modulesare equal. Each broadcasts informationto all others according to a set of prior-ities, and each module knows what in-formation to accept.

•One module is the master, whichdecides (based on its priorities) whichslave should report and when.

•All modules are like riders on acarousel, with the “free ride” tokenring revolving around them. When onemodule has useful information, it grabsthe ring and attaches a message;whichever other module needs it canpick it off the ring.

•There’s an “arbitration” system,usually based on the digital “spelling”of each message. The system sets thepriority for each data transmission (forexample, a digital message that endswith a 1 has a higher priority than oneending in a 0).

As a service technician, you reallydon’t care about the protocol itself, on-ly the effect it can have on diagnosis.

Why all the motor vehicle protocols?It all depends on how much data themaker wants to transmit to how manymodules and how fast that data has tomove along the bus. Every data buscould be lightning fast, which is usefulfor engine torque management, emis-sions, etc. But how fast a signal do youneed to turn on the a/c or operate apower window? And how much datadoes it really take to send a simple sig-nal to open a power door lock vs. oper-ating a complex emissions strategy? Asophisticated protocol can do a lot ofthings, but it may require more expen-sive modules to process information athigh speed. Why spend extra moneyfor simple requirements?

Most protocols (and therefore thedata buses and networks they serve) areproprietary and require specific soft-ware for diagnosis. Heard of GM’sE&C (entertainment and comfort) bus,introduced in the mid-’80s for opera-tion and diagnosis of radio, HVAC andsome other body systems? It’s a propri-etary data bus that requires specific di-agnostic software. Ditto for Chrysler’sCCD (Chrysler Collision Detection),which is used for chassis/body/engine

networks on many models through thepresent model year. Software for thesetwo are widely available in the after-market, but many other systems, partic-ularly safety-related, are dealer-only atthis time. Incidentally, CCD offers thefollowing good example of a fault intol-erance: If a module loses its ground,the network goes down (although manyindividual functions continue).

Data bus speed and electronic“width”—traffic patterns, toll booths,etc., prevent you from driving the post-ed speed all the time. It’s basically thesame with a data bus, where a modulemay be asleep, has to wake up and letthe other modules know it’s awake, per-haps awakening them, too. The speedlimits in an automotive data bus are notposted in mph, but usually in baud rate,which are kilobytes per second (kB/sec).The “width” of the highway also affectsdata transmission speed—a 32-bit sys-

tem means that four times as much datacan be transmitted than one wideenough for only 8 zeroes and ones.

Proving that speed isn’t everything,GM has introduced a master/slave ar-chitecture using a version of its low-speed OBD II data bus, for its newBravada/Trailblazer/Envoy sport/utes. ATruck Body Controller is the master,and there are 17 modules in physicallydifferent zones, providing a broad rangeof features, from battery rundown pro-tection through HVAC, all lighting,seats, antitheft systems, wipers, washer,memory seats and mirrors and doorlocks, including personalization of manyadjustments through the remote.

Super-fast data buses and networksare prone to produce a lot of electricalnoise (electromagnetic interference),which causes errors in data transmis-

sion. There are many ways for a databus to detect errors, such as by check-ing the length of a particular transmis-sion. But if there’s an error detected,data has to be retransmitted, whichslows everything down again. The alter-natives are costly (more powerful, moresophisticated) modules, two wires vs.one (more expensive than you’d think)and even shielding of wiring.

To keep the price right, a data busand network has to be no faster or morecomplex than necessary. Most designsare done in at least three basic ver-sions—slow, moderate and fast.

Enforcing the protocol—may beself-enforcing (arbitration, token-ring,master/slave). It also may be done withthe aid of electronic chips that decidewho goes and when. The ChryslerCCD data bus has such a chip in eachmodule, and the way everything workstogether is Chrysler’s proprietary de-sign.

Engineering Standards:Where They’ve Taken Usand What’s Coming SoonWhen the term standard is used, we’retalking about an engineering standard.Think of a protocol standard as sort ofa National Highway Act, in whichthere are broad guidelines, such asmaximum speed, lane width, etc. Buteach state and city gets to fill in the de-tails when it builds roads. The oldcliche “The devil is in the details” ap-plies to local highway laws, and cer-tainly to OBD II, as well.

OBD II was legally required acrossthe board by 1996, although some vehi-cles were equipped with the system asearly as ’94. The objective of OBD IIwas to create a generic protocol stan-dard for detection and diagnosis ofemissions-related failures and operatingconditions of specific systems. Everyonehad to use the same 16-pin diagnosticconnector and produce specific num-bered trouble codes that when loggedby the powertrain computer would re-port to a generic scan tool. In addition,certain data items called PIDs (parame-ter IDs) that were in the emissions con-trol area would have to be displayed.

What emerged was J1850 from theSociety of Automotive Engineers—

28 July 2001

NEGOTIATING THE MULTIPLEXING MAZE

As a service technician,you really don’t care

about the protocol itself,only the effect it canhave on diagnosis.

29July 2001

Just fo show you how far we’ve come in multiplexing, the new Lexus LS 430 has 29 computers (ECUs) wired to severaldata buses, which in turn are wired into networks (AVC is audio visual communications network, BEAN is body elec-tronics area network). Three J/B (junction box) computers also are used, serving as two front and one rear zonemodules, which provide diagnostic assistance (including convenient disconnection and testing points).

System Diagram

Backup Bus Fail-Safe Line

really 21⁄2 data bus protocols under asingle banner. One is a GM “Class 2Bus” protocol that operates at10.4kB/sec with one wire. A second isthe Ford “Standard Corporate Proto-col,” which operates at 41.6kB/sec with

two wires. Finally, there’s a Chrysleradaptation of the GM Class 2. TheGM and Ford protocols cause a databus to operate in totally different ways.These protocols not only feed the scan-ner, but operate the data bus, as well.

ISO 9141-2 (from the European-influenced International Standards Or-ganization). This is a single-wire sys-tem, but nothing like anything inJ1850. Modules talk only when asked,and only to the scan tool, not to each

30 July 2001

NEGOTIATING THE MULTIPLEXING MAZE

Chrysler’s CCD (Chrysler Collision Detection) uses achip in each module to ensure that messages get onthe data bus in a predetermined priority and with aclear path, so they don’t collide. The engine compart-ment node (data sensing and transmitting module)and lamp outage module provide data signals avail-able to the engine computer (SMEC), body computerand EVIC (electronic vehicle information center).

Single Module Engine Controller (SMEC)

Engine Compartment Node

EVIC Overhead Console

Bus � Bus �

� �

DiagnosticConnector

Lamp OutageModule

Premium BodyComputer

other, so it’s a master/slave setup. Andit’s even slower than GM and Chryslerversions of SAE J1850. It has a longwake-up call and allows lots of time foreach module to report each PID.

ISO 14230. This was considered anupgrade to ISO 9141-2 and was in useby 1997. It has a faster wake-up calland a system to bypass any PIDs thatare not supported by the data bus.

All existing protocols support burstmode, a setup where you can ask theon-board system to transmit in “multi-word” bunches, so perhaps a half-dozen PIDs can be transmitted and up-dated continuously. In standard mode,the scanner waits for each PID to be

reported one by one, then displaysthem all...slowly. Burst mode obviouslyis handy for diagnosing intermittents.

However, “supports” doesn’t neces-sarily mean “available.” While someOBD II vehicles actually provide burstmode, others don’t. It varies by model,so check your CD-ROM informationsystem. If burst isn’t supported, pickonly the most likely PID or two forevaluation, or the information willcome slooowly.

Basic CompatibilityIt should be obvious that all theseOBD II protocols involve differentcomputer languages, and with propri-

etary protocols, the electronics can becomplex. Cadillac Catera, for example,uses ISO 14230, J1850, GM’s E&Cand CAN. Just as you can learn a for-eign language, a scan tool can be pro-grammed to recognize all these proto-col languages.

But if you thought that when youbought your first OBD II scan tool thegeneric coverage would be complete,you’ve now found out otherwise. Forexample, an early-release generic scan-ner will not “hear” anything from a da-ta bus using ISO 14230. It will thinknothing is being transmitted, and tellyou that on its digital display. The toolneeds a software upgrade.

32 July 2001

NEGOTIATING THE MULTIPLEXING MAZE

Chrysler’s CCD is a twisted two-wire system, with the digital signals based on voltage differences between positive and neg-ative wires. This illustration shows all the signals that go into the node. They all come out (virtually at the same time, if nec-essary) on the two-wire (M1 and M2) data bus. Although it’s going away, the system is still used on some present-day trucks.

To Body Computer

Electronic VehicleInformation Center(Overhead Console) Engine

CompartmentNode

Note that almost all ISO systemson vehicles that underwent anythingbut a trim change went from ISO9141-2 to ISO 14230.

In-Depth CompatibilityRemember our cliche about the devilbeing in the details? One team of soft-ware engineers will read the protocolsand their in-depth rules one way, whileothers come up with slightly differentinterpretations. So long as they’re veryclose, everyone’s scan tool should dothe same on OBD II generic. An after-market company may choose to ignoresome PIDs if it feels they’re not usefulto the independent technician, or du-

plicated by other PIDs, but the key in-formation should be there.

Unfortunately, real problems havearisen with generic OBD II. Here area couple of examples:

•Chrysler trouble codes. There wasa programming error on early-produc-tion 2001 Ram Vans and Trucks, Dako-tas and Durangos, Jeep Wranglers andGrand Cherokees and Vipers. So ageneric scan tool won’t display six oxy-gen sensor heater codes or ambientsensor temperature. And on Viper, italso may generate a false P1394 code(for the leak detection pump switch).There’s a new program out there, butdon’t bet dealers will bother repro-

gramming, because the error doesn’taffect them. The problem doesn’t arisewith the DRB III (or aftermarket scantools using Chrysler’s enhanced mode).

•Hyundai and Kia. Late models ofthese vehicles use ISO 14230, but theyhave German engine computers andKorean transmission controllers. Whenyou plug in your scanner, you’ll see amessage about emissions “readinesstests,” saying whether they’re complet-ed or not. However, when you try tograb trouble codes or PIDs, you’ll runinto a problem. The transmission con-troller may get on the line and starttalking. But it has nothing meaningfulto say. Its jabber is interpreted by your

33July 2001

This basic GM-type OBD II layout shows all the wiring inputs to and outputs from the VCM (vehicle control module) andthe one-wire data bus to the 16-pin DLC (diagnostic link connector). In many vehicles, there also would be another da-ta bus connection to the ABS module so the VCM could use wheel speed sensor information as an input for vehiclespeed. Because the VCM does double-duty controlling both the engine and transmission, there’s no need for an addi-tional data bus connection to a transmission computer, as there is on many other vehicles.

GM DATA BUS (J1850)

scan tool as “no transmission,” which iswhat you’ll see on the display. You cantry plugging in your scanner and start-ing the engine once more, and thenagain, and again. At some point youmay be lucky and the engine controllerwill get on the line first, giving you ac-cess to the codes and PIDs.

Information Sharing:Meet the GatewayWith so many different data buses andnetworks in use, there has to be a wayfor them to share information accu-rately and without protocol conflicts.The powertrain electronics may wantto be awakened when the driver un-locks the car, for example. To permitan error-free exchange between databuses that operate with different pro-tocols and different speeds, there arespecial computers called gateways.

Gateway refers to a type of module,and how well it can do its job deter-mines the effectiveness of any attemptto get different computer data buses,modules, networks, etc., to talk to eachother. In effect, your OBD II scannerperforms a gateway function to its owndisplay, for the generic OBD II proto-cols. A gateway is like the guard at agated residential community. Beforehe raises the gate to let anyone in, hefinds out if the guest is invited. Or hemay just pass some information to aresident. The gateway module does thesame thing for communication be-tween data buses and networks thatare not compatible but need informa-tion from each other. But don’t neces-sarily blame the messenger (the gate-way) if the data doesn’t get through.The software may be faulty in one ormore modules.

CAN Is Here, and NowIt’s for DiagnosticsWhy do we need still another networkand protocol? CAN (Controller AreaNetwork) actually has been around foryears, used just for powertrain controlon many big-rig trucks and Europeancars. However, diagnostic data goesthrough a gateway to a J1850 or ISOdata bus. CAN itself is legal for emis-sions diagnosis in Europe now, and ap-parently will be approved for use in

43July 2001

NEGOTIATING THE MULTIPLEXING MAZE

When the Intelligent Data Bus standards are complete and motor vehicles actuallyare equipped with an IDB gateway, it will be possible to plug in and play aftermarketaccessories such as these video monitors for computer games and DVD movies,as was done as a demonstration in the back of this Lincoln Navigator (above). TheNavigator also has a port for plugging in a Sony PlayStation, shown below.

the U.S. by next year. When there’stalk about CAN, it’s really CAN C, thehighest speed network, although therealso will be widespread use of CAN B,a medium-speed network.

CAN C runs at very high speed,permitting ultra-fast emissions control,which explains its popularity. It’s atwo-wire system with multiple mastermodules, so failure of one has little ef-fect on operation. It’s intended to op-

erate at a 500kB/sec baud rate, about50 times faster than GM’s Class 2 databus version of J1850 and over 60 timesfaster than ISO 9141-2. When CAN Cis used for diagnostics, you can expecta near revolution in scanner perfor-mance. As one electronic architecturespecialist at Chrysler put it, “That’squasi-realtime.” Others are not quiteso exuberant, claiming that some drive-ability glitches come and go even faster,so don’t expect your multitrace oscillo-scope to gather dust just yet.

CAN B, the medium-speed network(nominally about 125kB/sec), will beused for body electrical systems andprobably will operate at just 83.3kB/secat Chrysler, according to its engineer-ing staff. Will CAN B and CAN C be

able to work together (doing a CAN-CAN?)? No, there’d be too big a dif-ference in “leg-kicking speed.” Therehas to be a gateway between them.

The bottom line on CAN C is thatyour present scan tool won’t be able todo CAN diagnostics, and you’ll likelyneed more than just new software.Can your scan tool be upgraded eco-nomically? Some have the internalhardware or plug-in configuration that

permits this; others don’t and will notbe upgraded, particularly older andlower-end scan tools from severalmakers. Before you buy somethingnew today, ask the questions regardingCAN diagnostic capabilities. If thesalesman doesn’t know, call the techni-cal service department at the toolmanufacturer.

Plug & Play Is On the WayIf you have a personal computer with“plug and play” software, you knowthe feature doesn ’t always work.(Some call it “plug and pray.”) Andso-called standard formats in graphics(TIF, JPG and PDF) sometimes doand sometimes don’t. Be prepared forthat in automotive accessories, in-

cluding the scanner software thatmay be marketed to diagnose trouble.The proposed standards are “open” toall, but how they’re interpreted willdetermine how effective they are.

Virtually all the vehicle makershave signed on to a proposed “plugand play” standard for add-on acces-sories called Intelligent Data Bus, orIDB. But like OBD II, it already hastwo distinct proposals for “the de-tails.” One is to use IEEE1394, theApple standard for personal comput-er systems better known as“Firewire.” It’s fast and sophisticated,and Ford has been showing a LincolnNavigator rear-seat entertainmentsystem (Sony PlayStation) based onit. Pluses: A lot of Firewire productalready is out, ready for plugging in,and more is in the pipeline.

But German manufacturers are sup-porting MOST (see the sidebar“Acronym Stew” at left), an alternative.The first set of specs from the IDB con-sortium hasn’t picked a winner. Will thefinal specs (due out in a year) do that?Not likely. It probably will be like SAEJ1850, and give both sides room to dealwith the IDB gateway.

IDB is for physical connections.Wireless standards also are in theworks for cell phones and other de-vices, such as Palm Pilot and its com-petitors.

If you got through to here, you’reentitled to ask: Have we told youeverything you need to know aboutmultiplexing? Truth is, we haven’teven come close. We gave you just theabsolute basics. However, causes ofproblems you’ve been encounteringshould start to clear up. And as thenew systems come out, you’ll pick updetails that will not seem as thoughthey’re in a foreign language. You’ll beable to ask the right questions whenyou buy or upgrade your test equip-ment. And we’ll tell you more, to helpyou negotiate the learning curve.

44 July 2001

NEGOTIATING THE MULTIPLEXING MAZE

If most of the following definitionscause your eyes to glaze over,

don’t worry. The guys who designthis stuff will not be the ones whopick catchy names to market it.

UART (universal asynchronous re-ceive-transmit). A type of serial datatransmission. As used in motor vehi-cles, it will communicate by voltagechanges under the (typically) 5-voltor 12-volt peak. It goes back to OBDI on GM vehicles, and is used in ISO9141-2/14230 for OBD II.

TTA or TTP (time-triggered ar-chitecture or time-triggered proto-col). A highly fault-tolerant com-puter control system that seems tobe the choice for steer-by-wire andbrake-by-wire systems.

WAP (wireless automotive pro-tocol). A Delphi proposal for awireless multiplex system to be

connected through a gateway tothe vehicle’s wired electronics.

ATRIsys (Automotive TechnicalResources Inc. system). Anotherproposal for a wireless multiplexsystem for the motor vehicle.

MOST (media-oriented systemtransport). A proposed plug-and-play standard using fiber-optic lines.

Bluetooth. Not an acronym, justa weird name for a protocol cur-rently in use for cell phones andsome other wireless devices. Thehardware that goes with it has arange of only about 30 feet. How-ever, that’s enough for the vehicle’selectronics to sense the presence ofa cell phone and tie it in. It’s a ra-dio frequency standard created bySwedish cell phone maker Ericsson.The name somehow honors aViking king named Harald Blatand.

Visit www.motor.com todownload a free copy of this

article. Copies are also availableby sending $3 for each copy to:

Fulfillment Dept., MOTOR Magazine,5600 Crooks Rd., Troy, MI 48098.

Acronym Stew