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RAMBLINı MAN

by Tom Cantrell

Start ı Little Network ı Mini-Message ı LO BAT ı Caravan ı Sources and PDF

Circuit Cellar is revered for going under the hood, and judging by the response, you take it literally. My road-warrior articles such as "Saab Story" (Circuit Cellar 57) and "On the Road Again" (Circuit Cellar 118) invariably generated a lot of comments. Clearly, automotive electronics is a popular topic. Besides the fact that itıs arguably the heavyweight of embedded applications, I suspect that many engineers are into cars as well as chips and can fully imagine the possibilities offered by bringing them together.

In a nutshell, whatıs going on under the hood in cars reflects the overall trendınetwork anything and everything thatıs got a moving electron.

At the top level, I think itıs safe to predict that cars ultimately will be Internet-enabled. But, TCP/IP and HTTP will go only so far. Under-the-hood networking remains the province of the car manufacturers who historically havenıt been afraid to roll their own standards and such.

Nevertheless, recent years have seen the automakers muddling through the process of rationalizing their networking schemes. With more than a bit of prodding from regulatory and pollution control authorities, networking standards have taken hold.

Not that the old proprietary parts counter and authorized service mentality has gone down without a fight. For instance, the historic U.S. car networking standard (SAE J1850) comes in different flavors, with each camp adding their own proprietary and incompatible tweaks.

Fordıs version of J1850 runs at 41.6 kbps over a two-wire differential pair using conventional PWM modulation. By contrast, GMıs version runs at 10.4 kbps on a single wire using a variable pulse width scheme. What a hassle for chips, parts, and tool suppliers!

Even as I write, the J1850 era is coming to a close. Yes, it was a kludge. Not surprising given its mishmash of proprietary roots. But, I wouldnıt go so far as some people and call it a failure. The fact is, J1850 irrevocably set a course towards ubiquitous VolksComputing and established the proving ground of daily drivers needed to make it happen.

The Europeans may have trouble agreeing on the Euro, but theyıve done a great job prodding things along towards CAN, the emerging one-world automotive networking standard (see Figure 1).

Figure 1ıMercedes S-Class automotive electronics is all about microcontrollers and networking. (enlarge)

But, networking (or multiplexing, as aficionados say) in cars is a hierarchical situation. Schemes like J1850 and CAN were conceived for serving high-level (so-called Class B or C) communication among major subsystems such as the engine, transmission, and ABS control units.

Unfortunately, based on my own experience under the hood, a major goal of networking (reducing the packaging, weight, reliability, and cost burden associated with the traditional wiring harness) remains elusive. J1850 and CAN may be eliminating some wires, but in many cases, theyıre inspiration for new features that (you guessed it) require new wires. For example, in my ı99, even the stereo is connected to J1850, offering speed-sensitive volume control.

The bulk of the wiring challenge doesnıt lie with such whimsically brainy features. Rather, itıs the low-tech stuff like switches and lights that are still mainly wired point-to-point like in yesteryear.

Forget electronically variable valve timing, active suspension, drive-by-wire, and the rest and consider something mundane like a driver-side door. Itıs home to a growing litter of switches (windows, lock, and mirrors), motors (windows and mirrors), actuators (locks and folding mirrors), lights (courtesy, marker, switch, and turn signal in mirror), and so on. The situation is repeated for other major assemblies such as seats, climate control, and lighting. J1850 and CAN are overkill, but running dozens of wires hither and yon hardly seems like progress. What to do?

NEXT

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