Eye On Electronics

Motor,  Jan 2005  by Dale, Mike

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When it comes to understanding data network construction and protocols, you can't afford to miss the bus, because the next one may be an express.

Computer networks are used so peripherals like printers can serve many users, such as in an office. Automotive networks are used to connect sensors, actuators and computers together with the least amount of wiring possible. More importantly, they allow information to be shared among independent computer systems in the vehicle. This is called multiplexing.

Multiplexing is a technique that uses a single data line to connect multiple sources of information or destinations for actuation. To make this work, it's necessary to come up with a system that controls when any given sensor can be asked for its information or when any actuator can be told what it should do. This is all about data buses and computer language systems called protocols.

Multiplexing is not new in automobiles. Versions of it have been around since the '70s. Originally, the idea was to minimize the weight and complexity of the wire harness. In a vehicle with a lot of high-end content, the cost of the wire harness could exceed that of the transmission. One reason is that assembly of the connectors, wires and terminal blocks is so labor-intensive. More importantly, electronic reliability is almost completely about connections. More connections mean more possible sources of trouble. Keeping the wire harness as simple as possible reduces weight, saves cost and makes vehicle assembly easier.

The common connection that allows multiple sources of information to communicate is called a data bus. The term bus originates from the name of a fuse company (Buss) that made early household electrical boxes. In that application, a single bar of copper was connected to the source of power. Individual fuses or circuit breakers led from this to different electrical circuits in the house. The electrical bus bar was really just an electrical connection common to all circuits.

Data buses can be formulated in a variety of ways (see the illustration). Each point on the network where information is exchanged is called a node. Each node must be able to receive instructions from the central computer (the master) and either send information to, or perform an action on, the individual actuators or sensors (slaves). Because of this, the total cost of the system depends on how many nodes have to be serviced. The cost also depends on how fast the information must be transferred.

The data bus itself can be created in several ways. A single copper wire that connects to all of the devices is one way. The problem is that we're dealing with digital data comprised of 1s and 0s. Nominally, the 1 value is 5 volts and the 0 value is 0 volts. In reality, the 1 can be anything more than 3.5 volts and the O can be anything less than 2.0 volts. It doesn't take much in the way of electromagnetic noise to make a mess out of things.

Real life data buses can be as simple as a single copper wire, if that wire is kept very short and is located in a part of the vehicle not subjected to much electromagnetic noise. More common is what's called a twisted pair. This is a pair of wires, one of them grounded, that wrap around each other in the direction of the wire's travel. This is a noise-canceling technique that has been around since the early days of radio.

There are other ways to form a data bus beyond the single wire or twisted pair. One is to use coaxially shielded cable, like the stuff used to hook up televisions to their satellites. The outer shield is grounded and the data passes safely through the protected inner wire. But coax is relatively bulky. In the case of an automotive wiring harness, it would be hard to route through the nooks and crannies of a body structure. It would also be more expensive than a single wire or a twisted pair.

Still another way to form a bus is to use fiber optics. The big advantage here is that fiber optics uses pulses of light that are unaffected by whatever is going on around them electromagnetically. Fiber optics also can handle far higher data exchange rates than the other types of busses. Of course, there are a couple of drawbacks. Of all the bus structures, fiber optics is by iar the most expensive. Data has to be converted into light pulses by turning LEDs on and off. It has to be reconverted on the other end so the computers can understand the information. Optical data buses, while they have huge advantages, can cost five times as much as a twisted pair of copper wires.

The term pmiocoZ is computerspeak for the rules and regulations that govern how information is exchanged on a data bus. So far there are about 75 different protocols in use on todays vehicles. They can be divided into four separate groups, based on their capabilities. Its important to understand that even though they can be grouped into four categories, none is compatible with any of the others.

The automakers are trying to change this. An outfit called AUTOSAR is a consortium of European, Japanese and U.S. automakers that has the goal of commonizing not only the software protocols but the computer hardware as well. As it stands today, all cars need sensors, computers and actuators. Each automaker has developed its own proprietary hardware and software. This misses out on the economies of scale that could be realized if the hardware and software were "open" in terms of their architecture.