Electronic Design

Communications Standard Drives Development Of Vehicle Networks

Adding remote-control features to an auto design can afford drivers and passengers greater comfort and convenience. Unfortunately, the electronics associated with these accessories can be expensive, often limiting them to luxury models. As remote functions increase, it becomes more difficult to maintain communications between them and the car's main body network.

To address this problem, automotive and electronics firms are developing a new communications standard—the Local Interconnect Network (LIN). This system will cut development, production, service, and logistics costs for in-vehicle electronics. As it gains popularity, this standard should replace current low-end multiplex solutions.

LIN is the brainchild of an industry consortium that includes Austin, Texas-based semiconductor giant Motorola Inc., and communications specialist Volcano Communications Technologies AB of Gothenburg, Sweden. Automakers such as Audi AG, BMW AG, DaimlerChrysler AG, Volvo Car Corp. AB, and Volkswagen are partners as well. The proposed network is an open communications standard for class-A serial buses. Although primarily intended for automotive applications, it may also benefit industrial or white-good electronics. The specification defines a communication protocol as well as tool and application programming interfaces.

In the automotive realm, the LIN bus links assemblies such as those found in door locks, mirrors, windows, seats, climate controls, and lighting (see the figure). Once connected to the LIN bus, these devices are available for various types of diagnostics and services. These assemblies used to be directly controlled by wires and switches. Now, they're guided by electronic control units, sensors, and actuators connected in a multiplexed controller area network (CAN). There are drawbacks to the CAN bus system, however. The cost per node is high, and as network traffic increases, its flow becomes harder to manage.

A subbus system, LIN complements CAN. It provides lower-cost connections within local network clusters that don't need CAN's performance, bandwidth, and complexity (see the table). LIN's communication protocol is based on ISO9141 NRZ transmission. It's a single-master and multiple-slave protocol transmitted over a single-wire bus. Power is provided by the car's 12-V battery. Message identification allows a multicast data transmission between any network node controlled by the master.

Manufacturing and component costs are lower using LIN than they are with CAN. LIN requires only one wire to CAN's two, and it eliminates the 5-V voltage regulator needed by CAN. It also uses UART/SCI interface hardware common to most microcontrollers, rather than the nonstandard CAN interface. In addition, a self-synchronization feature sidesteps the need for an external crystal in the slave nodes. As a result, mechatronic functions can be implemented using a single-chip solution. With LIN, hardware required to interface an electronic control unit to the vehicle network—wires, connector, and silicon—adds about $1 to the network cost. An interface to CAN costs nearly $2.

Besides reducing hardware costs, LIN simplifies traffic management by creating a hierarchical system. With LIN subsystems in place, CAN functions as a backbone that addresses only its main nodes. The LIN subbuses distribute messages between the main and local nodes within each LIN subsystem. In addition, messages within a subsystem don't have to be routed through the CAN backbone. The end result is a more controllable network.

Although news of the association was only recently an-nounced, work on the LIN standard has been in progress for some time. The coalition began as a workgroup in October of 1998, and the first specification documents were released last July. Release 1.0 of the standard is now available, with revisions soon to follow in release 1.1.

LIN-based applications are already in development and scheduled to reach production next year. It is expected that new car designs will initially contain three to ten LIN nodes each. Over the next decade, those numbers should rise to an average of twenty.

Motorola is one of the chip vendors that will supply microcontrollers, transceivers, and driver software for the LIN bus. The company also plans to supply smart connectors and actuators. Volcano Communications Technologies will offer development tools and software.

For more details, visit the LIN web site at www.lin-subbus.org.

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