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Serial data link communication throughout a vehicle has become an important area for new technology application of automotive engineering during the last several years. As the electronic content of vehicles increases, so does the need to transfer information between the various sensors, controllers, and actuators in that vehicle. This paper will discuss Class 2, General Motors' innovative version of the SAE J1850 Recommended Practice for Class B Data Communications Network Interface. An introduction to J1850 and related multiplexing topics will be followed by an explanation of the details of the Class 2 serial data link protocol. There are many benefits of using Class 2 including: the ability to standardize messages across vehicle types; one corporate database; the ability to change the size of the network easily due to the “open architecture” of the protocol, etc.

This paper is an overview of the current state (calendar year 2010) of in-vehicle multiplexing and what pertinent technologies are emerging. Usage and trends of in-vehicle networking protocols will be presented and categorized. The past few years have seen a large growth in the number and type of communication buses used in automobiles, trucks, construction equipment, and military, among others. Development continues even into boating and recreation vehicles. Areas for discussion will include SAE Class A, B, C, Diagnostics, SafetyBus, Mobile Media, Wireless, and X-by-Wire. All existing mainstream vehicular multiplex protocols (approximately 40) are categorized using the SAE convention as well as categories previously proposed by this author. Top contenders will be pointed out along with a discussion of the protocol in the best position to become the industry standard in each category.

The increasing features and complexity of today's automotive architectures are becoming increasingly difficult to manage. Each new innovation typically requires additional mechanical actuators and associated electrical controllers. The sheer number of black boxes and wiring are being limited not by features or cost but by the inability to physically assemble them into a vehicle. A new architecture is required which will support the ability to add new features but also enable the Vehicle Assembly Plants to easily assemble and test each subsystem. One such architecture is a distributed multiplex arrangement that reduces the number of wires while enabling flexibility and expandability. Previous versions have had to deal with issues such as noise immunity at high switching currents. The LIN Bus with its low cost and rail-to-rail capability may be the key enabling technology to make the multiplexed architecture a reality.

The current SAE classification system for serial data communication protocols encompasses Class A, Class B, and Class C categories. Because of the proliferation of applications and new protocols these three groups are not enough. This paper will introduce and discuss several new categories which are Diagnostics, SafetyBus, Mobile Media, and X-by-Wire. The serial data protocols that fall under these categories are for the most part brand new and will serve distinct and unique tasks. All existing common vehicular multiplex protocols (approximately 40) will be categorized using the SAE convention plus the new groupings. Top contenders will be pointed out along with a discussion of the protocol in the best position to become the industry standard in each category. Future vehicle applications having up to seven different data networks will be presented.

A previous SAE 2001 Congress paper, “Multiplex Bus Progression” [1] introduced the idea of categorizing vehicle serial data protocols into additional areas beyond the traditional SAE Class A, B, and C. This paper will expand on that idea, and provide a 2003 update to the Diagnostics, SafetyBus, Mobile Media, and X-by-Wire categories. All existing mainstream vehicular multiplex protocols (approximately 40) are categorized using the SAE convention plus the new groupings. Top contenders will be pointed out along with a discussion of the protocol in the best position to become the industry standard in each category at this time.

For a given CAN application the nominal data rate can be programmed in the range of about 5 Kb/s to 1 Mb/s. Each bit is made up of programmable segments and programmable sampling and synchronization elements. These parameters may be chosen so as to favorably influence the allowed propagation delay through the CAN link and the allowed oscillator frequency tolerance for a CAN node. This paper will investigate these issues and define several equations that demonstrate the interrelationships of the various CAN bit elements, and the effect that propagation delays and oscillator tolerance have on the fundamental ability of CAN receivers to successfully decode a CAN waveform. Graphs are given to illustrate the allowable CAN bus length and oscillator tolerance for various bit rates.