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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.

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.