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For the Model Year 1988 Chrysler Corporation was successful in introducing an effective Data Communications Class of Vehicle Multiplexing that reduced the average vehicle cost. Based on this concept a full vehicle multiplex network proliferated into most of Chrysler's product line. Success in vehicle multiplexing can be attributed to the extremely low component cost and the outstanding flexibility in the C2D multiplexing system. Presently Chrysler is replacing this system with the SAE J1850 Standard Data Communications Network. Chrysler and Harris Engineers were able to demonstrate significant improvements in performance and system cost. A new multiplexing term “Multiplex Effectiveness” is presented and explains how it plays a significant role in demonstrating the most effective architecture for future multiplexing endeavors. The conceptual use of SAE J1850 multiplexing capabilities such as Header Bytes, In-Frame Response and Error Detection will be discussed.

This paper proposes the adaptation of a modulation technique called Modified Frequency Modulation (MFM) to vehicle multiplexing. MFM was developed during the latter 1960's for use in magnetic disk drives. Disk drives use MFM encoding to achieve a maximum density of recorded data on a disk. The advantage in vehicle multiplexing is that the technique is synchronous with an average of 0.75 transitions per bit. Another advantage is that it can tolerate a large amount of rise and fall time wave shaping, which can significantly reduce radiated EMI. The paper will compare the EMI characteristics generated by NRZ, PWM, VPWM, Manchester, and MFM encoding. Included are typical encoding requirements such as symbol generation, arbitration capabilities, latency, invalid bit testing as well as encoding techniques effect on the host microcomputer.