SAE J1850

The SAE J1850 interface was intended for both diagnostic applications and on-board communication. Access was standardized earlier in the USA than in Europe, leading to an earlier introduction of OBD. The bus standardized by SAE J1850 was used for this purpose.

Strictly speaking, the J1850 standard comprises two standards. These differ at the physical layer and are incompatible. There is the so-called Ford variant and the variant used by General Motors, Chrysler, and other companies.

A distinctive feature of this standard is the use of pulse-width modulated signals instead of the conventional NRZ (Non-Return-to-Zero) method. Regarding the message format, a header is transmitted first, followed by a payload section ranging from 0 to 8 bytes. Finally, a trailer containing a checksum and several control bits is appended. The standard defines an “in-frame response,” allowing the control unit to respond directly within the frame in the case of short responses to a request.

Due to the characteristics of J1850, the system is quite complex to implement and typically requires a dedicated communication controller. The bus has since been superseded by the CAN bus.