Almost simultaneously with the K-Line in Europe originated in the USA SAE J1850 interface. On the one hand, it was also intended for diagnostic applications, on the other hand, it was provided, in contrast to the K-Line, and for the on-board communications.

SAE J1850

The SAE J1850 interface was standardized in 1994 and was, as the K-line interface that accepts the OBD. The interesting thing about the J1850 standard is that it actually contains two different standards in a standard. On the one hand, the solution that the company favored Ford. On the other hand, a solution prepared by General Motors, Chrysler and some other companies was used. These two versions are incompatible at the level of the physical layer, although they can be found in the same standard. Only the data link layer is the same for both versions.

Comparison of SAE J1850 PWM and VPWM

Since 2007, all new vehicles in the U.S. CAN be used as a diagnostic interface.

Physical- und Data Link Layer

For the bit coding uses J1850 not the usual NRZ process, but pulse-width modulated signals, as illustrated. In PWM varies the pulse duration, while the pulse itself is constant. In VPWM is additionally changed the pulse rate.

SAE J1850 - Bit coding

The message format, so the question of the composition of the individual bits of an entire message is the same in both cases. There is a header that can be a long 1-3 bytes. There is a trailer at the end again with a checksum and a number of other control bits. The user data is between 0 and 8 bytes.

SAE J1850 - Botschaftsformat

As a special feature of this format within the SAE J1850 message format is the possibility of a so-called in-frame response. That is, a controller can respond at very short answer to a question already within such a message frame. J1850 is to be implemented on the basis of pulse width modulation, and of specific in-frame responses relatively complicated and can be put into practice only with the help of a special communication controller.