The J1850 PWM and J1850 VPW protocols are two of the legacy lower-layer physical standards defined under OBD-II, which is mandated by US federal law for all petrol cars sold from model year 1996 onward. J1850 PWM uses two-wire differential signalling on OBD-II DLC pins 2 (bus positive) and 10 (bus negative) at 41.6 kbps and was used primarily in Ford vehicles up to approximately model year 2007. J1850 VPW uses a single wire on pin 2 only at 10.4 kbps with variable-pulse-width encoding, and was used in General Motors and some Chrysler vehicles over the same era. The RS-232 adapter cable described here converts either bus to a PC serial port using third-party signal conditioning not covered by SAE J1962 or the J1850 standard itself.
Notes
- The OBD-II Data Link Connector (DLC) is a 16-pin trapezoidal connector specified by SAE J1962 / ISO 15031-3. Mandatory pins across all variants are pin 4 (chassis ground), pin 5 (signal ground), and pin 16 (battery positive, 12 V on Type A vehicles).
- J1850 PWM: differential two-wire bus; pin 2 bus positive (B+) active at 5 V, pin 10 bus negative (B−) active at 0 V; nominal data rate 41.6 kbps; maximum message payload 12 bytes including CRC.
- J1850 VPW: single-wire bus on pin 2; idle/passive at 0 V, active at 7–8 V; bit timing via 64 µs (logic 1) and 128 µs (logic 0) pulse widths; start-of-frame pulse 200 µs; data rate 10.4 kbps; bus maximum 35 m, up to 32 nodes.
- Both J1850 variants were superseded by ISO 15765-4 CAN (pins 6 and 14), which became mandatory for all US-market cars from 1 January 2008.
- The maximum permitted OBD adapter cable length is 5 m per SAE J1962.
- The RS-232 conversion side of this cable is a proprietary third-party interface; its signal conditioning circuit is not defined by any SAE or ISO OBD-II standard.
References
- SAE J1962_201607: Diagnostic Connector — SAE International
- How OBD-II Vehicle Diagnostics Work, Part 2: A Closer Look — Circuit Crush
- Automotive Diagnostic Standards — x-engineer.org