connector or cable wiring scheme
9 pin D-SUB female connector at the computer
16 pin car OBD2 special connector at the car (looking into the car-side connector)
ISO 9141-2 and ISO 14230-4 (KWP2000) are two of the five original OBD-II data-link protocols, both using the K-line on pin 7 of the SAE J1962 16-pin Data Link Connector (DLC) as a single bidirectional half-duplex serial wire at 10.4 kbaud with 8N1 framing. The optional L-line on pin 15 is used only during slow initialisation on some ISO 9141-2 implementations. These protocols were prevalent on European and Asian vehicles from roughly 1996 to 2007, including Volkswagen, Audi, Skoda, and Seat (VAG group), as well as Alfa Romeo, BMW, Fiat, Honda, Mercedes-Benz, and many others; CAN (ISO 15765-4) replaced K-line as the mandatory OBD-II protocol from the 2008 US model year. The cable described here connects pin 7 (K-line) and optionally pin 15 (L-line) of the J1962 DLC to a 9-pin DB9 female RS-232 connector via a dedicated K-line transceiver IC (such as the NXP MC33290 or ST L9637) or a discrete transistor level-shifter.
Notes
- The K-line idles high at battery voltage (~12 V) via a pull-up resistor to VBat; a dominant (low) bit pulls the line toward 0 V. The K-line is not electrically compatible with RS-232 or 3.3/5 V CMOS logic; a dedicated K-line transceiver is required between the bus and a microcontroller UART or serial port.
- ISO 9141-2 uses a 5-baud slow initialisation: the tester transmits the target address at 5 bps on the K-line; the ECU responds with sync byte 0x55, keyword bytes KW1 and KW2; the tester acknowledges with the bitwise complement of KW2; normal 10.4 kbaud communication then begins.
- ISO 14230-4 (KWP2000) adds a fast-init option: K-line pulled low for 25 ms then released for 25 ms, after which 10.4 kbaud communication begins immediately.
- The SAE J1962 DLC provides pin 4 (chassis ground), pin 5 (signal ground), and pin 16 (unswitched battery +12 V, minimum 4 A) mandatory on all vehicles.
- Type A connectors (12 V systems) and Type B connectors (24 V systems) are distinguished by an interrupted centre groove on the Type B variant that prevents insertion of a Type A male plug.
References
- SAE J1962: Diagnostic Connector (January 1995)
- ISO 14230-4:2000 — Road Vehicles: Diagnostic Systems, KWP2000 Part 4
- SAE J1962_201607: Diagnostic Connector (July 2016). SAE International.
- MC33290: ISO K Line Serial Link Interface — Datasheet (NXP/Freescale)
- L9637: Monolithic Bus Driver with ISO 9141 Interface — Datasheet (STMicroelectronics)