connector or cable wiring scheme
9 pin D-SUB female connector
Components:
- R1-R10 - 560 ohm, 0.25W
- T1 - BC556 or 2N3906
- T2,T3 - BC546 or 2N3904
This adapter connects the OBD-II Data Link Connector (DLC) K-line diagnostic bus — defined by ISO 9141-2 and ISO 14230-4 (KWP2000) — to a standard RS-232 PC serial port. The K-line physical layer operates at 10.4 kbaud (8N1, half-duplex) on OBD-II pin 7, with pin 15 providing an optional L-line for wake-up initialisation. ISO 9141-2 uses a 5-baud slow initialisation sequence, whilst KWP2000 additionally supports a fast-init method. These protocols were common on European, Asian, and Chrysler vehicles from the late 1990s through to around 2007, before CAN (ISO 15765-4) became mandatory in the United States from 2008. The RS-232 conversion circuitry is a third-party interface, not defined by the OBD-II standards themselves.
Notes
- The OBD-II DLC is defined by SAE J1962 / ISO 15031-3 as a 16-pin trapezoidal female connector. Type A (12 V) and Type B (24 V) variants are distinguished by an interrupted centre groove on Type B.
- Mandatory OBD-II pins used by this adapter: pin 4 (chassis GND), pin 5 (signal GND), pin 16 (battery +12 V).
- Pin 7 (K-line) idles high at approximately 12 V via a pull-up resistor to battery voltage; a logic low represents a transmitted bit.
- Pin 15 (L-line) is a unidirectional wake-up line from tester to ECU, used only during the initialisation phase of ISO 9141-2; it may be omitted in minimal implementations.
- Resistors R1–R10 (560 Ω, 0.25 W) provide current limiting and voltage level shifting between the 12 V K-line signals and RS-232/TTL levels; T1 (BC556 / 2N3906, PNP) and T2/T3 (BC546 / 2N3904, NPN) perform the logic-level conversion.
- Maximum adapter cable length from the DLC is 5 m per SAE J1962.
- This adapter does not support CAN-based OBD-II (ISO 15765-4) or J1850-based protocols (SAE J1850 PWM/VPW).
References
- SAE J1962_201607: Diagnostic Connector — SAE International
- ISO 9141-2 and K-Line: 5-Baud Handshake Guide — OBD-Cable.com
- How OBD-II Vehicle Diagnostics Work, Part 2 — Circuit Crush
- Automotive Diagnostic Standards — x-engineer.org