- CAN-dat - K-line-dat - OBD-dat
ISO 9141-2 (K-Line) Protocol Overview
ISO 9141-2 is a diagnostic communication protocol primarily used in European and Asian vehicles manufactured between the late 1990s and the mid-2000s. It is the "language" used by an OBD-II scanner to talk to the Engine Control Unit (ECU).
1. Physical Layer & Pins
The protocol relies on a single-wire or dual-wire serial bus found on the standard 16-pin OBD-II connector:
- K-Line (Pin 7): The bidirectional line used for all data exchange (queries and responses).
- L-Line (Pin 15): An optional unidirectional line used only for "waking up" the system in older implementations.
2. Key Technical Specifications
- Signaling: Asynchronous serial communication (UART).
- Voltage Levels: Based on the vehicle battery (0V for Logic Low, 12V for Logic High).
- Baud Rate: Usually fixed at 10.4 kbps.
- Initialization: Requires a specific "5-baud init" (very slow pulse) or a "Fast Init" sequence to establish a handshake before data flows.
3. Comparison with Modern CAN Bus
| Feature | ISO 9141-2 (K-Line) | ISO 15765 (CAN Bus) |
|---|---|---|
| Data Speed | 10.4 kbps | 500 kbps |
| Architecture | Master-Slave (Request/Response) | Multi-Master (Broadcast) |
| Typical Era | 1996 – 2007 | 2008 – Present |
| Hardware | Single wire (K) | Twisted pair (High/Low) |
4. Implementation for DIY Projects
Since you are familiar with motor drivers and IMU sensors, you can think of the K-Line as a standard UART signal shifted to 12V.
If you wanted to interface a 3.3V microcontroller (like an ESP32) with a car's K-Line, you cannot connect them directly. You would need a Level Shifter or a dedicated K-Line transceiver chip (such as the L9637D or MC33660) to handle the 12V swings without frying your MCU.