The K-line, operating under the ISO9141-2 standard and also known as Keyword 2000 or CARB 9141, is a single-wire, low-speed serial communication system prevalent in many cars and commercial vehicles. It’s primarily used for diagnostic connections, facilitating communication between a vehicle’s Electronic Control Modules (ECMs) and diagnostic equipment like scan tools and data loggers within the OBD2 system.
Unlike modern networks like CAN bus, the K-line network relies on a master-slave communication principle. In a CAN bus system, all ECMs are essentially equal, capable of both sending and receiving messages across the network. However, in a K-line or ISO 9141 compliant network, the direction of message flow is crucial. A master ECM dictates network control, managing message direction and timing based on which ECM is transmitting (speaking) and which is receiving (listening). This means ECMs cannot transmit simultaneously; they must await permission from the master ECM, ensuring orderly communication across the diagnostic line.
As illustrated in Figure 3, K-line networks utilize a single wire for all communication. Messages are transmitted in binary format, represented as pulsed voltage signals on this single wire. The K-line voltages pulse between two distinct values to represent binary code – a series of ones and zeros. These binary codes are interpreted based on the voltage levels shown in Figure 4.
It’s important to note that in K-line communication, logic 0 is represented by the battery voltage, which can be above 12V. Furthermore, K-line message structure differs from CAN bus. While CAN bus transmits complete messages at once, K-line may send messages in segments. While CAN bus networks operate continuously for both communication and diagnostics between ECMs during vehicle operation, K-line is specifically designed to support diagnostic equipment interactions. However, when no diagnostic tool is connected, the K-line wiring might be repurposed by other ECMs for communication at different baud rates and synchronization patterns, showcasing its versatility beyond solely OBD2 diagnostics.
