K-Line

The K-Line was one of the first bus systems to be specified in ISO 9141 and used in vehicle diagnostics for communication with multiple control units.

In principle, it works like the RS-232 serial interface familiar from the early days of personal computing, but it operates at different voltage levels. Using a simple level converter, a computer could even communicate directly with the vehicle.

In the automotive industry, the typical baud rate of 10,400 was widely used for standard diagnostic operations, but other typical values—such as 2,400, 9,600, and 19,200—were also common, and for the latest control units of this generation, baud rates of up to 115,200 in flash mode were also quite common.

In order for a diagnostic tester to establish a connection to an ECU, the connection must first be initialized. This process is referred to as “pinging.” Either the 5-baud init or the fast init (which takes approximately 1.8 seconds) is used for this purpose. During this initialization, the address of the desired control unit is also transmitted, meaning that only the addressed control unit establishes communication. This transforms the purely serial question-and-answer protocol with a single control unit into a type of bus system in which multiple control units can be addressed, but only one at a time per K-Line. During initialization, a second line was sometimes required. However, this line led a shadowy existence and was not specified for actual diagnostic communication.

KWP 1281 and KWP 2000 are often cited as the first diagnostic protocols on the K-Line. KWP 1281 was used by Volkswagen. It was not until KWP 2000 that the protocol became widely adopted, thanks to ISO 14230. One of the main arguments in favor of KWP 2000 was that this protocol made it possible to update control modules with new software and data versions in a standardized manner — a process known as “flashing.” Today, this process is more commonly referred to as a software update.