The K-Line is the oldest European diagnostic interface. It was already standardized the end of the 80s.

K-Line

Their standard comprised mainly the electrical properties, the type of bit transmission and the type of rendezvous. The diagnostic protocol itself was not specified. It was at that time, manufacturer-specific. This changed gradually. In 1994 the European Union, the American emissions legislation in the form of the diagnostic interface. As this is OBD-2 as a European on-board diagnostics known. And on this basis, the general vehicle diagnostics in the form of the KWP 2000 (Keyword Protocol 2000) was specified and standardized.

K-Line - Standardization

Physical Layer and bus topology

The K-line interface uses a character-based protocol with a bidirectional one-wire bus. The physical layer looks almost exactly like the LIN bus system. We are working with battery voltage levels. Optionally, there is an additional one-way line, called the L-Line.

Bus-und Physical Layer Topologies

The communication protocol in the diagnosis is essentially a master-slave protocol. The diagnostic tester as the master sends the requests (requests). One or more control devices as slaves reply with a corresponding response message (response). In the case of OBD as bit rate 10.4 kbit / s fixed.

The K-line protocol is easy to implement. Both on the ECU as well on the test page there is virtually entirely of software. The only required hardware component is a UART, which today brings each PC, each ECU and each microcontroller.

Data Link Layer

In contrast to CAN, which can send a control device at any time be in the K-Line, first to establish a connection. Only then can take place the data exchange. Moreover, at the end of the connection must be dismantled.

K-Line - Fast irritation to ISO 14230 (duration 100 ms)

To establish the connection are two methods in use: First, the newer, so-called fast-stimulation that was only with the KWP 2000 protocol is standardized, as shown above. This sends a message to the diagnostic tester to the controller and the controller responds with a series of bytes that contain a core component called a keyword. With this keyword, the control unit diagnostic tester, the details of the Protocol shall, with the desire to communicate with the controller. This method only works with a fixed baud rate, by the legislature for the OBD prescribed baud rate of 10.4 kbit / s.

K-Line - Address irritation (5 baud = 2.5 s)

For systems that do use a different baud rate and in older systems, the 5 Baudreizung used, see figure above. This takes about 2.6 seconds with a relatively long time. This diagnostic tester and controller communicate initially with 5 baud. Here they exchange information using the keyword through the communication protocol and baud rate, the control unit off. Then the baud rate can be a faster, baud rate supported by the control unit to be changed.

K-Line - Message Format

The data exchange works in the request-response procedure. That is, the diagnostic tester sends a request to the master control unit. The controller responds with a response. For both the same message format is used, see Fig. The first byte (format byte) contains information about the format of the following message. Then follow two bytes of address information and a byte that indicates how long the following user data. The two address information and the length specification is optional. The format byte is communicated to be as address and length, as shown below. The user data may comprise 1-255 bytes. This is followed by one byte checksum of all data bytes to detect errors.

K-Line message format - format byte

The error control based on monitoring of the message format, message length and checksum. On error, a control unit ignores a received request and sends a response. The diagnosis can recognize about timeouts and sends its request again. The timing parameters for the messages in accordance with the standards specified in figure below. These are the distances between the individual bytes of a data block, the distances between request and response and the minimum distance of a response to the nearest legal request.

K-Line - Troubleshooting

For the OBD, the legislative requirements to fulfill, there are a number of limitations:

• Header consists of 3 bytes (target, source address, no length byte)
• Embassies have max. 7 bytes of user data
• Tester Request Target Address is always functional
• ECU-response target Address physically accessible
• Only default timing parameters are allowed
• Only 10.4 kbit / s. For
• Other:
  • Transmission time of 255 bytes of data at 10.4 kbit / s of 250 ms to 5.5 s
  • Protocol implementation almost entirely in software - high interrupt load
  • ISO 9141 was the basis of many proprietary protocols (KW 71, KW 81, KW 500, etc.)