K line
The K-line is a bidirectional one-wire bus for data transmission in automotive engineering according to ISO 9141 and ISO 14230-1 . In conjunction with the L line of the vehicle diagnosis, the K line can also be operated unidirectionally .
The L line is designed according to the standards only for irritation (su, initialization) of the control unit and thereafter remains at high level. Such an initialization is uncommon in practice and is handled by all common vehicles via the K line. Some luxury vehicles from the manufacturer Audi use the L line to exchange user data, which does not conform to the standard and for which a special adapter (usually referred to as KKL or K / KL or similar) is required.
In more modern vehicles, the CAN bus is now used instead of the K line , which, depending on the emissions standard , is even legally required as an OBD interface.
application
The K and L lines are mainly used for external communication, for example for on-board diagnosis of control units in the workshop. The transmission can only take place in one of the two directions at a time, but also in the other after the completion of a data telegram. Nevertheless, the characteristic of a bus is retained so that communication with several control units is possible via the same cable: A specific control unit is addressed with its ID during the stimulation (see below) and then only communicates with it until another control unit is addressed. Transfer rates of 9,600 or 10,400 baud are typically used.
The bus works with levels that are defined relative to the on-board voltage (12 or 24 V ) of the vehicle and, depending on the direction of the signal (vehicle to the diagnostic device or vice versa) for a logical 0 ( low ) 0-30% and for a logical 1 ( high ) 70–100% of the battery voltage.
In practice, the interface can be operated on almost any commercially available PC using a simple so-called K-line adapter that is connected to a normal serial RS-232 or USB interface . Since most adapters also support the L line, they are often referred to as a KL interface . An integrated ISO component (e.g. L9637D from STMicroelectronics, Si9243AEY from Vishay or MC33290D from Freescale) can be used to convert signals to RS232-compatible signal levels . Alternatively, there are outdated circuits with optocouplers , whereby the design by Jeff Noxon is, so to speak, the forefather of such circuits. However, circuits with optocouplers occasionally cause problems when coordinating the signal level.
irritation
The initialization of the communication between the external diagnostic computer and a control unit takes place in the form of what is known as stimulation. This can be done in two different ways:
Fast initialization
The stimulation occurs with a logical 0 for a duration of 1.8 s ± 0.01 s. After this stimulation, normal communication is started.
5 baud initialization
The stimulation takes place here with extremely slow 5 Bd., Namely with telegram bytes that deliberately violate the parity conditions. Only after this stimulation phase is normal communication started at the final speed.
Another variant
Functionally practically identical, but less common and with a slightly different area of application is the W cable . It is mostly used for communication between the immobilizer and the engine control unit , less often for the transmission of diagnostic data.
Logs
The KWP1281 or KWP2000 (KWP = Key-Word-Protocol) is mainly used as the communication protocol. The advantage of the KWP2000 is that it is possible to flash control units, i.e. to overwrite the firmware with newer software versions , via the diagnostic connection.
literature
- Werner Zimmermann, Ralf Schmidgall: Bus systems in vehicle technology - protocols, standards and software architecture. 5th edition, Springer Vieweg, 2014, ISBN 978-3-658-02418-5 .
- Florian Schäffer: Vehicle diagnosis with OBD. Elektor, ISBN 978-3-89576-173-7 .
- Florian Schäffer: OBD - vehicle diagnostics in practice. 1st edition, Franzis, 2012, ISBN 978-3-645-65156-1 .
Web links
- Jeff's KL interface with optocoupler
- Circuit design for a KL and a KKL interface with ISO component (RS232 and USB)