System Description
The immobilizer system consists of a passive transponder (for mutual recognition), mounted in the car key head, and the antenna unit «Smartra» (Smart Transponder Antenna).
Block «Smartra» includes a built-in inductive antenna and electronic circuits connected to the ignition switch. Block «Smartra» connected to the engine control unit by a special communication line.
This block control «Smartra» is the most appropriate, because engine start is controlled by the engine control unit.
Pic. 7.77. Block diagram of the immobilizer antenna unit
The principle of operation of the engine control unit system
The engine control unit recognizes the ignition key using a special coding algorithm that operates in parallel in the transponder. The engine can only be started if the algorithm results match. The engine control unit stores data from all transponders supplied with the vehicle.
Engine control unit and block «Smartra» communicate with each other via a special communication line. The engine control unit establishes communication over the K line or with the unit «Smartra», or with other devices (e.g. with a maintenance tester) by turning on the multiplexer and running special communication procedures. The multiplexer is part of the engine control unit hardware. Block «Smartra» communicates via a transponder built into the ignition key. Communication is carried out at a radio frequency of 125 kHz. Block «Smartra» installed on the ignition switch next to the frame transceiver radio antenna. The radio frequency signal received by the loop antenna is converted by the unit «Smartra» into messages for serial communication, and the messages received from the engine control unit are converted into an RF signal transmitted through the antenna to the transponder.
Block «Smartra» does not perform transponder identification or coding algorithm calculation. It is an advanced interface that converts the received radio frequency data stream from the transponder into a sequence of messages issued to the engine control unit, and vice versa.
Transponder - type «Hitag2» with advanced coding algorithm. When programming the key, special vehicle data is entered into the transponder, which is recorded in its memory device. The recording procedure is one-time, so the contents of the transponder memory cannot be changed. The data is entered as a 9-byte string, compiled at the factory. The contents of the transponder memory are divided into two chains, which are called «identifier» And «key password».
After this programming, the transponder's memory is locked and its contents cannot be read or changed. The transponder transitions from the state «clean» into a state «programmed».
In addition, each transponder has its own 32 bit IDE identification number. The transponder IDE numbers are not repeated. The IDE number is entered at the factory and can only be read.
The key identity and password are not transmitted from the engine control unit to the transponder or vice versa. Only the results of the encoding algorithm operation are transmitted. It is almost impossible to calculate special vehicle data from the encoding result.
For key programming or other purposes, the engine control unit is connected to a test device, which can be a factory service tester used at service stations. Communication is carried out according to the protocol of line K of the engine control unit.