7.4.1. Immobilizer

Description of system
The system of an immobilizer consists of the passive transponder (for a mutual identification) mounted in a car key head and the antenna Smartra block (Smart Transponder Antenna).
The Smartra block includes the built-in inductive antenna and electronic schemes connected to the ignition switch. The Smartra block is connected to the control unit of the engine of the special communication line.
Such way of control of the Smartra block is the most expedient since the control unit of the engine operates start of the engine.

Fig. 7.77. Flowchart of the antenna block of an immobilizer

Principle of action of system of the control unit of the engine
The control unit of the engine makes an ignition key identification by means of the special coding algorithm operating in parallel in the transponder. Start of the engine is possible only at coincidence of results of action of an algorithm. Data of all transponders attached to the car are stored in the control unit of the engine.
The control unit of the engine and the Smartra block are reported among themselves on the special communication line. The control unit of the engine establishes connection on the line K either with the Smartra block, or with other devices (for example, with the test device at maintenance) by turning on of the multiplexer and start of special procedures of communication. The multiplexer is a part of the hardware of the control unit of the engine. The Smartra block carries out communication via the transponder which is built in the ignition key. Communication is carried out on a radio frequency 125 kHz. The Smartra block is installed on the ignition switch near a frame send-receive radio antenna. The radio-frequency signal accepted by the frame antenna will be transformed by the Smartra block to the messages for serial communication and accepted from the control unit of the engine of the message will be transformed to the radio-frequency signal transmitted via the antenna to the transponder.
The Smartra block does not make an identification of the transponder or calculation of an algorithm of coding. It represents the advanced interface which will transform the data flow received on a radio frequency from the transponder to the sequence of the messages given on the control unit of the engine and vice versa.
The transponder – the Hitag2 type with an advanced algorithm of coding. When programming a key special data of the car which register its memory device are entered into the transponder. The procedure of record – disposable therefore contents of memory of the transponder cannot be changed. Data are entered in the form of the chain in 9 bytes made at manufacturer. Contents of memory of the transponder are divided into two chains which are carried by the name "opoznavatel" and "password of a key".
After this programming memory of the transponder is blocked and its contents cannot be considered or changed. The transponder passes "programmed", "pure" in a state, from a state.
Besides, each transponder has own identification number IDE in 32 bits. Numbers IDE of transponders do not repeat. Number IDE is entered at manufacturer and can only be read out.
Opoznavatel and the password of a key are not given from the control unit of the engine to the transponder or back. Only results of action of an algorithm of coding are transferred. By result of coding it is almost impossible to calculate special data of the car.
For programming of keys or other purposes the control unit of the engine connects to the test device as which the factory tester for maintenance used at service stations can serve. Communication is carried out under the protocol of the line K of the control unit of the engine.