interface with EWS4

The CAS serves as an antitheft alarm system and enables the start of BMW vehicles.
A transponder chip is integrated in each of the vehicle keys. The ignition lock is surrounded by a ring aerial.
The transponder chip is powered by this coil from the CAS control module. This means that no battery is necessary in the key. The power supply and data transfer take place in the same way as a transformer between the loop aerial at the ignition lock and the transponder chip.

Data interchange between CAS and DME/DDE
The CAS and DME/DDE communicate across the CAN bus and CAS bus.

CAS-DME/DDE interface
A secret key, protected against manipulation, is stored in the CAS control module and in the DME/DDE control module.
From this point onwards, the DME and CAS control modules can only be replaced with control modules specifically ordered for each vehicle. A calibration of EWS in the vehicle is not possible.

It is not possible to replace the DME/DDE or CAS control modules for test purposes.

The following procedure takes place after inserting the vehicle key in the ignition lock:
· The transponder in the key is powered via the loop antenna and sends the key data to the CAS control module.
· The CAS control module checks the correctness of the key data and only then enables activation of the starter by the DME/DDE.
· The DME/DDE uses a random number and the secret key to calculate a request. The DME/DDE sends this request across the CAN bus and the CAS bus to the CAS.
· The CAS uses the request and the secret key to calculate the response. The response is sent by the CAS across the CAN bus and the CAS bus to the CAS.
· The DME/DDE itself also calculates the response that the CAS expects. The DME/DDE then checks that the response it receives from the CAS matches the response it has calculated itself.
· If the responses match, the engine start is enabled.

The following faults are monitored in the engine control module:
The DME/DDE monitors both the signal transfer on the CAN bus/CAS bus and the signals themselves. Engine start is inhibited if a fault is detected.
 

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