An apparatus includes a magnetic head having an array of transducers. In an exhaust path 48a, a vacuum pump and exhaust gas filtration device are provided. The DSP’s are further configurable to act as a master processor and a slave processor relative to another DSP. The state includes a state in which the engine is not running. A measurement module measures the frequency of the signal at the output of the last chain of lags after updating the control words, and the control module can deduce from the frequency measurements the bits making up the signature of the circuit.