Originally Posted by
NSEU
Actually, now that I think about it, most engines, including GE, have some engine sensor data hardwired to the aircraft, so the EECs don't necessarily need to be powered. The CF6, for example, had N1, N2, vibration and oil quantity. But I'm sure the engines were still above the required N2 to maintain PMA power.
In the early days of FADEC, the rotor speeds were generally fed via analog as well as digital data bus - with the analog signal only used when digital was unavailable.
However, as the reliability of the FADECs has been proven, the need for analog backup has slowly gone away. A big reason why the analog rotor speeds went to aircraft was for use in the engine vibration system, but on the GEnx (both 787 and 747-8), the engine vibration data is calculated on the engine using the "Engine Monitor Unit" - EMU - so those vibe related parameters are no longer sent to the airframe).
I'm not sure that the 787 EICAS gets any backup analog rotor speed signals...