Originally Posted by
FrequentSLF
The FDR trace show only one cockpit pitch trim control input position, it does now show BOTH thumn switches, that might lead to assume is an elaborated signal that is taken at the servo motor controller. Of course the traces might have been elebaorated before been published, however the information published does not validate a theory of the trace representing the thumb switch at the source
Well, the trim schematic shows the thumb switch outputs for "up" and "down" connected together, and there are direct connections that go to the DFDAU (Digital Flight Data Acquisition Unit). This strongly suggests that the FDR traces are based on the actual position of the switches. I took a section of the schematic and colored in green and turquoise the paths of the signal from thumb switches to the DFDAU:
