From an A+C point of view, have just read the MM info on 737-300 FDR system. Just about everything that moves and more besides send a signal to the Digital Flight Data Aquisition Unit (DFDAU). This unit converts all signals into a digital format 1 signal type of thing. This signal is then sent to the FDR. I guess that in the event of a crash, the FDR digital data can be backtracked and decoded into numerous individual position sensors. The DFDAU also has a floppy disk inside that is periodically changed. This could also possibly be retrieved. As said at the beginning, this is an A+C point of view and therefore isopen to correction by anyone knowing better.