gEDA-user: BLDC motor "FG" signals

[peter at peter-b.co.uk]

Hi folks,

I'm designing a BLDC motor driver at the moment (probably around an
integrated circuit such as Toshiba's TB6571FG).

However, many datasheets show a differential "FG" input to the speed
controller, which I assume must be some motor output -- but none of the
datasheets explain exactly what it is or what it should be connected to.

Does anyone have any idea what this signal is?  Googling produces no
useful results.  I'm guessing it must be some sort of pulse-per-revolution
tacho-like output, but I'd rather be certain...

Thanks in advance,

                       Peter