1950s Airspeed Indicator/Continuous-Rotation Servo Motor?

http://www.mycockpit.org/forums/atta...id=11126&stc=1Hello, I have a general question relating to SIOC and a continuous-rotation RC servo motor: how does SIOC know what the instrument's "beginning" value is upon startup?

My 1950s-era airspeed indicator (see photo) has a "hundreds" needle and a spinning wheel for "tens/ones." The needle and spinning wheel are geared together. Large movement of the spinning wheel equals small movement of the needle.

I think the RC servo motor must be continuous-rotation, and must drive the "tens/ones" spinning wheel.

Here's my question: let's say I end a flight with 180 knots indicated airspeed. The instrument will be "stuck" indicating 180 knots (because that's where the motor drove the "tens/ones" mechanism to). Now, I start a new flight at 0 airspeed (on the ground); I'm guessing the airspeed indicator will still show 180. The instrument needs to be driven back to 0 airspeed. But, how would SIOC know that?

In a general sense, must I first manually tell SIOC that the airspeed indicator reads 180 knots, and therefore it must drive the needles back by 180 knots (to zero)? Such as: Var 1 Value 180 (and then write code to drive the instrument back to zero knots)?

I would imagine the same issue exists for a directional gyro, but I couldn't find an example. Thank you for any general guidance.