A neutral start interlock for a hydrostatic drive system that uses potentiometers to electronically monitor at least the position of the hydrostatic displacement control such that the engine will not start unless such control is in the neutral position. The storage is temporary by discharging the capacitor through a resistor Rf parallel to the capacitor. Left and right interdigital transducers of which the polarities are inverted relative to each other are connected to an unbalanced signal terminal to provide a balanced-to-unbalanced conversion function. A similar procedure is followed for an incoming call to a mobility device, which can be forwarded to the subscriber’s fixed-site device.