A gearing radius of a worm wheel is set corresponding to a neighborhood region of a steering neutral position, in such a way as to be relatively large. When the cyclic redundancy check data of a received block fails, the soft values of the block are stored along with the first half of a signalling message so as to facilitate soft combining. The sensing circuit generates an output signal having a first logic level in response to the sense current being greater than the reference current and generates the output signal having a second logic level in response to the sense current being less than the reference current.