An apparatus for supporting a detecting shoe of a sensor for rotation around a drill pipe stem for detecting identification numbers encoded on the drill pipe sections of the stem as they are lowered into a borehole. The apparatus comprises an outer plate means supported for rotation and having an opening in which is located an inner plate also having an opening for receiving the drill stem. Arms couple the inner plate to the outer plate means for rotation therewith and also support the inner plate for lateral movement in the opening of the outer plate means. The detecting shoe is coupled to the inner plate by an arm which moves the shoe against the drill stem for reading purposes. Means also is provided for operating the shoe arm to move the shoe outward to allow drilling or other operations to be carried out.