A sensor apparatus for a movable barrier system having a rotatable drum and an elongate member that winds up on and pays out from an external surface of the rotatable drum. The sensor apparatus includes a base portion, a sensing portion, and a controller. The sensing portion senses a first spaced apart proximity of the elongate member relative to the sensing portion and a second spaced apart proximity of the elongate member relative to the sensing portion. The controller detects a change in the proximity of the elongate member relative to the sensing portion without the elongate member contacting the sensing portion.