A method and assembly to produce data pulses in a drilling fluid in a drill string. The assembly comprises a shear valve that includes a valve member mounted in a valve passage in fluid flow communication with a fluid flow conduit of a drill string to which the assembly is connectable. The valve member is connected to a reciprocation mechanism comprising a rocker, a driven crank arrangement, and a slider member that provides a sliding coupling between the crank arrangement and the rocker. The slider member is pivotally connected to the crank arrangement, is keyed to the rocker for angular displacement about a valve axis, and is radially slidable relative to the rocker, so actuation of the crank arrangement causes angular reciprocation of the rocker, and hence of the valve member, about the valve axis, to produce data pulses in the drilling fluid.