An assembly to produce data pulses in a drilling fluid in a drill string, the assembly comprising: a housing having a hollow interior, the housing being connectable to the drill string to place a valve passage defined by the hollow interior of the housing in fluid flow communication with a drilling fluid conduit defined by the drill string; a shear valve mounted in the valve passage to produce data pulses in the drilling fluid by varying obstruction by the shear valve of the valve passage, the shear valve comprising a valve member (240; 608) which is angularly displaceable about a valve axis that is longitudinally aligned with the drill string, to vary obstruction of the valve passage; and a reciprocation mechanism operatively connected to the valve member (240; 608) to actuate angular reciprocation of the valve member (240; 608) about the valve axis, the reciprocation mechanism comprising: a rocker that is drivingly connected to the valve member (240; 608), the rocker being mounted to be substantially coaxial with the valve axis and to be angularly displaceable about the valve axis, a driven crank arrangement mounted to rotate about a crank axis substantially parallel to and transversely spaced from the valve axis, and a slider member which extends radially between the crank arrangement and the rocker, the slider member coupled to provide a sliding coupling between the crank arrangement and the rocker, the slider member pivotally connected to the crank arrangement, the slider member keyed to the rocker for angular displacement about the valve axis, and the slider member coupled to be radially slidable relative to the rocker, so that angular reciprocation of the slider member about the valve axis due to rotation of the crank arrangement results in angular reciprocation of the rocker and hence of the valve member (240; 608). a housing having a hollow interior, the housing being connectable to the drill string to place a valve passage defined by the hollow interior of the housing in fluid flow communication with a drilling fluid conduit defined by the drill string; a shear valve mounted in the valve passage to produce data pulses in the drilling fluid by varying obstruction by the shear valve of the valve passage, the shear valve comprising a valve member (240; 608) which is angularly displaceable about a valve axis that is longitudinally aligned with the drill string, to vary obstruction of the valve passage; and a reciprocation mechanism operatively connected to the valve member (240; 608) to actuate angular reciprocation of the valve member (240; 608) about the valve axis, the reciprocation mechanism comprising: a rocker that is drivingly connected to the valve member (240; 608), the rocker being mounted to be substantially coaxial with the valve axis and to be angularly displaceable about the valve axis, a driven crank arrangement mounted to rotate about a crank axis substantially parallel to and transversely spaced from the valve axis, and a slider member which extends radially between the crank arrangement and the rocker, the slider member coupled to provide a sliding coupling between the crank arrangement and the rocker, the slider member pivotally connected to the crank arrangement, the slider member keyed to the rocker for angular displacement about the valve axis, and the slider member coupled to be radially slidable relative to the rocker, so that angular reciprocation of the slider member about the valve axis due to rotation of the crank arrangement results in angular reciprocation of the rocker and hence of the valve member (240; 608). a rocker that is drivingly connected to the valve member (240; 608), the rocker being mounted to be substantially coaxial with the valve axis and to be angularly displaceable about the valve axis, a driven crank arrangement mounted to rotate about a crank axis substantially parallel to and transversely spaced from the valve axis, and a slider member which extends radially between the crank arrangement and the rocker, the slider member coupled to provide a sliding coupling between the crank arrangement and the rocker, the slider member pivotally connected to the crank arrangement, the slider member keyed to the rocker for angular displacement about the valve axis, and the slider member coupled to be radially slidable relative to the rocker, so that angular reciprocation of the slider member about the valve axis due to rotation of the crank arrangement results in angular reciprocation of the rocker and hence of the valve member (240; 608).