A mud pump is provided that includes a pump shaft having substantially circular eccentric lobes. Each lobe is rotatably mounted in a connecting rod that, in turn, moves a slide in a horizontal and linearly manner. A piston rod is operatively couples a pump module to one or both sides of each slide frame and a pump module. As the pump shaft turns, each lobe causes the slide to move side to side. As the slide moves side to side, each piston rod operates a pump module whose outputs can be coupled to a common manifold.