This paper presents a kinematic model of three ports differential mechanism using the block diagram technique. In contrast to more complex conventional approaches, the block diagram technique allows for the characterization of the input-port output-port speed kinematic relationship of the planetary gear systematically. The dedicated closed loop relationship can be found by simply applying methods known in the control community, such as Mason's rule. By way of exemplary implementation, this paper adopts a South Pointing Chariot, which is a two-input port differential, as a case study. A feedback connection kinematic model of the South Pointing Chariot system is established using the block diagram technique.