This paper features a Rapid Control Prototype - RCP- for a Shunt Active Power Filter -SAPF- using a three-phase power inverter board and a Digital Sign Processing board -DSP-. In order to demonstrate the benefits of working RCP with Simulink and its real-time toolbox -RTW-, we suggest a control algorithm in closed loop under constant power source constraint, based on p-q theory, to compensate a nonlinear load: in this case, an uncontrolled six-pulse full-bridge rectifier with inductive - resistive load on the DC side.