Using arduino mega2560 with MATLAB/Simulink in extrenal mode (real time)

problem of using arduino mega 2560 in matlab/Simulink in real time. when I put scope in the model Simulink and I activate mode extrenal the signal frequency in the digital output does not like the signal which I need it; ex: I need 500hz, I get less than 500HZ (about 50hz). If I desactiavate extrenal mode, I haven't problem. if I cut scope from model, I haven't also any problem.(extrenal mode activate)

Please help us.