could s-function based on discrete state behave the same as the model constructed by blocks in simulink ?

I need to use s-function to simulate electric motor, which is defined by four ordinary differential equations (ODEs). I have tried s-function based on continuous state, and it can solve ODEs at a acceptable accuracy compared to the model constructed by blocks in simulink. Now I need to use discrete solver to improve the simulation speed, so I must change the continuous state to discrete state. But I find that the output of the s-function has always one sample time delay compared to the model constructed by blocks. This because in every simulation loop the s-function always compute the output at first and update the discrete state at second, so the output is actually the last sample time's state, not the current. How can I make it act exactly the same as the model? Bow~~ and thanks a lot.

[Ryan G]

Are the initial conditions in both the s-function and the block the same?