  We solve the RSOS($p$) models on the light--cone lattice with fixed boundary
conditions by disentangling the type II representations of $SU(2)_q$, at
$q=e^{i\pi/p}$, from the full SOS spectrum obtained through Algebraic Bethe
Ansatz. The rule which realizes the quantum group reduction to the RSOS states
is that there must not be {\it singular} roots in the solutions of the Bethe
Ansatz equations describing the states with quantum spin $J<(p-1)/2$. By
studying how this rule is active on the particle states, we are able to give a
microscopic derivation of the lattice $S-$matrix of the massive kinks. The
correspondence between the light--cone Six--Vertex model and the Sine--Gordon
field theory implies that the continuum limit of the RSOS($p+1$) model is to be
identified with the $p-$restricted Sine--Gordon field theory.
