In order to model the processes taking place in systems with Josephson contacts, a differential equation on a torus with three parameters is used. One of the parameters of the system can be considered small and the methods of the fast—slow systems theory can be applied. The properties of the phase—lock areas — the subsets in the parameter space, in which the changing of a current doesn’t affect the voltage — are important in practical applications. The phase-lock areas coincide with the Arnold tongues of a Poincare map along the period. A description of the limit properties of Arnold tongues is given. It is shown that the parameter space is split into certain areas, where the tongues have different geometrical structures due to fast—slow effects. An efficient algorithm for the calculation of tongue borders is elaborated. The statement concerning the asymptotic approximation of borders by Bessel functions is announced.