In the presence of a battery source and a radiation field, a superconducting weaklink can be represented by a simplified model consisting of a resistance shunted Josephson junction driven by a constant current source. Using such a model, we give a general criterion for the occuring of constant voltage current steps in the current-voltage characteristics of the weaklink. Following this criterion, we have also discussed the absence of sub-harmonic current steps. For the case while the radiation field is very weak, the current step heights are calculated by pertubation method. The current step heights are also obtained by numerical compution for the case of strong field.