If the RCS-900 protection adopts the 2 048 kbit/s multiplex channel， slips may occur in the receiving code of the protective equipment while the retiming function of the channel is on and the communication clock of the protective equipment is set in the inner clock mode. The impact of the retiming function on service is studied and the service transmission results in different modes of combination of the retiming function and the transmission clock are tested. The results show that part of the 2 048 kbit/s channels can perform the retiming function， combine the reference timing and service data for transmission to the protection equipment. The protection equipment communication clock should adopt the“slave-slave” mode when the retiming function is on and at least the communication clock on one side of the protection equipments should adopt the inner clock mode with the retiming function off.