The RCS-900 series protection equipment uses synchronous communication to exchange information by the multiplex channel or fiber optic core in 64 kbit/s or 2 048 kbit/s. This paper describes the scheme of pattern transfer， the function of clock pick-up from the receiving wave， clock setting for information transmission， and how the protection equipment maintains bit synchronization. By introducing the relationship between the transmission clock and the receiving clock in the PCM terminal， it is also pointed out that the protection equipment should adopt the“slave-slave” communication clock mode for the 64 kbit/s multiplex channel， and“master-master” or“master-slave” mode for the fiber optic core mode. For 2 048 kbit/s multiplex channel， the protection equipment should adopt the“slave-slave” mode while the retiming function is on and“master-master” or“master-slave” mode while the retiming function is off.