In order to avoiding the phenomena of false alarm and missed alarm of in frame synchronization circuit, by the use of tri-state logic, it designs frame check/ protection circuit to assure the reliability of synchronous circuit. 为避免帧同步电路的虚警、漏警现象,采用三态逻辑判断方法设计了帧同步校验/保护电路来确保帧同步电路的可靠性。