| 摘要 |
PURPOSE:To prevent pseudo synchronism at the deterioration of a transmission line by providing a counter counting a frame pulse in a prescribed number of frames at all times and a means bringing the state to the asynchronous state when the counter counts a value exceeding the prescribed number in the prescribed number of frames to a frame synchronism circuit. CONSTITUTION:An output of a protection circuit 4 is at H level in the asynchronous state and when a pseudo synchronism pulse as shown in Fig. (a) A is generated, the pulse is inputted to a frame counter 1 and a pulse having the same period as that of the frame pulse is outputted as shown in Fig. (b). The count number of an FP counter 5 goes to 1 as shown in Fig. (c) with a frame pulse as shown in Fig. (a) B, the output level goes to an L level, and an output of an FF6 goes to an L level as shown in Fig. (d). Neither frame pulse nor an output of the counter 1 is inputted to the circuit 4, the output goes to H level and the state is brought into the asynchronous state. The pseudo synchronism is prevented in this way. |
