PURPOSE: To prevent the state of disabled communication by providing a detector for transmission start state and a circuit supervising the number of transmission characters to a transmission circuit part to detect earlier the stall state to the control of the software or the firmware.

CONSTITUTION: A multiplexer 6 transmits an F pattern character for establishing synchronism produced by an F pattern generator 5 to a terminal device side through a line via a driver 7. When the software or the firmware of a communication processing device control section 1 is stalled because of bug or any other cause and the multiplexer 6 cannot transmit the F pattern being the final character of one block, a transmission character counter 12 overflows the preset number of transmission characters. When the transmission character counter 12 overflows, an interruption start circuit 15 is started and the fault of the transmission operation is informed to the communication processing unit control section 1 by interruption.