PURPOSE: To attain simulation of a failed line by providing another line control circuit as a loop test channel to make the loop test easy.

CONSTITUTION: Line control circuit channels 12a, 12b are operated by the program control of a microprocessor 7, the channel 12b is not operated at a normal transmission and the A side input of the selector 14a is connected to an RD of the channel 12a. In case of loop test, the B side input of the selector 14a is connected to the RD of the channel 12a, the output of the channel 12a enters the channel 12b via a shift register 13a and processed and inputted to the channel 12a via a shift register 13b. The control code table for the channel 12b is changed by the program in the connection shown in Figure and the error in an optional transmission control code is simulated.