PURPOSE: To attain loopback in each controller by remote control from a line extending device by alternately using one-bit synchronizing bits on a frame basis for synchronization and remote control.

CONSTITUTION: Each transmitted frame is composed of eight bits. Normally the synchronizing bit F is allocated to a 1st bit, data to following six bits and a transmission request signal RS to a final one bit. When loop indication information is transmitted, a loop indication bit L is inserted to a 1st bit in every two frames. Reception data from a terminal station device is inputted to an SYD 7 to detect the synchronization between bits and frames. According to the instructions, the reception data is accumulated in an RBF 8. If a loop indication (local) is issued to an indication information generator circuit LG 6, the LG 6 transmits a control signal to a gate circuit G4, switches selectors SELs 1, 2, 11 and 12, and constitutes a loopback on the terminal side and a loopback circuit on the side of the terminal station device.