DATA TRANSMITTER

PURPOSE:To atain loop-back return control with simple constitution, without the provision of a transmission line for control, by providing a means for performing the loop-back return indication via a data transmission line. CONSTITUTION:A data transmitter 1 is provided with A and B transmission line interface sections 3, 4 connected to an A data transmission line 15 and a B data transmission line 14, and a logical circuit section 2 is connected to the interface sections 3, 4 via switches 17, 18. The circuit 2 is provided with a normal loop returen indicating signal reception circuit 5, a loop-back return section 6, and switches 7, 8. A monitor device 16 is connected to the transmission lines 14, 15, and the normal loop reture indication signl is trnasmitted to the transmitter 1 via the transmission lines 14, 15. Thus indication signal is received at a reception circuit 5 and a loop-back returen indication signal line 9 is set. With the loop returen indication, the circuit 2 changes over the switches 7, 17 and 8, 18 to attain the loop-back return control with simple constitution.