PURPOSE: To obtain a simple and economical-constitution data transmission circuit by which data receiving and transmission are performed in parallel simultaneously.

CONSTITUTION: Input data from terminal T13 is synchronized with the negative wave of data pulse AC power source G1, and input controlling circuit CONT11 becomes conductive, and the negative wave flows through G1-diode D13-CONT11- photo coupler PS12-D12-G1, and data is outputted to terminal T12 by operation of PS12. When data is inputted to terminal T14, CONT12 becomes conductive synchronously with the positive wave of G1, and the positive wave flows through G1-D14-CONT12-PS11-D11-G1, and data is outputted to terminal T11 by operation of PS11. When G1 has the positive wave, data is transmitted from the B side and is received by the A side; and when G has the negative wave, data is transmitted from the A side and received by the B side. Thus, data transmission and receiving can be performed in parallel simultaneously.