PURPOSE: To enable to check mis-transmission, by comparing the transmitted data with the data produced actually on the transmission line at the transmission section.

CONSTITUTION: An output terminal 2 of a transmission section 1 and an input terminal 4 of a reception section 3 are tied with a line 5. The data read out from a data memory 6 is loaded to a shift register 7 with the load pulse. When noise is on a transmission line 5, noise is included in the data Di appeared on the collector of a transistor 13. The transmission data Do and the data Di are fed to an exclusive OR gate 14 to produce the detection pulse COM corresponding to the noise. A flip-flop 39 is set with the first leading of the detection pulse COM and reset with the load pulse PD. A restart pulse SS is output from an NAND gate 18, then the data Do is again transmitted.