PURPOSE: To enable detection of the break signal in the two-wire and semi-double circuit by producing the time frame with every fixed time under data transfer via the conversion interface and then generating the break signal during that period.

CONSTITUTION: The two-wire and semi-double circuit is formed between CPU1 and terminal unit 7 via circuit control unit 2 plus conversion interfaces 3 and 6. Then the time frame is produced with every fixed time under data transfer via interface 3 and irrespective of the control given from CPU1. And unit 7 generates the break signal showing the interruption to CPU1 in the time frame mentioned above, and CPU1 detects the break signal sent from unit 7 within the time frame.