in an on - line system composed of a high rank unit and a plurality of terminal units connected thereto , various causes may produce a time - out in one of the terminal units . when a data link for a terminal unit is not established in a prescribed time resulting in a time - out , the status of the communication line during the attempted data communication is analyzed by a circuit provided for discriminating the cause of the time - out . using the result of analysis , it is determined whether time - out has resulted from a busy condition of the line so as to diminish undesired work attendant upon occurrence of the time - out as far as possible and for obtaining a system with a high operation rate .