there has been provided , in a computer system , a control unit having authority over a plurality of subordinate units with differing response times . the control unit addresses and issues command instructions to selected ones of the subordinate units then waits for an acknowledgement of completion from the addressed units before going to the next step in its routine . to prevent the control unit from being hung - up in the event of a failure of response from the subordinate unit , time - out means are provided for releasing the control unit for further operation . since the response times of the individual subordinate units are unknown to the control unit , a unique time - out circuit is provided for each of the subordinate units , each time - out circuit being commensurate with the response time of the associated subordinate unit .