One or more option units connecting with a communication system that includes a control part configured to control a communication are provided with an upper side signal line connecting with a lower side signal line of an option unit connected in an upper position, a lower side signal line connecting with an upper side signal line of an option unit connected in a lower position; and a control circuit. A potential of the upper side signal line is changed from the second potential in response to a potential change from a potential in a normal state which is different from a potential in an initial state of the lower side signal line.