A connection control circuit having the function of controlling connection and
disconnection of an information storage apparatus to and from a transmission line
is constructed such that it uses a comparator to determine whether or not the level
of operating clock signal output from a controller of the information storage apparatus
is within a predetermined range, and if it is detected that the level of the operating
clock signal departs from the predetermined range, the connection control circuit
outputs a control signal to a by-pass circuit to disconnect the information storage
apparatus from the transmission line loop. This enables the information storage
apparatus to be disconnected from the loop even if the controller of the information
storage apparatus does not function normally due to an abnormality of the clock
or an abnormality of the power supply.
