In an open-loop monitoring system, a host device is operable to output encoded signals at a coupling end to a single transmission line. Each of monitoring devices includes: a control unit identifying a monitoring signal from the encoded signals from the transmission line; and a detecting unit detecting, in response to the monitoring signal, whether each of electronic devices coupled thereto is activated or deactivated and transmitting a reply signal associated with states of the electronic devices to the transmission line based on a detection result made thereby. The host device receives the reply signal from each monitoring device through the transmission line and the coupling end to obtain state information associated with the electronic devices and determines whether the transmission line is cut off based on the reply signals from the monitoring devices.