In a closed-loop monitoring system, a host device is operable to output encoded signals at a coupling side to a single transmission line. Each of monitoring devices includes: a current inducting generating unit generating induced encoded signals when the encoded signals pass through a signal current generating element coupled across an input side coupled to the transmission line; a control unit identifying a monitoring signal from the induced encoded signals generated by the current inducting generating unit; and a detecting unit detecting, in response to the monitoring signal, whether each of electronic devices coupled thereto is activated ox 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 side to obtain state information associated with the electronic devices.