A control device constituting at least a part of a control system, the control device comprising: a master control unit (10, 40); and at least one slave control unit connected to the master control unit (10, 40) via a communication line, wherein the communication line includes: a downlink for transmitting data from the master control unit (10, 40) to the slave control unit; and an uplink for transmitting data from the slave control unit to the master control unit (10, 40), the master control unit (10, 40) includes: a circuit for, upon completion of data transmission from the slave control unit via the uplink, transmitting an instruction for deactivating the uplink to the slave control unit via the downlink; and a circuit for, in order to obtain data from the slave control unit, transmitting an instruction for activating the uplink to the slave control unit via the downlink, and the slave control unit includes a circuit for keeping the downlink active, and for activating/deactivating the uplink in accordance with an instruction from the master control unit (10, 40) transmitted via the downlink, wherein the slave control unit includes a conversion circuit (220a, 220b) for converting a signal transmitted over the uplink, deactivation of the uplink involves blocking of a power source to the conversion circuit (220a, 220b), the master control unit (10, 40) includes a reception circuit for receiving a signal transmitted over the uplink, and the master control unit (10, 40) blocks a power source for the reception circuit during a period in which the uplink is deactivated. a master control unit (10, 40); and at least one slave control unit connected to the master control unit (10, 40) via a communication line, wherein the communication line includes: a downlink for transmitting data from the master control unit (10, 40) to the slave control unit; and an uplink for transmitting data from the slave control unit to the master control unit (10, 40), a downlink for transmitting data from the master control unit (10, 40) to the slave control unit; and an uplink for transmitting data from the slave control unit to the master control unit (10, 40), the master control unit (10, 40) includes: a circuit for, upon completion of data transmission from the slave control unit via the uplink, transmitting an instruction for deactivating the uplink to the slave control unit via the downlink; and a circuit for, in order to obtain data from the slave control unit, transmitting an instruction for activating the uplink to the slave control unit via the downlink, and a circuit for, upon completion of data transmission from the slave control unit via the uplink, transmitting an instruction for deactivating the uplink to the slave control unit via the downlink; and a circuit for, in order to obtain data from the slave control unit, transmitting an instruction for activating the uplink to the slave control unit via the downlink, and the slave control unit includes a circuit for keeping the downlink active, and for activating/deactivating the uplink in accordance with an instruction from the master control unit (10, 40) transmitted via the downlink, wherein the slave control unit includes a conversion circuit (220a, 220b) for converting a signal transmitted over the uplink, deactivation of the uplink involves blocking of a power source to the conversion circuit (220a, 220b), the master control unit (10, 40) includes a reception circuit for receiving a signal transmitted over the uplink, and the master control unit (10, 40) blocks a power source for the reception circuit during a period in which the uplink is deactivated.