A transmission circuit is disposed in at least one of a control panel and a plurality of terminal devices of a fire protection/security system, wherein the control panel individually polls the terminal devices to read in terminal information from the terminal devices, to judge and/or display the information, or to control the terminal devices. The transmission circuit includes a signal sending device for sending a signal to a signal line, a memory device for storing a signal to be sent to the signal line, and a coincidence decision device for deciding a coincidence between the signal sent from the signal sending device and the signal stored in the memory device.