can work as a stand alone unit or be linked together with an efm 3 unit the efm 2 unit housing contains four holes for four terminals the alarm works in two modes i e at home mode and out night mode one terminal is used for the dc power supply one terminal is used to control the valve solenoid one terminal is used as the serial interface terminal and one terminal is used for driving the buzzer and the led each unit will use its efm id number for identification it can also connect to household alarm panel via dry contact which was just added the efm 3 unit is a stand alone unit with a two wire serial interface and a telephone line interface it communicates with the efm 2 units by a two wire serial interface and with the computer station by a telephone interface radio cell phone the efm 3 unit housing contains four holes for four 4 terminals one terminal is used for the dc power supply two terminals are used for the serial interface terminals and one terminal is used for the earphone plug to the telephone jack of the telephone line interface terminal the handheld unit hhu 1 works with the efm 2 unit only it can store the data.