the present invention concerns an alarm device for computer units which , via a network of twisted pair type , are connected to a common hub . the computer units are arranged to be connected to a first alarm unit corresponding to the respective computer unit . this incorporates an input for connection to the hub , an output for operational connection to a network board in a computer unit corresponding to the alarm unit , and a connection for interconnection with a second alarm unit or a connection unit . in the device according to the invention , an alarm center is operationally connected to the hub , which is arranged to transmit a request generated in the alarm center to the first alarm unit and to notify the alarm center when a change occurs in the computer units associated with the alarm centers .