fig1 shows an alarmed computer network according to the invention together with a basic coupling of the latter . the network shown in the drawing incorporates five different computer units ; two personal computers 13a , 13b , two monitors 14a , 14b , and one printer 15 , which are connected to hub 3 . alarm center 1 is connected to a vacant output on the hub via connection adapter 2 . the other outputs on the hub are intended for the computer units arranged in the network . each computer unit is provided with one alarm unit to alarm center 1 via the common hub . a first alarm unit 4 of the type shown in fig2 is connected to personal computers 13a , 13b . first alarm unit 4 shown in the drawing incorporates a first connection 25 , a second connection 23 , a third connection 22 , an address unit 11 , a switch coupling 16 , and a microswitch 12 . first connection 25 comprises an input to alarm unit 4 and , in the embodiment shown , consists of a connection for a databus consisting of eight conductors , such as e . g . and 8 - modular jack . second connection 23 comprises an output from first alarm unit 4 , whereby six of the above - noted eight conductors are connected to one network board in respective computer unit 13a , 13b . the remaining two conductors 9 are connected to third connection 22 on first alarm unit 4 and are arranged to facilitate interconnection with further alarm units 5 , 7 or connection unit 10 . in this embodiment , the third connection comprises one connection for six conductors in the same way as for second connection 23 . in a functioning alarm device , these six conductors are operationally connected to each other in such a manner that the alarm center is included in a closed line . a specific alarm address to the alarmed unit is provided through circuit address unit 11 . this address unit is connected to two conductors in such a way that the latter form part of the line through first alarm unit 4 and any further alarm units 5 , 7 . switch coupling 16 on the input to address unit 11 is arranged so that the connection between address unit 11 and alarm center 1 is interrupted when the alarm unit is opened or removed . circuit microswitch 12 is arranged in an alarm loop arranged for the latter . microswitch 12 is connected to the computer in such a manner that the latter is arranged in a detachable part of the computer case , whereby unauthorissed opening of the computer is detected . if the computer is opened , the microswitch is affected so that the alarm loop through the latter is interrupted . the alarm center can thereby obtain information about this failure and whether the computer unit may have been exposed to any other type of effect . this embodiment of the alarm center , however , leaves open the possibility of opening the computer without the alarm being activated using a bypass of address unit 11 . this bypass is of course desirable to enable authorised measures to be taken in the computer , whereas they cannot be executed without permission from the alarm center . to alarm e . g . a monitor directly connected to a specific personal computer , a second alarm unit 5 of the type shown in fig4 can be used . this alarm unit is not addressable , but incorporates only two different cable connections and one microswitch 12 . as distinct from first alarm unit 4 shown in fig2 this unit is not intended for direct connection to the hub via the computer network , but instead envisages connection to an addressable first alarm unit 4 or an addressable second alarm unit 7 , which are connected to a second computer unit . a further addressable second alarm unit 7 is shown in fig3 . as distinct from monitors 14a , 14b , printer unit 15 is provided with addressable second alarm unit 7 . this is due to the fact that the printer may be arranged so that it is separated from the other equipment , e . g . in a separate printer room . the monitor , however , is most often directly connected to a respective personal computer . the second alarm unit 7 used is shown in fig3 and substantially agrees with that shown in fig4 except that second alarm unit 7 intended for the printer is provided with addressable alarm circuit 11 . this alarm unit is also arranged in connection to computer unit not directly connected to the network . to facilitate coupling of an unlimited number of alarm units in one line , each alarm unit is arranged so that two conductors couple through the request transmitted from the alarm center to any further alarm units . to enable a closed line to be formed to the different alarm units , connection unit 10 is used , which loops the conductors in the alarm units . the connection unit is also used for connection of the alarm loop through one or more alarm units in which microswitches are arranged . this connection unit is shown in fig5 . as shown in fig1 each of the units included in the computer network is thus provided with its own alarm unit , which is arranged in conjunction with the unit &# 39 ; as connection to the computer network . the alarm center is arranged so that an alarm is activated from the alarm unit when one of cables 8 connecting the respective personal computer to the computer network hub is removed . if the computer is disconnected from the network , the alarm databus line is interrupted , and the alarm centre cannot any longer maintain contact with the different address units arranged in the alarm units . through each alarm unit being linked with a specific address unit , the alarm center can identify which part ( s ) of the computer equipment has been disconnected from the alarm device and consequently disconnected from the network . the alarm center is correspondingly arranged so that an alarm is activated if cable 9 between the personal computer and monitor is disconnected or if the connection the printer is affected . different address units 11 mean that the alarm center can detect a disconnected address unit . should there be any unauthorised opening of opening - protected equipment , the alarm center will give an alarm signal through corresponding microswitch 12 is the opening - protected equipment transferring from closed to interrupted position and the alarm loop thereby being interrupted , which affects the status message for this alarm unit to the alarm center . the alarm signal is also received even when the whole alarm unit is removed from the computer unit to which it is fixed . the alarm center used in the device according to the invention regularly transmits a request to each of the alarm units connected to the hub which has a specific address . these alarm units report their respective address values and statuses to the alarm center . so long as the switch coupling is not affected in the respective alarm unit , the alarm center detects the address values from each of the alarm units connected to the different computer units . the alarm center is also arranged to establish the status of a specific alarm unit . if the microswitch interrupts the alarm loop , the status is affected , whereby the alarm center may e . g . find that the alarm loop has been interrupted at specific address unit 11 . fig6 shows how a number of alarm units can be connected so that the alarm device according to the invention can effectively cover a number of computer units . the interconnection shown in the drawing is suitable e . g . for a situation where personal computer 13a is connected to monitor 14a and two printers 15 arranged in another room . first alarm unit 4 is in this situation connected to personal computer 13a and engaged between the latter and hub 3 . the other alarm units 5 , 7 are connected to first alarm unit 4 via cables 9 specifically arranged for this purpose so that a closed line is formed to each alarm unit . alarm unit 5a next following first alarm unit 4 is intended for monitor 14a and comprises a second non - addressable alarm unit 5a , since the monitor is directly connected to personal computer 13a . since the printers are arranged at a separate location in relation to the personal computer and monitor , one of the alarm units connected to the latter comprises second addressable alarm unit 5b . the line to the alarm units and alarm loop is closed by one alarm unit , which is directly connected to last alarm unit 5b . each of the computer units directly connected to the network , such as e . g . personal computers 13a 13b in fig1 are thus connected to an addressable first alarm unit 4 , which is coupled between the hub and corresponding computer unit and which couples through unaffected the computer signals from the hub to the computer unit and vice versa . other lower - level computer units , such as e . g . monitor and printer , which are connected to a computer unit connected to the network , are provided with either addressable second alarm unit 7 or non - addressable second alarm unit 5 depending on the physical placement of this unit . the alarm unit which is located far from the network in connection terms and hub is provided with above - noted connection unit 10 , which interconnects the conductors and this alarm unit so that a closed line is formed to each alarm unit and so that an alarm loop is formed through the latter . fig7 shows an embodiment of the alarm system where first alarm unit 4 is arranged inside the openable case which encloses the computer unit . in a preferred embodiment , the alarm center is integrally arranged with the network board 30 of the computer unit . the network board 30 arranged for the alarm function then diverts two conductors for an alarm function , whereas the other conductors in the connected databus are used for network communication of the computer unit . the two alarm conductors are coupled in the previously indicated manner in an alarm circuit , which is connected to microswitch 12 at a suitable part of the openable case of the computer unit . as indicated above , the alarm circuit also incorporates an address unit 11 , whereby alarm unit 4 can be given a unique address . this address unit 11 is integrated in the network board 30 . to allow connection to further alarm units 5 , 7 or to a so - called connection unit 10 , the computer unit is equipped with an extra output , which is only used to achieve the alarm function in the network . in a second embodiment of the device according to the invention , first alarm unit 4 is arranged in a build - on board 31 to the network board 30 . much as for a network board with an integrated alarm unit , installation of alarm unit 4 is extremely simple . in this embodiment , however , the network board 30 is required to be arranged for this type of build - on board 31 . much as for a network board with an integrated alarm unit , the computer unit must also here be provided with a special output for the alarm function . a first alarm unit 4 can also be installed in a computer units whose network board has not been adapted for an alarm function . a special alarm unit 4 is then coupled , e . g ., via cable 32 , between the network connection of the computer unit and its network board 30 , as illustrated in fig1 . placement of alarm unit 4 inside the computer unit case is optional . two of the conductors in the new databus which are not used for network communication are connected to an alarm circuit , with the other conductors being allowed to pass through unaffected to the network board 30 . the alarm circuit incorporates a microswitch 12 , which rests on the openable case of the computer unit in such a manner that microswitch 12 transfers from closed to interrupted position if the case or part thereof is removed . the computer unit is provided with an extra alarm output , whereby further alarm units 5 , 7 can be connected in series to first alarm unit 4 . no further alarm units 5 , 7 are then used in connection unit 10 connected to the noted output . the alarm circuit is closed by this connection unit 10 . the alarm device according to the invention is especially suitable for application in networks of twisted pair type in ethernet type networks , in which a pair of conductors connected to each computer unit in the network is not used for communication with the hub . the invention , however , should not be regarded as being limited to this advantageous application , but is also applicable to each type of computer network in which one or more computer units are connected to a common hub and in which at least two conductors are unused in each connection .