Abstract:
An arrangement for electrically connecting two pieces of electronic equipment includes a tube-like element that connects a first cage with a second cage. The tube-like element is electrically connected to the two cages and with cables extending between the two cages. The inner diameter is adapted to the combined diameter of the cables.

Description:
FIELD OF INVENTION 
     The present invention relates to an arrangement for mutually coupling electrically two pieces of electronic equipment, each having good EMC properties and retaining said properties. 
     DESCRIPTION OF THE BACKGROUND ART 
     It is known to enclose sensitive electronic equipment in an electrically conductive cage, a Faraday cage, to obtain good electromagnetic compatibility (EMC) with respect to said equipment, e.g. protection against electrostatic discharges (ESD). 
     In those situations when duplicate systems are used, two such pieces of equipment, will normally stand side-by-side so that the first piece of equipment is able to take over the functions of the second piece of equipment should this second piece of equipment malfunction or require inspection. It is, of course, possible to enclose both pieces of equipment in one and the same cage, thereby protecting both pieces of equipment. However, problems arise when the first piece of equipment needs to be repaired or serviced. In order to obtain access to this equipment, it is necessary to open the cage, which will impair the EMC properties with respect to the second piece of equipment, therewith risking disablement of the second piece of equipment and therewith a total breakdown. It is therefore necessary to enclose each piece of equipment in its respective cage. 
     In order for equipment that has good EMC properties in general to communicate with other equipment via cables for instance, it is necessary to provide holes in the cage that encloses the equipment and that constitutes the actual basis for said good EMC properties. Such holes enable radiation to leak into and out of the cage, resulting in poorer EMC properties. 
     When two cages stand side-by-side, electrical discharges, ESD, from one piece of equipment in the first cage are able to pass out through the hole therein and wander through the air or along electric conductors and in through the hole in the second cage and there disable the equipment enclosed by this second cage. This is, of course, highly unfortunate and presents a specific problem to the application of duplicated systems. 
     There are many different causes of electrostatic discharges, ESD. For instance, the equipment may be subjected to such discharge by a person coming into contact with the equipment or solely being present in the proximity thereof. In order to prevent such an occurrence, there are often provided special grounding or earthing points at which a person is able to connect up an electrically conductive arm band and therewith obtain the same ground, or earth, as the equipment, thereby preventing an ESD. Unfortunately, an ESD will sometimes occur before the person concerned has been able to ground his body at a grounding point, which may have catastrophic consequences in the case of sensitive equipment. A second person can cause an ESD, even though the first person is grounded or earthed. If the first person has had time to open the cage, the EMC properties of the equipment enclosed therein will be greatly impaired, and even if the actual discharge does not cause problems, the electromagnetic radiation caused by the discharge may play havoc and create problems with respect to nearby equipment, by passing through holes that carry electric cables. 
     Computers that are used for switching purposes in telephony are often duplicated, so that if the first computer is disabled, the other computer will take over. When repairing the first computer, it is extremely important that no electrostatic discharges caused by a careless technician will also disable the other computer, which would result in a total breakdown in operations. The problem is particularly serious in respect of new types of IC circuits that are extremely sensitive to static discharges. 
     SUMMARY OF THE INVENTION 
     The invention solves the aforesaid problem of enabling part of a duplicated system to be serviced without impairing EMC properties, e.g. without impairing ESD protection, of the other part of the system, by minimising the open hole area and sealingly enclosing the shield of the connection cables with a grounded electric conductor, and electrically connecting said conductor to the cages. 
     More specifically, the solution is achieved with the aid of a tube-like element which connects the two cages. The inner diameter of the tube-like element is adapted to the diameter of the cables received in the hole so as to obtain a sealed joint. The holes provided in the sides of the cages are sufficiently large to accommodate an electric contact element of reasonable size, meaning that the holes may be substantially larger than the thickness of the cables received therein. These holes are covered with an electrically conductive member, for instance a steel plate, such as to adapt the hole opening to the diameter of the cables. 
     The object of the invention is to prevent an electrostatic discharge occurring in one cage or the equipment encased thereby from wandering through the air or through electric conductors to the other cage or the equipment encased thereby, so that the EMC properties of one part of a duplicated system will be retained even when the protection in the other part is partially non-existent, so as to be able to pass cables effectively from one piece of equipment to another while retaining EMC properties, e.g. ESD protection, and to be able to carry out repairs and servicing on one side of a duplicated system without impairing the EMC properties of the other side. 
     The advantages thus afforded are good EMC properties between the two parts of a duplicated system, minimisation of the risk of disabling both sides of duplicated equipment as a result of an electrostatic discharge. Other advantages are that the solution is simple and inexpensive, that the solution enables cables having large electric contact devices in relation to cable thickness to be passed from one piece of equipment to other pieces of equipment, that ESD spikes will not wander from one piece of equipment to the other, and that the one cage will still provide good EMC properties and be protected against radiation caused by ESD, for instance, even when the other cage is open and has lost its ESD protection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which 
     FIG. 1 illustrates two cages in perspective; 
     FIG. 2 is a perspective view of one embodiment; 
     FIG. 3 is a perspective view of one part of one embodiment; 
     FIGS. 4-6 illustrate alternative embodiments schematically and in cross-section; and 
     FIGS. 7-8 illustrate schematically parts of different embodiments. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates schematically first electronic equipment  19  enclosed in a first cage  16 , second electronic equipment  20  enclosed in a second cage  17 , and cables  10  which function to connect the two pieces of equipment  19 ,  20  electrically. The sides of the cages are comprised of an electrically conductive material and may include small holes for ventilation purposes, for instance. The diameter of the holes determine how much electromagnetic radiation is able to leak from the cage and consequently the requirements set by the authorities or determined by the sensitivity of peripheral equipment to radiation will determine the size of the holes. 
     FIG. 2 is a perspective view of one embodiment of an inventive arrangement. Reference numeral  1  identifies one side of the first cage and reference numeral  2  identities one side of the second cage. Reference  5   a  identifies a first plate having a first U-shaped bar  5   b  located on its lower edge. Reference  4   a  identifies a second plate which has a second U-shaped bar  4   b  located on its upper edge. Reference  3  identifies a third plate. Reference  6  identifies screws and reference  7  identifies a first hole in the first cage, and reference  8  identifies a second hole in the second cage. FIG. 3 illustrates the cables  10  and the cable shields  11 . FIG. 5 illustrates an electrically conductive stocking  12 . 
     FIG. 6 illustrates an electrically conductive tube  13  and two nuts  14  and  15 . FIG. 8 illustrates shutter means  18 . Mutually corresponding elements in the various Figures have been identified with the same reference signs. 
     The second plate  4   a  is attached to the bottom edge of the second U-shaped bar  4   b  flush with the short side, as shown in FIG.  2 . The second U-bar  4   b  is inserted into the first hole  7 , such that the second plate  4   a  will lie in abutment with the cage side beneath the first hole  7 . The length of the second U-bar  4   b  is adapted to the distance between the cages, so that the opposite short side of the U-bar will lie in abutment with the side  2  of the second cage. The size of the holes  7 ,  8  may be adapted to the cable contact devices, which can be substantially larger than the cables  10  themselves, so as to enable cables and contact devices to be drawn readily between the two cages  16 ,  17 . 
     The first U-bar  5   b  is attached to the first plate  5   a  in the same way as that in which the second plate  4   a  and the second bar  4   b  are attached to each another. The first U-bar  5   b  is inserted into the second hole  8 , such that the first plate  5   a  will be in abutment with the side  2  of the second cage over the second hole  8 , and such that the opposing short side of the U-bar will be in abutment with side  1  of the first cage. The first bar  5   b  will therewith face downwardly. The first bar  5   b  is slightly smaller than the second bar  4   b  therewith enabling it to fit into the second bar  4   b . Because the plate  5   a  is slotted, the first plate  5   a  can be attached to the cage side so that the first bar  5   b  will press against the cables with the plate covering the upper part of the second hole  8  at the same time. When the first plate  5   a  is in an upwardly displaced position, cables fitted with contact devices can be led from the first cage  16  to the second cage  17 . The cables  10  are peeled so as to expose the cable shields  11  between the first and second cages  16  and  17 . 
     Subsequent to having drawn the cables between the cages, the first plate  5   a  is pressed downwards so as to obtain good contact between the cable shields  11 , the second U-bar  4   b  and the first U-bar  5   b . This will result in the formation of a tight tube between the first and second cages  16  and  17  and also in electrical contact between the formed tube, the cable shields, and the first and the second cages. 
     The plate  3  is fastened in the first side  1  and moved down so as to seal the first hole  7 . The second plate  5   a  functions to seal the second hole  8  on the second side  2  of the second cage in a similar manner. 
     FIG. 3 shows the second plate  4   a  and the third plate  3  in position with the cables drawn through the hole. A different method of fastening the plate  3  has been used in this case. In this embodiment, the plate  3  moves behind two smaller plates  9  which are removably attached to the first cage side  1 . Subsequent to having moved the plate  3  down to its active position, the plates  9  are secured more firmly with the aid of spring elements for instance, or with the aid of screws as in the illustrated case. 
     FIG. 4 illustrates this embodiment in cross-section prior to the third plate  3  being secured but subsequent to having moved down the first plate  5   a.    
     FIG. 5 shows an alternative embodiment in which there is used an electrically conductive stocking  12  of the kind typically used to shield cables. The stocking is attached to side  1  of the first cage with the aid of an annulus, which is secured with screws. The stocking is thereafter kept loose and the cables and contact devices drawn through the stocking. Subsequent to having drawn all cables through the stocking, the stocking is tightened to reduce its diameter and thereby sealingly embrace the cables, and is then secured to side  2  of the second cage. 
     FIG. 6 illustrates a solution that utilizes a tube  13  which is secured with the aid of nuts  14 ,  15 . In this case, the internal diameter of the tube itself need not be adapted to the actual diameter taken-up by the cables  10 . On the other hand, the active diameter can be adapted by filling the cavity defined between the cable shields  11  and the inner wall surface of the tube with an electrically conductive material, for instance steel wool or buckled or crunched aluminium foil. 
     FIG. 7 shows the first plate  5   a  attached to the second side  2  of the second cage. 
     FIG. 8 illustrates a shutter means  18  that can be used as an alternative to the plate for adapting the hole opening to the diameter presented by the cables.