Abstract:
An intrinsically safe connection unit with a network interface for intrinsically safe appliances in explosion-risk areas, having a housing, a voltage supply connection, plug connection for connecting an intrinsically safe appliance via a transmission cable, and a decoupling circuit. For allowing even a multiplicity of network subscribers to be connected in the explosion-risk area with less wiring complexity, a plurality of plug connections of the same type are provided as network interface, each being preceded by a separate decoupling circuit, and the voltage supply connection builds a central feed connection with separate supply cores for each plug connection each having at least two plug contacts for data communication and at least two plug contacts for supplying power to the connectable appliances via the transmission cable. The invention relates also to a Controller and a network interface for use therewith.

Description:
The invention relates to an intrinsically safe connection unit with a network interface for intrinsically safe appliances in explosion-risk areas, in particular for mining, such as coal mining, having a housing, having a voltage supply connection on the housing, having at least one plug connection on the housing for connection of a transmission cable by means of which an intrinsically safe appliance can be connected to the connection unit, and having a decoupling circuit, which is connected upstream of the at least one plug connection and is arranged in the housing. The invention furthermore relates to an intrinsically safe controller for connection to a connection unit with a network interface, wherein the controller is provided with an appliance housing, with a plug connection which is arranged on the appliance housing for a transmission cable, and with a decoupling circuit which is arranged in the appliance housing and is connected downstream from the plug connection. Finally, the invention also relates to a network interface for explosion-risk areas, in particular for mining, having a circuit stage on the electronics side and having a circuit stage on the plug side, which is connected to a plug connection via a decoupling circuit. 
     BACKGROUND OF THE INVENTION 
     US 2010/0283560 discloses a device for use in explosion-risk areas, in which two appliances can be connected to one another without any problems via a transmission line, for data interchange purposes, by providing decoupling networks on the input side of the appliances, which are dimensioned and designed such that the energy which can be stored in the decoupling network cannot exceed a predeterminable maximum value. This is intended to effectively prevent the creation of ignition sparks, which could lead to an explosion of a gas mixture in explosion-risk areas, while complying with the respectively applicable explosion protection standards. The network interface is also intended to comply with the Ethernet transmission standard. The transmission element in DE 10 2007 036 964 A1 builds on a standard Ethernet circuit, in which the transmission elements for the decoupling network are formed with capacitors, in order to limit the energy which can be stored in the transmission elements. 
     DE 10 2005 042 257 B4 discloses data communication being provided in the explosion-risk mining field on the one hand via wireless LAN (WLAN) and on the other hand by commercially available standard components, in which case the electrical Ethernet data communication which is used internally in the appliances is converted via medium converters to optical signals, in order then to use these optical signals in the network infrastructure of the mine. The standard components are encapsulated, potted, in an intrinsically safe form in the central unit, and are accommodated in a housing, which is licensed as being intrinsically safe, and is composed, in particular, of stainless steel. 
     In the meantime, the demand for automation in explosion-risk areas, for example in coal mining, have become as great as those above ground. In general, centrally arranged control stations are used for machine control and monitoring of mobile machines which are used in hazardous areas, and are positioned in those areas in which there are no problems with explosion protection. In order to allow above-ground control stations, for example, to also control and monitor complex automation processes, a large number of different sensors and actuators, including measurement and diagnosis systems, are required, although these have to be used in the explosion-risk area. Even in explosion-risk areas, there are therefore demands to allow an interface network which has become established in normal areas of application, in particular a network based on the Ethernet protocol, to be used. Ethernet based on the use of glass fibres is nowadays used predominantly in explosion-risk areas. The maintenance and servicing effort is considerably greater here. The glass fibre cable does not offer the capability to provide an electrical supply to the communication subscriber. 
     SUMMARY OF THE INVENTION 
     The invention therefore has an object of overcoming this problem and of allowing even a multiplicity of network subscribers to be connected in the explosion-risk area, with less wiring complexity. 
     In order to achieve this object and others, a connection unit is proposed in which a plurality of electrically conductive plug connections of the same type are provided as network interface connections on the housing, with a separate decoupling circuit being connected upstream of each network interface connection, with the voltage supply connection on the housing preferably being in the form of a central feed connection with separate supply cores for each plug connection, and wherein each plug connection has at least two plug contacts for data communication and at least two plug contacts, which are connected to the associated supply cores, for supplying power to the appliances which can be connected to the connection unit via the transmission cable. An intrinsically safe network switch is also provided with the intrinsically safe connection unit, in which switch each plug connection is used both for data communication and the power supply of the connected appliances, sensors, actuators or the like. Each network subscriber connected to the connection unit can also be operated on the one hand for signalling purposes via an electrically conductive and preferably copper-based network interface and, at the same time, where the network subscriber requires a power supply, the power supply to this network subscriber can also be ensured via the same plug connection. It is self-evident that transmission cables which are suitable for this purpose should then also be used here. 
     In the connection unit according to the invention, it is preferably possible to connect only one specific appliance to one plug connection, or to use one plug connection as a feed connection for a supply circuit having a plurality of subscribers. When a supply circuit is connected to a plug connection, all the connected appliances can then be supplied with power via the associated supply cores for this plug connection. The integration of signal and voltage supply cores in a single plug connection makes it possible to considerably reduce the wiring required in the explosion-risk area. All the necessary connections for individual network subscribers or appliances can be carried in one transmission cable, if necessary also carried in a protected form in a flexible hydraulic tube, as a transmission cable. 
     In one preferred refinement, the connection unit on the housing has a total of X plug connections for the combined data and power supply, with the preferably central feed connection having 2*X supply cores and two additional supply cores; the additional supply cores are preferably used for separately supplying power to the decoupling circuits, in particular to electronics which are normally connected upstream of the decoupling circuits. The connection unit preferably has a central feed connection, which has two supply cores for each plug connection. The electronics within the connection unit are galvanically isolated via two additional supply cores from the supply cores for supplying the individual plug connections with power. In the particularly preferred refinement, X=6 plug connections are formed on the housing, as a result of which the feed connection has 14 poles. Seven voltage circuits can then be supplied via this feed connection, with one voltage circuit being provided for supplying power to the electronics in the connection unit. 
     In order to additionally allow DC-isolated data transmission and/or data transmission for particularly long connecting paths, for example of several kilometers, as well by means of the connection unit. It is also advantageous to provide at least one optical connection, preferably two optical connections, for optical data communication on the housing of the connection unit, in addition to the plug connections. It would optionally also be possible to provide additional plug connections on the housing, without plug contacts for supplying power, and/or additional plug connections with blind contacts, which are not connected to supply cores, in order if required to allow network subscribers to be connected directly to the connection unit, which network subscribers are used for pure diagnosis functions and monitoring functions and do not require a separate power supply. 
     In order to ensure that the electrical resistance of the supply cores is as low as possible, possibly with a limited cable cross section, it is particularly advantageous for each plug connection to be provided with in each case two plug contacts for each power potential and with four plug contacts for data communication, that is to say with a total of 8 plug contacts. Furthermore, it is preferably to provide that the central feed connection can be connected to a power supply unit via a power supply cable having (2*X+2) cores, in particular a 14-core power supply cable, which power supply unit has X+1 separate power supply in a preferably pressure-resistant housing, with the output side of each power supply being connected to a feed connection of the power supply cable via two supply cores. Each plug connection within the connection unit is also supplied with power from a separate power supply; at the same time, the electronics for the decoupling circuits are also supplied with power by a separate power supply, in which case, in fact, this power supply provides the power supply for the electronics and for all the decoupling circuits. 
     The above objects are also achieved by an intrinsically safe controller for connection to a corresponding connection unit, wherein the controller is characterized in that the plug connection has at least two, and preferably four, plug contacts for data communication and at least two plug contacts for supplying power via the transmission cable, and wherein the plug contacts for the power supply are connected to electronics, which are arranged in the housing, for the decoupling circuit and/or to the decoupling circuits which are arranged in the appliance. A corresponding controller can also be connected to one of the plug connections on the connection unit, wherein the power supply for the controller is provided via the plug contacts for the power supply to the connection unit, and the associated plug contacts on the plug connection of the controller. According to one advantageous refinement, the controller can be provided with a secondary plug connection, which is designed to be identical to the plug connection and is likewise connected to the electronics in the controller, wherein the plug contacts for the power supply on the secondary plug connection are directly electrically connected to the plug contacts on the plug connection. The power supply to the secondary plug connection is also passed through the appliance. In order to comply with the requirements for intrinsic safety, it is particularly advantageous for the electronics which are connected upstream of the decoupling circuits in the connection unit or the intrinsically safe controller each to have an associated power limiting circuit on the input side, by means of which the current which is supplied to the power supply for the electronics and the decoupling circuits, and the voltage which is applied to the electronics or the decoupling circuits, are limited to a predeterminable input current and a predeterminable input voltage for the electronics. The output current and the output voltage from the power limiting circuit, which at the same time form the input voltage and the input current for the electronics and the decoupling circuits for the network interfaces, are limited by this measure to maximum values at which spark formation or any other state which is a hazard in compliance with explosion protection can definitively and effectively be precluded. By way of example, the power limiting circuit can limit the power to less than 3 watts. 
     It is advantageous for the connection unit and the intrinsically safe controller to be equipped with network interfaces which comply with the requirements for intrinsic safety and allow licensing for explosion-risk areas. In order to achieve this, it is particularly advantageous for the decoupling circuit to have a circuit stage on the plug connection side and a circuit stage on the electronics side, providing DC voltage decoupling for the circuit stage on the plug connection side, wherein the circuit stage on the electronics side has a resistance network for radio-frequency power limiting and/or the circuit stage on the plug side has a capacitor network for direct-current power decoupling from other plug connections of the same type. In normal circumstances, both a resistance network and a capacitor network are used. A network interface such as this may in particular be in the form of an Ethernet interface for transmission of the Ethernet IP protocol, which has PHY semiconductor components for data communication, in a manner known per se. A capacitor network with capacitors preferably connected in series allows the direct current power in the signal path downstream from PHY components to be limited. With preferably two resistances in each case in each signal path of a resistance network, the power of the radio-frequency circuit, via which in principle communication data can flow at a data rate of 100 Mbit/s, can be limited. These measures allow any desired number of appliances to be connected together by appropriate network interfaces in intrinsically safe networks. 
     A corresponding network interface with a circuit stage on the electronics side and with a circuit stage on the plug side, in which the circuit stage on the electronics side has a resistance network for radio-frequency power limiting and/or the circuit stage on the plug side has a capacitor network for direct-current power decoupling from other network interfaces is of independent inventive importance. In a network interface such as this, even if it forms a component of a connection unit or of a controller, it is particularly advantageous for each circuit stage on the electronics side to have a transmitter part for bidirectional data communication, preferably with a suitable Tx-PHY semiconductor component, and a receiver part, with a suitable Rx-PHY semiconductor component, wherein a transmission element of the decoupling circuit is connected downstream from a two-pole transmitter output, and a separate transmission element of the decoupling circuit is in each case connected upstream of the two-pole receiver input, and decouples the circuit stage on the plug side from the circuit stage on the electronics side. A capacitor can be connected between the transmission element and the plug contact of data communication on the plug connection for each contact pole of a transmitter output or receiver input in the circuit stage on the plug side, for voltage decoupling, in particular DC voltage decoupling, and/or resistors, as port elements with a 2-port characteristic, can be connected between each contact pole and the associated transmission element for radio-frequency power limiting. Capacitors between the plug contacts on the plug connection and the respective transmission element allow effective direct-current power coupling to be achieved even when appliances which are connected to one another are fed via different power supplies. Possible destruction of the transmission elements by external influences can therefore not occur. An additional resistance network with resistors which are arranged between the transmission element and the PHY semiconductor components at the same time makes it possible to limit the radio-frequency power, with minimal matching losses with respect to the terminating impedances. The combination of the abovementioned features in particular allows licensing for use in explosion-risk areas. 
     It is particularly advantageous for a capacitor to be connected in the capacitor network between each plug contact for data communication and the circuit stage, in which case the capacitors in the capacitor network should preferably each comprise two series-connecting capacitor elements, for redundancy reasons. 
     The connection unit, the controller and the network interface are preferably provided with a plug connection which has eight plug contacts, of which two plug contacts are in each case provided for each power potential, and four plug contacts for data communication. 
     These and other objects, aspects, features, developments and advantages of the invention of this application will become apparent to those skilled in the art upon a reading of the Detailed Description of Embodiments set forth below taken together with the drawings which will be described in the next section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  uses a schematic circuit diagram to illustrate a connection unit having a plurality of plug connections, which each comprise network interfaces, a power supply unit with a plurality of power supplies for supplying voltage to the connection unit, as well as a subscriber circuit which is connected to the connection unit and has two intrinsically safe controllers; 
         FIG. 2  uses a schematic circuit diagram to illustrate further details of the connection unit shown in  FIG. 1 , together with the connected controller and the intrinsically safe network interface used in it; 
         FIG. 3  shows a detail view of a part of the network interface according to the invention as shown in  FIG. 2 , for bidirectional data communication; and 
         FIG. 4  shows a plug contact configuration which is preferably used in the connection unit, the controller and/or the network interface. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring now to the drawings wherein the showings are for the purpose of illustrating preferred and alternative embodiments of the invention only and not for the purpose of limiting same,  FIG. 1  uses a schematically simplified sketch to illustrate the basic configuration of a network infrastructure for a network  100  for explosion-risk areas using an intrinsically safe connection unit  10  according to the invention, as well as in this case two intrinsically safe controllers  50  according to the invention, which in  FIG. 1  are connected together to form a subscriber chain on the network  100 . Power is supplied both to the connection unit  10  and to the intrinsically safe controllers  50  via a power supply unit  1  which preferably has a pressure-resistant, encapsulated housing  2 , in which a plurality of separate power supplies  3 A,  3 B,  3 C, which are annotated in their totality with the reference symbol  3 , are arranged. The number of power supplies  3  in the power supply unit depends on the number of plug connections  11 , network interfaces  15  and voltage circuits which can be connected to the connection unit  10 . In the illustrated exemplary embodiment in  FIG. 1 , only three of the preferably six plug connections  11  which are provided are illustrated, with these plug connections being annotated with the reference symbols  11 A,  11 B,  11 C in  FIG. 1 . In one preferred refinement, a power supply unit  1  is used which has a total of seven separate power supplies  3 , wherein each of these power supplies  3 H,  3 A,  3 B,  3 C is supplied with power by means of a central power line  4 , which is connected to an alternating-current input side  5  of the power supply unit  1 . Each intrinsically safe power supply  3 A,  3 B etc. converts the power supplied to it to a DC voltage and a direct current, for which reasons a DC voltage supply for the individual assemblies in the connection unit  10  can in each case be provided via the cores  6 ′ on the secondary side of the power supply unit  1 . As is still to be explained, the downstream controllers  50  are also supplied with power via the power supplies  3  in the power supply unit  1 . The individual cores  6 ′ of the power supply cable  6 , that is to say fourteen cores  6 ′ when there are seven power supplies, are preferably arranged within a single supply cable in order on the one hand with the housing  2  of the power supply unit  1  can be provided with a preferably central voltage supply output  7  for the power supply cable  6 , and on the other hand in order that a central feed connection  12  can be provided on the connection unit  10 . Each feed pole which is connected to one of the cores  6 ′ is in turn connected at the feed connection  12  via a separate pair of supply cores  13 A,  13 B,  13 C to an associated plug connection  11 A,  11 B,  11 C. 
     The connection unit  10  has a housing  14  which is preferably composed of metal and is closed, forming a seal, by a metal cover which is covered by means of a seal. Within the housing  14 , a network interface  15 , which is annotated with the reference symbol  15  overall, is provided at least for each of the plug connections  11 A,  11 B,  11 C, with the network interface associated with the individual plug connections  11 A,  11 B,  11 C in  FIG. 1  being annotated correspondingly with the reference symbols  15 A,  15 B,  15 C. In addition to the preferably six plug connections  11 , which are each connected via separate supply cores  13 A,  13 B,  13 C to the central feed connection  12 , the connection unit  10  also has two optical connections  16 , as well as a blind plug connection  17  here as well, which, although it has a network interface  15 ′, is not, however, connected to one of the power supplies  3  in the power supply unit  1 . Such blind plug connections  17  could also be omitted and in this case are intended only to indicate that additional components can be integrated in a connection unit  10 . 
     In the illustrated exemplary embodiment, electronics  18  are connected upstream of all of the network interfaces  15  in which case these electronics  18 , which are connected upstream of the network interfaces  15 A,  15 B,  15 C,  15 ′, are supplied, possibly together with further circuit elements accommodated on printed circuit boards and the like in the connection unit  10 , via the separate supply cores  13 H with the power from the first power supply  3 H from the power supply unit  1 . The individual network interfaces  15  are also supplied with power via a separate power supply  3 H and in each case independently of that power supply  3 A,  3 B,  3 C via which power is supplied to the downstream controllers  50  on the same plug connection  11 A,  11 B. In addition to a power supply, each of the plug connections  11  also allows data communication via the associated network interface  15 . Power is supplied for the entire data communication process within the network  100 , and in particular within the connection unit  10 , separately from the power supply for the individual controllers and subscriber circuits. As can also be seen from the illustration of the connection unit  10  in  FIG. 1 , each network interface  15 , which preferably allows data communication based on the Ethernet IP protocol, in each case has two signal paths  19  for data communication in one direction, and two separate signal paths  20  for data communication in the other direction. Each plug connection  11 , as well as the blind plug connection  17 , therefore in the illustrated exemplary embodiment has four plug contacts  21 , which are illustrated by means of the relatively small circles, for data communication. In addition to the plug contacts  21  for data communication, there are at least two plug contacts  22 , which are indicated by means of the large circles, on each plug connection  11 , in order to allow the associated supply cores  13 B,  13 C and the power supplied via them to be tapped off at the respective plug connection  11 A,  11 B. In order to minimize the electrical resistance of the supply cores with limited cable cross sections, it is particularly advantageous for each of the two plug contacts  22  for the power supply to be duplicated, such that there are a total of eight plug contacts  21 ,  22  on each plug connection  11 A,  11 B, for example for a plug connection  11 A as illustrated in  FIG. 4 , specifically the four plug contacts  21  and the four plug contacts  22  for the power supply. Two of the plug contacts  22  for the power supply in this case form the conductor for the GROUND potential, while the two further plug contacts  22  form the voltage line with the voltage potential of preferably 12 volts. A fixed predetermined position of the individual plug contacts, for example by arranging one of the GROUND plug contacts  22  in the centre, makes it possible to ensure that a connection of a plug to a plug connection  11  can only ever be made in the same manner and with predetermined contact being made, without errors being able to occur as a result of incorrect plug connections. The geometry of the distribution of the plug contacts  21 ,  22  on the plug connection  11  also ensures that a cable such as a transmission cable with its eight-pole cable plug can be connected to the plug connection  11  in only one specific position. 
     The configuration of the intrinsically safe controllers  50  for use in the network  100  will now first of all be explained briefly with reference to  FIG. 1 . Both of the intrinsically safe appliances  50  illustrated in  FIG. 1  have a plug connection  51  on the input side, whose configuration is identical to the plug connection  11  on the connection unit  10 , as a result of which reference is made to the description there. The plug connection  51  also has eight plug contacts  61 ,  62  with contact wiring as explained for the plug connection  11  with reference to  FIG. 4 . The plug contacts  62  for the voltage supply are on the one hand connected to electronics  58 , which are arranged in the housing  54  of the intrinsically safe controllers  50 , and on the other hand are connected directly to a secondary plug connection  71 , which is once again configured in the same way as the plug connections  11  and  51 . Because of the configuration, the secondary plug connection  71  could also form the input side of the controller  50 . The sketch in  FIG. 1  shows well that the two plug contacts  62  on the plug connection  51  for the power supply for the controller  50  are connected via the two wiring lines  63  to the plug contacts  82  on the secondary plug connection  71 . Once again, a network interface  65  is arranged on the plug connection  51  behind the plug contact  61  for signal transmission, and is configured in the same way as the network interface  15  in the connection unit  10 . There are also plug contacts  81  for data communication on the secondary plug connection  71 , once again with a network interface  65  being connected upstream of these plug contacts  81  within the housing  54 . All of the network interfaces  65  of the intrinsically safe controller  50  are controlled via the electronics  58 . 
     All of the network interfaces  15 ,  15 A,  15 B,  15 C in the connection unit  10  or  65  in the intrinsically safe controller  50  are configured identically to one another, and each have a decoupling circuit with a transmission element  30  for both transmission directions, and the configuration of the decoupling circuit will now be explained first of all with additional reference to  FIG. 3 . 
       FIG. 3  illustrates only the circuit arrangement for one signal direction of a network interface  15  or  65 , with the circuit part which is illustrated in the left-hand half and has the Tx-PHY semiconductor component forming the transmitter part, and the right-hand circuit part with the Rx-PHY semiconductor component forming the receiver part of the network interface. A transmission element  30  is in each case located between the PHY semiconductor modules and the plug connections  11  and  51 , which are only partially indicated, which transmission element  30  may consist, in a manner known per se, of a core with windings arranged on it, as is familiar to a person skilled in the art for the configuration of corresponding network interfaces, in particular interfaces for the Ethernet protocol. According to the refinement of the network interface  15 ,  65  according to the invention for use in explosion-risk areas, a circuit stage  31  on the electronics side is in each case provided between the transmission element and each PHY which is arranged on the associated electronics ( 18 ,  58 ,  FIG. 1 ). Furthermore, a circuit stage  32  on the plug connection side is arranged between the transmission element  30  and the plug connection  11  or  51 . Each of the circuit stages  31  on the electronics side has, as illustrated, a resistance network, which is annotated with the reference symbol  35  overall, while the circuit stage  32  on the plug connection side has a capacitor network  36 , in this case consisting of the two capacitors C 1 , C 2  in the transmitter part and the two capacitors C 3 , C 4  in the receiver part. Although the figures in each case illustrate only one capacitor C 1 , C 2 , C 3 , C 4  in each signal path in the capacitor network  36 , the capacitors are, however, preferably duplicated for redundancy reasons and each consist of two series-connected capacitances in order that power decoupling can still be ensured in the event of a fault, if one capacitor C 1 , C 2 , C 3 , C 4  is shorted, despite the second, series-connected capacitor. Since one capacitance is arranged in each transmission path or signal path with at least one capacitor C 1 , C 2 , C 3 , C 4 , this ensures that no DC power can be transmitted. The capacitances should be chosen to be as small as possible, in order to minimize the physical size as well, in which case, however, the RF signal to be transmitted should be subject to as little interference as possible in order to allow a 100 Mbit/s signal to be transmitted and received without interference. For this purpose, by way of example, the capacitors in each transmission path may together have a capacitance of about 16 nF, although larger capacitances may also be used. 
     A 100 Mbit/s transmission rate is standard for an Ethernet interface. Two resistors R 5 , R 6 ; R 7 , R 8 ; R 9 , R 10  as well as R 11 , R 12  are arranged as components of the resistance network  35  upstream of each transmission element  30  in each signal path for the transmission path, and limit the power of the radio-frequency signal in order that only desired appliances, and any desired numbers of appliances, with this interface specification (point-to-point link) can be interconnected in intrinsically safe networks. Said resistors R 5  to R 16  in the resistance network  35  ensure limiting of the power coupling with respect to the RF power (radio-frequency AC voltage power), in particular in conjunction with an intrinsically safe voltage supply for the transmitter semiconductor modules Tx-PHY and the receiver semiconductor modules Rx-PHY. In addition, the resistance network  35  also contains the resistors R 1 , R 2 , R 3 , R 4 , as is also known for a “traditional” Ethernet network interface. Because of the arrangement of the resistors, each resistance network  35  is in the form of a 2-port network both for the transmitter part with the associated Tx-PHY and for the receiver part with the associated Rx-PHY, with the resistance networks  35  being designed such that a terminating impedance z 0  of about 100Ω exists in each case, allowing, RF power limiting at the same time. By way of example, the resistors R 5  to R 16  may be 3Ω resistors, and the resistors R 1  to R 4 , for example, 47Ω resistors. The resistances of the resistors should be chosen such that, on the one hand, adequate RF power limiting is achieved, while the attenuation losses are not excessive at the same time. For a maximum voltage for the individual PHY semiconductor modules of about 2.7 volts, for example, the total power in the signal paths can thus be kept below an upper limit. 
     Reference will now be made to  FIG. 2 , in which some of the components described further above for an intrinsically safe, underground network  100 , are illustrated once again in detail and in extracted form. However,  FIG. 2  shows only plug connection  11 A on the connection unit  10  as well as the plug connection  51  on the controller  50 . In addition, only the intrinsically safe power supply  3 H for the electronics  18  in the connection unit and the power supply  3 A for feeding power at the first plug connection  11 A, and therefore the power supply for all of the appliances connected to the plug connection  11 A, as well as the electronics  58  in the controller  50 , are also illustrated from the power supply unit  1 . In order to ensure the power limiting, as described further above, for the electronics  18  in the connection unit  10  and for the electronics  58  in the controller  50 , the electronics  18 ,  58  preferably each have an associated intrinsically safe power limiting circuit  39  in the connection unit  10  and, respectively, power limiting circuit  69  in the controller  50 . The power limiting circuit  39  limits the current supplied by the power supply  3 H for the electronics  18  in the connection unit  10 , and the voltage applied, to a predeterminable input current of, for example, about 1 ampere and approximately 2.7 volts. Correspondingly, the power limiting circuit  69  limits the current supplied by the power supply  3 A for the electronics  58  in the controller  50  and the voltage applied to it to a predeterminable input current of, for example, about 1 ampere and to an input voltage of approximately 2.7 volts. The power limiting circuit  39 ,  69  may also be associated with an input protection circuit with diodes, in which case both the input protection circuit and a current limiting circuit are preferably in each case duplicated in order to ensure the protection function still exists if one of the circuits malfunctions. Each controller  50  should be provided with a corresponding power limiting circuit  69 .  FIG. 2  indicates particularly clearly that the network interfaces  15  and  65  have the same configuration and each have a transmitter part and a receiver part, configured in the same way as one another, in each case with a capacitor network  36  and a resistance network  35 . 
     All of the transmission between the plug connection  11 A on the connection unit  10  and the plug connection  51  on the controller  90  takes place by means of a schematically illustrated transmission cable, which preferably has eight cores, for the reasons mentioned further above. A power cable for connecting the power supplies to the connection unit preferably has 14 cores, in order to allow a power supply unit  1  with seven power supplies  3  to be connected to a connection unit  10  with six current-carrying plug connections  11 . 
     Numerous modifications will be evident to a person skilled in the art from the above description and are intended to be covered within the scope of protection of the dependent claims. The figures illustrate an Ethernet interface as a preferred exemplary embodiment. However, the selected circuit principle can also be applied to similar interfaces, such as an interface based on the RS485 circuit principle. In addition, a greater or lesser number of plug connections could be provided in each connection unit, and each connection unit could also have a plurality of blind plugs or the like, in addition to the optical waveguide connections as optical connections. 
     Further, while considerable emphasis has been placed on the preferred embodiments of the invention illustrated and described herein, it will be appreciated that other embodiments, and equivalences thereof, can be made and that many changes can be made in the preferred embodiments without departing from the principles of the invention. Furthermore, the embodiments described above can be combined to form yet other embodiments of the invention of this application. Accordingly, it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.