Abstract:
A circuit arrangement for a field device includes an input, an output and a current-limiting element. The circuit arrangement is designed to transmit a useful signal along a useful-signal path from the input to the output. The input and the output are galvanically separated from each other. The current-limiting element is arranged outside the useful-signal path.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/754,233 filed Dec. 27, 2005 and of German Patent Application Serial No. 10 2005 062 422.7 filed Dec. 27, 2005, the disclosures of which are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the general technical field of metrology. In particular, the present invention relates to a circuit arrangement, a field device, and a method for operating a circuit arrangement, in which arrangement, device and method a current-limiting element is arranged outside a useful-signal path.  
       BACKGROUND INFORMATION  
       [0003]     In metrology it is common for so-called field devices, which convert process values to electrical signals, to be connected to feed devices or evaluation devices by way of bus systems. In this way the measuring signals that have been picked up by the field devices can be transmitted to the feed devices over long distances. The so-called HART® bus standard provides a technique, which is common today, for connecting field devices to feed devices. By way of the HART® bus standard it is possible to transmit measured values that have been determined by a field device to a feed device, with such transmission being either analogue or digital. However, feed devices and evaluation devices can also be separate devices, wherein, in the case of data transmission, the data is transmitted to the evaluation device. In the case of a digital bus, such transmission can be bidirectional.  
         [0004]     For the setting of parameters, also called parameterisation, of a field device it may be necessary that a programming device is to be connected to the field device. Since wiring between a field device and a feed device is usually fixed, i.e. it can be detached only with considerable difficulties, it is possible, for the parameterisation, setting of parameters, or scanning of values from the field device, to connect a parameterisation arrangement or device in parallel with the existing fix-connection bus system and in this way to access individual values or parameters of the field device.  
         [0005]     During coupling of a programming device to the field device two current circuits are interconnected. Due to different potentials between the current circuits it is possible for charge equalisation between the current circuits to occur. Such charge equalisation can be prevented by direct-current suppression.  
         [0006]     However, direct-current suppression cannot prevent the occurrence of high short-term current peaks. Such high current peaks can result in damage to the coupled circuits.  
         [0007]     Furthermore, the current peaks can result in an ignition-triggerable or incendive spark, which is to be avoided in particular if the coupling of the two circuits is to be used in a potentially explosive environment.  
         [0008]     Installing a resistor in a connection line of the parameterisation device to the HARTS bus nowadays prevents excessive current flow, which can otherwise result in an ignition-triggerable spark. By the resistor the current is reduced to a non-hazardous extent so that sparking can be prevented. However, the signal quality is negatively affected by a current-limiting resistor.  
       SUMMARY OF THE INVENTION  
       [0009]     According to an exemplary embodiment a circuit arrangement or assembly for a field device is provided, wherein the circuit arrangement comprises an input, an output, and at least one current-limiting element, wherein the circuit arrangement is designed to transfer a wanted- or useful-signal along a useful-signal path from the input to the output, wherein the input and the output are separated from each other by direct-current separation or direct-current suppression, and wherein the at least one current-limiting element is arranged outside the useful-signal path. The at least one current-limiting element may be a short-circuit current-limiting element which reduces the extent of a current that flows during a short circuit to a permissible level. In particular, within the context of this application the term “direct-current suppression” refers to a situation where for charge carriers there is essentially no way to flow from one current circuit to another (directly adjacent) current circuit.  
         [0010]     The at least one current-limiting element may reduce a current such that the circuit arrangement, in particular part of the circuit arrangement, may be operated in a potentially explosive environment. The current-limiting element may therefore provide explosion protection.  
         [0011]     According to one exemplary embodiment a parameterisation arrangement with explosion protection is provided, which parameterisation arrangement comprises an input and an output. Furthermore, the parameterisation arrangement comprises at least one current-limiting element. The parameterisation arrangement is designed to transmit a useful signal along a signal path from the input to the output, wherein the input and the output are separated by direct-current suppression or direct-current separation (see above for direct-current separation), and wherein the at least one current-limiting element is arranged outside the useful-signal path. In the context of this application parameterisation may in particular relate to the setting of parameters, e.g. a parameterisation arrangement or a parameterisation assembly may be a arrangement or assembly which is suitable for setting parameters of another device, e.g. a field device.  
         [0012]     By the at least one current-limiting element part of the circuit arrangement may be used in an explosion-protected environment. The part of the circuit arrangement that extends in an explosion-protected environment may be an output or an output line of the circuit arrangement, which output line is connected to the output.  
         [0013]     Limiting the short-circuit current may prevent an ignition spark from arising as a result of excessive current, during coupling of the circuit arrangement, for example to a bus. In this process an undesired current may flow, for example if the bus coupling connections accidentally touch each other.  
         [0014]     By arranging the at least one current-limiting element outside the useful-signal path, a situation may be avoided in which at least one current-limiting element influences the signal during transmission of the useful signal in a main path. Nevertheless, if a short circuit occurs, the at least one current-limiting element can limit the extent of this short-circuit current so that it is adequately small.  
         [0015]     Direct-current separation of the input from the output of the circuit arrangement may concretely decouple the output from the input. Different potential levels between the input and the output can consequently essentially not equalise, as a result of which the rise of undesirable currents and the generation of sparks may also be prevented or reduced. Furthermore, direct-current separation or direct-current suppression may be used for filtering direct-current signals. Most of the time, direct-current separation may form a barrier to a direct-current signal. It may thus be possible to avoid a situation in which direct-current signals propagate by way of direct-current separation.  
         [0016]     Measuring devices are frequently used in potentially explosive environments. If, for example, gas pressures or liquid levels of flammable liquids are to be measured with measuring devices, there may be an increased danger of explosion because the potentially explosive materials may spread in an uncontrolled way and may easily ignite.  
         [0017]     In order to reduce the danger of explosion, so-called explosion protection classes have been determined which classify the danger of work to be carried out. These explosion protection classes regulate limiting values such as, for example, maximum permissible electrical voltages or currents that may occur in the context of measuring in potentially explosive environments. Excessive currents, for example short-circuit currents, may create spark-over; likewise, an excessive voltage differential may result in spark-over. In the context of potentially explosive materials, such as for example gases, the sparks may cause explosions. However, the danger of spark-over may be reduced by direct-current separation or of at least one current-limiting element.  
         [0018]     According to an exemplary embodiment of the invention a field device with a circuit arrangement with the above-described features is created. In the context of this application the term “field device” also refers to a measuring device such as a pressure measuring device or a fill level measuring device. According to a further exemplary embodiment, in particular a measuring device with a parameterisation arrangement or a circuit arrangement is stated, wherein the circuit arrangement comprises the features stated above. A measuring device that comprises a circuit arrangement may make it possible for an inexpensive parameterisation device to be used for the parameterisation of the measuring device. However, a parmeterisation device that may be connected to a measuring device with the circuit arrangement may need not itself comprise direct-current separation for decoupling two current circuits. A measuring device with the circuit arrangement which circuit arrangement can be designed as a parameterisation adaptation arrangement may (on one input) be used for connecting a parameterisation device and may provide a corresponding interface. The output of the circuit arrangement may be firmly connected to a measuring bus of the measuring device. In this setup the circuit arrangement may be arranged in the measuring device. Just as the output may be matched to a potentially explosive environment, the input of the circuit arrangement may also be adapted for use in a potentially explosive environment.  
         [0019]     According to an exemplary embodiment a method for operating a circuit arrangement for a field device is provided, wherein the method comprises transmitting a useful signal along a useful-signal path from an input to an output of the circuit arrangement; separation of the input from the output by using a direct-current suppression; and arranging of at least one current-limiting element in the circuit arrangement and outside the useful-signal path.  
         [0020]     Exemplary embodiments of the invention can be implemented both by a computer program, i.e. software, and by one or several special electrical circuits, i.e. in hardware, or in any hybrid form, i.e. by software components and hardware components.  
         [0021]     Because the at least one current-limiting element may be wired outside the useful signal, e.g. the at least one current-limiting element is arranged outside the path on which the useful signal is transmitted, influencing the useful signal by the current-limiting element during signal transmission of the useful signal may be prevented.  
         [0022]     Because it may be possible to avoid a situation where in a clamping circuit a resistor that can be used for current reduction or current limitation is located in the useful-signal path, it may also be possible to avoid negatively influencing a useful signal that is transmitted through the resistor.  
         [0023]     According to a further exemplary embodiment a circuit arrangement is provided, wherein the output of the circuit arrangement is designed to be connected to a bus. By such a design of the output a disconnectable connection may be provided between a circuit arrangement and a bus. During measuring operations, the circuit arrangement may be coupled to the bus and/or decoupled from the bus by an output that is designed to be connected to a bus. With an output of the circuit arrangement, which output is designed to be connected to the bus, any interference with a measuring process or with some other transmission on the bus can be reduced.  
         [0024]     According to another exemplary embodiment a circuit arrangement is provided whose output is a HART® bus or an I 2 C bus. By designing the output as a HART® bus or I 2 C bus or field bus it may be possible for the circuit arrangement to flexibly connect to commonly used measuring bus systems.  
         [0025]     According to a further exemplary embodiment a circuit arrangement is provided, wherein the output of the circuit arrangement is designed so as to comprise two wires, and/or for connection of a two-wire bus. In this context the term “two-wire” may mean that useful information is transmitted by way of two signal lines. The output itself can comprise several connections (including more than two connections).  
         [0026]     In metrology two-wire connections frequently occur. Measuring signals may be transmitted by way of such a two-wire connection, which may be formed as a bus system. Parameterisation or configuration of field devices may also take place by way of this two-wire line. Field devices without an operator terminal of their own for configuration are possibly connected to a programming device by way of an externally connected circuit arrangement. In this arrangement the two-wire design of the output or of the interface of the circuit arrangement may have advantages.  
         [0027]     According to a further exemplary embodiment a circuit arrangement is provided, wherein at least part of the circuit arrangement is designed for use in a potentially explosive environment. If the circuit arrangement itself is not operated in the potentially explosive environment, it may be possible that at least the output, in particular a line connected to the output, may be operated in a potentially explosive environment. In order to be able to operate a line in a potentially explosive environment by the circuit arrangement, the circuit arrangement may comprise a protective device. To this effect the circuit arrangement, in particular an output stage of the circuit arrangement, may be designed to render an output, in particular a line connected to an output, operable in a potentially explosive environment.  
         [0028]     Potentially explosive environments can be specially classified safety regions in which there is a particular danger of explosion as a result of the type of measuring materials, and/or materials which are measured, used.  
         [0029]     According to a further exemplary embodiment a circuit arrangement is provided whose input is a universal serial bus (USB) connection or an RS232 connection.  
         [0030]     PCs or laptops or notebooks can be used for parameterisation. The provision of USB or RS232 connections may be advantageous for connection to the interfaces of such computers, as well as for connection to the interface of a PDA (personal digital assistant) or of other parameterisation devices. Designing the circuit arrangement by an interface that is compatible with the USB standard or the RS232 standard may make it possible to connect a computer or a programming device to the circuit arrangement. In conjunction with a correspondingly matched output, the circuit arrangement may thus provide an interface converter function, which may convert the signals of the interfaces among each other.  
         [0031]     According to a further exemplary embodiment, a circuit arrangement is provided, in which the useful signal is a parameterisation signal for at least one field device, which may, for example, be a fill level measuring device or a pressure measuring device. Matching the parameterisation device to a fill level measuring device or a pressure measuring device may make it possible to parameterise a fill level measuring device or a pressure measuring device.  
         [0032]     According to yet another exemplary embodiment, a circuit arrangement is provided in which direct-current suppression is generated by a capacity (for example by a capacitor, a capacitor bank or parasitic capacitance). The term capacity in particular refers to a capacitor. A capacitor may be a barrier to a direct-current signal, while alternating signals of a certain (adequately high) frequency may propagate across this barrier.  
         [0033]     According to a further exemplary embodiment a circuit arrangement is provided in which the at least one current-limiting element is a resistor, in particular a resistor with an ohmic component, furthermore in particular an essentially purely ohmic resistor. The resistor may delay the discharge of a capacitance. The charge of the capacity cannot be released through the resistance limiter in a very short time. Discharge of the capacity may be spread over an extended time. The discharge currents that flow in this process may be correspondingly small in order to meet the requirements of a device suitable to provide explosion protection. Incidents of spark-over may thus be avoided.  
         [0034]     In addition to the resistor, for example Zener diodes may ensure voltage limitation and thus may ensure short-circuit current limitation or discharge current limitation. In the case of a short circuit, small currents that are not dangerous can thus flow. Due to the small currents the circuit arrangement can be connected to a bus that leads to the potentially explosive environment. In this arrangement the summation of currents of interconnected devices should be smaller than the permissible current at the highest voltage in the circuit.  
         [0035]     A voltage in a HART® bus system can, for example, be a voltage of 30 volt. In the case of a short circuit, a short-circuit current of, for example, 131 mA may then flow. The use of resistors together with Zener diodes, whose disruptive discharge voltage is 6 volt, may reduce the short-circuit current.  
         [0036]     According to a further exemplary embodiment, a resistor is arranged between the useful-signal path and a reference point, for example an electrical reference potential, such as the mass potential or the supply voltage, of the circuit arrangement. In this way the path of a short-circuit current may be determined. The short-circuit current may be led away so that it is separate from the useful-signal path. For example, the path of the useful signal may be led to a mass potential via the resistor.  
         [0037]     According to a further exemplary embodiment, a circuit arrangement is provided, wherein the useful-signal path comprises at least one diode. If several diodes are used, the diodes can be arranged in a series circuit. By way of their forward voltage the diodes may determine a working point of the transmission.  
         [0038]     The use of three diodes or more may reduce the failure probability of the function of the diodes. In this way, the requirements of devices that are to be operated in a potentially explosive environment may be met. For, even if two diodes should fail, the blocking function of the diodes may be maintained by the diode that is intact.  
         [0039]     According to exemplary embodiments a circuit arrangement, a field device and a method for operating a circuit arrangement are provided which may provide an interference-immune circuit arrangement for a field device.  
         [0040]     This need may be met by a circuit arrangement, a field device and a method for operating a circuit arrangement with the features according to the independent claims.  
         [0041]     Many improvements of the invention have been described with reference to the parameterisation arrangement or the circuit arrangement. These designs also apply to the method for operating the circuit arrangement. 
     
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       [0042]     Below, exemplary embodiments of the present invention are described with reference to the figures:  
         [0043]      FIG. 1  shows a functional block diagram of a measuring arrangement with a connected parameterisation arrangement according to an exemplary embodiment of the present invention.  
         [0044]      FIG. 2  shows a detailed functional block diagram of a measuring arrangement with a connected parameterisation arrangement according to an exemplary embodiment of the present invention.  
         [0045]      FIG. 3  shows a circuit diagram of an explosion protection circuit for coupling a useful signal to a measuring bus according to an exemplary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0046]     The illustrations in the figures are diagrammatic and not to scale. In the following description of FIGS.  1  to  3  the same reference signs are used for identical or corresponding elements.  
         [0047]      FIG. 1  shows a functional block diagram of a measuring arrangement with a connected parameterisation arrangement according to an exemplary embodiment of the present invention.  
         [0048]     The field device  101  or measuring device  101  (for example a fill level measuring device or a pressure measuring device) is connected to a feed device  102  and/or an evaluation- or display device  102  by way of a measuring device bus  103 , for example a field bus, HART® bus or VBUS. By way of the measuring bus  103 , the field device  101  and the feed device  102  exchange bi-directional information, such as for example measured values.  
         [0049]     For the programming or parameterisation of the field device  101  an additional device can be connected to the measuring device bus  103 . In  FIG. 1  the parameterisation arrangement  105  is connected by way of the connection  104 , which is disconnectably connected to the measuring device bus  103 . In this arrangement the connection  104  is routed at least partly in an environment to which the requirements of an explosion protection class apply. By way of the disconnectable connection the parameterisation arrangement  105  can be connected or unclamped at any time. The communication between the measuring device  101  and the feed device  102  is not influenced by clamping or unclamping of the connection  104 .  
         [0050]     When connecting or disconnecting the connection  104  to/from the bus  103 , in particular as a result of unintended touching of lines, spark-over can occur, which must be avoided in particular in potentially explosive environments, e.g. areas which are exposed to explosive conditions. To prevent dangerous sparks, which could trigger an explosion, from occurring when coupling or uncoupling the connection  104  to the measuring device bus  103 , the parameterisation arrangement  105  comprises an explosion protection circuit  106 . The explosion protection circuit  106  concretely ensures physical matching of the matching signals of the parameterisation arrangement  105  to the signals of the measuring bus  103 , and also ensures adequate protective measures for using at least part of the parameterisation arrangement  105  or the connection  104  in a potentially explosive environment.  
         [0051]     The parameterisation functions are provided by a parameterisation device (not shown in  FIG. 1 ). To this effect the parameterisation arrangement  105  provides a connection  107  for a parameterisation device, for example a PC or a PDA with corresponding software. Connection of the external parameterisation device to the interface  107  can, for example, take place by way of a USB interface or by way of an RS 232  interface.  
         [0052]      FIG. 2  shows a detailed functional block diagram of a measuring arrangement with a connected parameterisation arrangement according to an exemplary embodiment of the present invention.  
         [0053]      FIG. 2  again shows the field device  101 , which is connected to the feed device  102  by way of the measuring device bus  103 . In  FIG. 2  the measuring device bus  103  is shown as a two-wire bus. It can thus be a HART® bus. By using the parameterisation arrangement  105  or the circuit arrangement  105  the useful parameterisation information  201  is to be coupled to the measuring device bus  103  as a useful parameterisation signal  202  and is to be conveyed to the field device  101  for parameterisation purposes. To this effect the useful parameterisation signal  201  is fed to the input  107  of the parameterisation arrangement  105  and is conveyed to the measuring device bus  103  by way of the output  209  of the parameterisation arrangement  105 .  FIG. 2  shows only two useful data connections of the interface  107 . The interface  107  can comprise further connections such as a power supply line.  
         [0054]     While  FIG. 2  only shows the flow of useful signals or work signals from the input  107  to the output  209 , a useful-signal flow, for example a feedback signal from the field device, can also take place in the opposite direction. A parameterisation device is connected to the input  107  of the parameterisation arrangement  105 . By an interface conversion device  203  the input signal is converted to a bus signal at the output  210  of the interface conversion device  203 . The interface conversion device  203  comprises galvanic separation (not shown in  FIG. 2 ).  
         [0055]     The parameterisation arrangement  105  is coupled to the bus line  103  by a line pair  104 . The line pair  104  is routed at least in part in a potentially explosive environment  212 .  
         [0056]     The first current-limiting element  205 , the second current-limiting element  211  and the three diodes  204  are used to ensure current-limit values to make it possible to operate part of the parameterisation arrangement  105  in a potentially explosive environment, in particular in order to make it possible to operate the line  104  in a potentially explosive environment. The first current-limiting element  205 , the second current-limiting element  211  and the three diodes  204  can carry out the functions of a explosion protection circuit  106 . The current-limiting elements  205 ,  211  and the diodes  204  protect the direct-current suppression  206  or the direct-current separation  206  or the capacitor  206 . In the case of a fault occurring, the capacitor  206  can become low-resistant. This means that the capacitor lets direct current flow through, and only provides ohmic resistance to the direct current.  
         [0057]     At the output  210  of the interface conversion device  203 , diodes  204  are located in the useful-signal path. Although  FIG. 1  shows three diodes  204 , any desired number of diodes can be used. The number of diodes depends on the selected protection level. If three diodes  204  are used, two diodes  204  can fail without this resulting in the loss of the function of the diodes  204 . In this context, fail of a diode  204  means that the blocking function of the diode  204  is lost and the diode  204  becomes conductive with low resistance.  
         [0058]     The diodes  204  are connected in series. The cathode of a first diode  204  is connected to the output  210  of the interface conversion circuit or voltage conversion device  203 . The cathode of a second diode  204  is connected to the anode of the first diode  204 . On the anode of the second diode  204  the short-circuit current-limiting resistor  205  and the direct-current separation element  206  and/or the capacitor  206  are connected. The current-limiting resistor  205  connects the potential node  207  to the mass potential. Also at the connection point  207  the positive supply voltage +Vss is connected by way of the resistor  211 . The positive supply voltage +Vss changes the two diodes  204  to a conductive state so that a useful signal that emanates from the output  210  of the voltage conversion device  203  is conveyed to the node  207 . In the case shown in  FIG. 2  the diodes are conductive because +Vss is connected to the node  207  by way of the resistor  211 . Not shown in  FIG. 2  is a switch, in particular a transistor, which can be arranged between +Vss and the resistor  211 , as a result of which the diodes  204  only become conductive if the switch is switched on and thus +Vss is present at the node  207 . By the switch, for example the diodes  204  can be brought to a conductive state when a signal is to be transmitted to the bus.  
         [0059]     By using the direct-current separation  206 , direct-current fractions are filtered out of the useful parameterisation signal, as a result of which, in particular, above all a direct current is prevented from flowing from the bus to the parameterisation arrangement  105 . On the way from the output  210  via the diodes  204  and the direct-current separation  206 , the useful parameterisation signal bypasses the resistor  205  and the resistor  211 . As a result of this the current-limiting resistor  205  cannot interfere with the useful parameterisation signal. Interference with the useful parameterisation signal is thus prevented. By way of the output  209  of the parameterisation arrangement  105  the useful parameterisation signal  202  is conveyed to the measuring device bus  103  or to the field device  101 .  
         [0060]     In addition to the state of signal transmission in an interference-free scenario,  FIG. 2  shows the malfunction case of a short circuit, in dashed lines, by the short circuit  208 . In the case of a short circuit  208  discharge of the capacitor  206  takes place by way of the mass line.  
         [0061]     A capacitor is deemed to be an unsafe component for the purpose of explosion protection. A situation can arise in which in the case of a fault the capacitor  206  becomes low-resistant. A current caused by +Vss could thus flow, by way of the line  104 , to the bus  103  or to the short circuit  208 .  
         [0062]     In the case of a short circuit the capacity  206  discharges via the resistor  205 . The resistor  205  also limits the short-circuit current, which occurs as a discharge current of the capacity  206 , to a small current that is permissible in a potentially explosive environment. In this way the parameterisation arrangement  105  or the interface converter  105  can also be connected to lines that lead in the potentially explosive environment.  
         [0063]     In the case of a short circuit  208  and a low-resistant capacitor  206  it is also possible for an increased current flow to occur through the potentially explosive environment  212 , from +Vss via the resistor  211 , the capacitor  206  and the short-circuit  208  to mass. This current is kept adequately small by the resistor  211  so as not to exceed the value permissible for a potentially explosive environment. Consequently the circuit arrangement  105  is adapted for operation in a potentially explosive environment  212 .  
         [0064]     There is triple safeguarding against failure of the diodes  204 . An increase in the number of diodes  204  increases failure safety. The diodes prevent current from flowing from the output  310  of the interface conversion circuit  203  via the direct-current separation  206 .  
         [0065]      FIG. 3  shows a circuit diagram of an explosion protection circuit for coupling a useful signal to a measuring bus, according to an exemplary embodiment of the present invention.  
         [0066]     At its output  303  the HART® bus driver module  301  provides an output signal that corresponds to the HART® bus protocol. This output signal is conveyed to the positive input of the operational amplifier  305  by way of the capacity  302 .  
         [0067]     The signal to be transmitted, which signal is provided at the output  303  of the module  301 , is a parameterisation signal in the HART® bus format. At this point the signal is FSK (frequency shift keying) modulated. In other words the signal is essentially free of direct-current fractions.  
         [0068]     The operational amplifier  305  is connected as a driver module in voltage follower switching. In this switching type, impedance matching of a high impedance at the input of the operational amplifier  305  takes place with a low impedance at the output of the operational amplifier  305 . As a result of the high input impedance of the operational amplifier  305  the output  303  of the integrated circuit arrangement  301  is only subjected to light loads.  
         [0069]     By way of the resistors  306  and  307  in voltage divider switching, which resistors  306  and  307  are also connected to the positive input of the operational amplifier  305 , a fixed direct-voltage level is provided to the positive input of the operational amplifier  305 . The capacitor  302  filters direct-current fractions from the FSK signal that has been provided at the output  303 . The alternating-current signal is modulated upon the direct-voltage signal on the positive input of the operational amplifier  305 , which direct-voltage signal has been generated by the voltage dividers  307  and  306 . At the output  310  of the operational amplifier  305  a signal is available onto which the useful signal has been modulated.  
         [0070]     Further processing of this signal then depends on the state of the transistor  311 . By way of the pullup resistor  309  the base of the transistor  311  has been determined to a value that depends on the supply voltage +Vss. By way of the resistor  308  the base of the transistor  311  and a connection of the resistor  309  are connected to the output  304  of the module  301 . By way of the output  304  a switching signal of a microcontroller can be applied, and it can be determined whether the useful signal is to be switched through to the output  209 . In order to prevent, in the case of a fault, too large a current from flowing between +Vss and mass via the capacitor  206  and parts of the line  104 , the resistor  211  limits the current.  
         [0071]     In particular two cases can occur.  
         [0072]     If in a first case a positive level is present at the output  304 , the transistor  311  blocks. Consequently the potential point  207  is present at mass potential, via the resistor  205 . The anode of one of the three diodes  204  is connected to the point  207 . The diodes  204  are connected in series. Because of the negative signal level on point  207  in relation to output  310  the diodes  204  block. Transmission from the output  310  of the operational amplifier  305  by way of the diodes  204  is not possible because no current can flow via the diodes, upon which current the signal of the output  310  of the operational amplifier  301  can be modulated. It is thus not possible for an output signal or a useful signal  202  to be present at the output  209 .  
         [0073]     In a second case a negative signal level is provided at the output  304 . By providing this signal level, useful signal transmission by way of the parameterisation arrangement is ought to be possible. If a negative signal level is present at the output  304 , this negative signal level is conveyed by way of the resistor  308  to the base of the transistor  311 , as a result of which the transistor  311  becomes conductive. In this way the supply voltage +Vss can be conveyed to the node  207  by way of the transistor  311  and by way of the resistor that is connected to the collector of the transistor  311 . As a result of the now positive voltage at the node  207  in relation to output  310 , the three diodes  204  are brought to a conductive state if the total disruptive discharge voltage of the diodes  204  is exceeded. In this way the signal that is present at the output  310  of the operational amplifier  305  by way of the output  303 , the capacity  302  and the operational amplifier  305  can reach the node  207 . This signal comprises the modulated-on useful parameterisation signal in HART® bus code. The useful signal reaches the capacity  206 , which filters from the useful signal direct-current fractions that may be present.  
         [0074]     In the useful signal&#39;s direct path by way of the capacitor  206  to the bus  103 , the current-limiting element  205  and the current-limiting element  211  are bypassed, and by way of the output  209  the useful parameterisation signal  202  can be conveyed to the HARTS bus.  
         [0075]     In the case of a short-circuit of the output  209  the capacitor  206  is discharged by way of the current-limiting element  205  and the current-limiting resistor  211 , with a current that does not present a hazard in a potentially explosive environment  212 . The explosion protection circuit stated in  FIG. 3  is thus suitable for coupling or transmitting a useful signal or a parameterisation signal in a potentially explosive environment. The danger of a hazardous short-circuit current occurring by short circuiting the connections of the output  209  when the output  209  is connected to a measuring signal bus  103  is thus reduced.  
         [0076]     By the resistor  211  the bus is protected against impermissibly high current from the parameterisation arrangement  105 . If the capacitor  205  were to become low-resistant as a result of a defect, an impermissible current could flow from the output  310  of the operational amplifier  305  to mass, by way of the capacitor  206  and the short circuit  208 , if as a result of a defect all three diodes  204  are conductive in their direction of blockage. However, concurrent failure of all three diodes is deemed to be improbable in relation to meeting explosion protection requirements.  
         [0077]     Current limitation by means of a resistor  211  prevents the formation of an ignition-triggerable spark at the output  209  in the case of a short circuit  208 . Current limitation by the resistor  205  also prevents an excessive flow of current to the bus line, which current is caused by +Vss. The bus line  103  may lead to the potentially explosive environment  104 .  
         [0078]     The parameterisation arrangement itself can partly be operated in the potentially explosive environment  212 . In this arrangement the parameterisation arrangement is operated already in the potentially explosive environment  212  when the output  209  is situated in a potentially explosive environment  212 . The diodes are operated for a useful signal flow in the direction of flow. For considerations of current limitation, to prevent an undesirable current from flowing from the operational amplifier output  310 , operation in the direction of blockage takes place.  
         [0079]     In addition it should be pointed out that “comprising” does not exclude other elements or steps, and “a” or “one” does not exclude a plural number. Furthermore, it should be pointed out that features or steps which have been described with reference to one of the above exemplary embodiments can also be used in combination with other features or steps of other exemplary embodiments described above. Reference signs in the claims are not to be interpreted as limitations.