CN114648868A

CN114648868A - Signal transmission system

Info

Publication number: CN114648868A
Application number: CN202011515584.8A
Authority: CN
Inventors: H-J·许布纳; F·贝特格; E·布克
Original assignee: Tool Manufacturing Co ltd
Current assignee: Tool Manufacturing Co ltd
Priority date: 2020-12-21
Filing date: 2020-12-21
Publication date: 2022-06-21
Anticipated expiration: 2040-12-21
Also published as: CN114648868B

Abstract

A signal transmission system has at least one signal value generator with an electronic unit and a current source, a line and a control and evaluation unit, the signal value generator feeding an applied current in a range between a predetermined first value and a predetermined second value into the line, the control and evaluation unit having a load and a microprocessor, in which a voltage drop across the load or a current flowing through the load is detected and evaluated. The load is connected in series with at least one electrical component in the control and evaluation unit, a direct voltage drop occurs across the electrical component when a current flows through the electrical component, the at least one electrical component can optionally be bridged by means of a switch, and the microprocessor compares the voltage U1 which drops across the load when the electrical component is bridged or the current flowing through the load with the voltage U2 which drops across the load when the electrical component is connected in series with the load or the current flowing through the load and identifies an error if a threshold value is exceeded or fallen below.

Description

Signal transmission system

Technical Field

The invention relates to a signal transmission system having at least one signal value generator, a line and a control and evaluation unit, wherein the signal value generator has an electronic unit and a current source and feeds an applied current into the line in a range between a predetermined first value and a predetermined second value, in particular between 4mA and 20 mA. The control and evaluation unit has a load and a microprocessor and detects the voltage drop across the load. The invention further relates to a measured value generator and an evaluation unit for a corresponding signal transmission system.

Background

Signal transmission systems are used in particular in process control and plant engineering to transmit process values from different locations to a mostly central monitoring and control location. For this purpose, the corresponding signal value generator is located on site, for example on the corresponding machine or facility or in the environment of said machine or facility. The signal value generator can be, for example, a measured value generator, which then also has at least one sensor for detecting measured values. In addition, however, the signal value generator may also be, for example, a data collector, a display unit or an actuator, which likewise can transmit values to the control and evaluation unit via the line. In the following, the measured value generator is mostly referred to as signal value generator, but the invention should not be limited thereto.

The measured value generator detects the respective measured value by means of the respective sensor and then transmits said measured value, optionally after corresponding amplification and normalization, via a line to a remotely arranged control and evaluation unit. The measured values to be detected can be, for example, temperature, pressure, gas concentration or medium velocity. In principle, exactly one measuring sensor can be assigned to exactly one control and evaluation unit. However, the control and evaluation unit is generally configured such that a plurality of, for example four, eight, sixteen or even more, measured value generators or signal value generators, which can be configured and monitored via the control and evaluation unit, can be connected to the control and evaluation unit. The measured values determined at different, partially inaccessible or remote locations can thus be monitored and displayed centrally at one location.

In existing installations, analog transmission measurement value generators are usually used, which are connected to the control and evaluation unit, mostly in a star connection. In this case, the transmission of measured values by means of an applied direct current in the range from 4mA to 20mA has established a de facto standard for the transmission of analog measured values from a measured value generator via lines to a control and evaluation unit for many years in practice. In this way, the analog measured values can be transmitted continuously, so that the current measured values of each individual measured value generator can always be called up or displayed at a control and evaluation unit, even if a plurality of measured value generators are connected to the control and evaluation unit.

The connection of the individual measured value generators to the control and evaluation unit is usually carried out here by means of a two-wire or three-wire line. The advantages of using a three-wire line here are: two cores may be used to supply energy to the measurement generator, while the applied current corresponding to the measured value is transmitted via the third core. Due to the normalization, the smallest measured value of the predetermined measuring range corresponds to a current of 4mA transmitted via the line, while the largest measured value corresponds to a current of 20 mA. This allows both monitoring of line breaks (current =0 mA) and exceeding of the measurement range (current >20 mA). In both cases, the evaluation electronics of the control and evaluation unit know that the incoming current values are not allowed to be evaluated and displayed as actually present measured values.

Even if the transmission of measured values by means of an applied current has proven to be very valuable for many years because of its very low reliability and its ability to be affected by electromagnetic interference, it brings with it the following disadvantages: it is almost impossible to transmit other information than the actual measurement values and the above two error states. Furthermore, information can be transmitted in one direction only, i.e. from the measured value generator to the control and evaluation unit. Furthermore, additional error recognition of the component or the connection of the component is often required for secure transmission.

These two disadvantages can be eliminated by replacing the analog-transmitted measured value generator with a digitally transmitted measured value generator, which is usually not connected to the control and evaluation unit via a two-or three-core star connection, but via a four-core bus connection by means of a ring or individual strings. Although this solution can be implemented without problems when constructing a new installation, in the case of existing installations all components, i.e. all measurement sensors, control and evaluation units and in particular all lines, need to be replaced, which not only entails a high cost expenditure, but often also requires a long interruption of the ongoing process. Therefore, complete replacement of the signal transmission system is generally not considered in the case of existing installations.

Disclosure of Invention

The invention is therefore based on the object of further developing the signal transmission system described at the outset in such a way that additional information about the signal transmission system or its components can be detected and transmitted as simply as possible. In this case, the replacement of one component (for example, the control and evaluation unit) does not necessarily necessitate the replacement of other components (for example, individual signal value generators or individual lines).

This object is achieved in the case of the signal transmission system described at the outset with the features of claim 1. In the case of the signal transmission system according to the invention, the load in the control and evaluation unit is connected in series with at least one electrical component, across which a direct voltage drop occurs when a current flows through the electrical component. In this case, the microprocessor determines the voltage drop across the load in both states of the switch, compares these voltages with one another, and identifies an error when a threshold value is exceeded or undershot, as a result of which the output of a corresponding error message can be initiated. Thus, the microprocessor determines both the voltage U1 that drops across the load when the electrical component is bridged and the voltage U2 that drops across the load when the electrical component is connected in series with the load. Instead of the respective voltage dropped across the load, the current flowing through the load can also be measured. In both cases, the microprocessor can determine the voltage drop across the load, so that the two voltages can also be compared accordingly.

The term "electrical component" generally refers to a component over which a dc voltage drop occurs when an electrical current flows through the electrical component. The component may be not only a single device, but a plurality of individual devices connected to one another (e.g., electrically connected in series with one another). In the simplest case, the electrical component may be a resistor. As the electrical component, for example, a diode or a series circuit of a resistor and a diode may be provided.

Since the supply current is fed into the line from the measurement generator in the case of the signal transmission system, the current source via the measurement value generator normally keeps the current constant even if the input resistance determined by the load and the electrical component at the control and evaluation unit and thus the load relationship on the line changes as a result of the electrical component being switched on in the control and evaluation unit. The change in the active load due to the switching on of the electrical component is therefore normally compensated for by the current source in the measured value generator, so that the current flowing through the load in the control and evaluation unit is independent of whether the electrical component is connected in series with the load or not. The voltage U1 dropped across the load when the electrical component is bridged should substantially correspond to the voltage U2 dropped across the load when the electrical component is connected in series with the load. If this is not the case or if the difference between the two voltages exceeds a predetermined threshold value, the microprocessor recognizes this as an error and outputs a corresponding error message. For example, an error may be, for example, a defect in the measurement generator itself or the presence of faulty wiring.

The above-described measure according to the invention, i.e. arranging the electrical components in the control and evaluation unit such that they can be connected in series or bridged with the load by means of switches, is an effective but very simple circuit technical measure for determining errors in the signal transmission system. In particular, this measure also does not require the replacement of the existing analog transmission measurement value generator and the replacement of the existing analog transmission measurement value generator with a digitally transmitted measurement value generator. Likewise, it is not necessary to replace the lines already laid between the individual measurement value generators and the control and evaluation unit.

If the load of the control and evaluation unit is referred to as a load, the load switching is effected in a simple manner by the arrangement of the electrical components and the switch. The resistance value of the load and the resistance value of the electrical component are preferably of the same order of magnitude here. In particular, the resistance value of the load may substantially correspond to the resistance value of the other resistor used as the electrical component, or may only slightly differ from the resistance value of the other resistor.

According to an advantageous embodiment of the signal transmission system according to the invention, the switches in the control and evaluation unit are switched in a clocked manner, wherein the clock represents or corresponds to a predefined serial data stream. The above-described load switching is then performed in a clocked manner in dependence on the serial data stream, which can be used for transmitting data from the control and evaluation unit to the signal value generator. By switching the switch in a clocked manner, a voltage change is superimposed on the constant current fed into the line, which voltage change can be detected by the signal value generator if the signal value generator is designed correspondingly. The control of the switches for connecting the electrical component in series with the load or bridging the electrical component is preferably carried out by a microprocessor arranged in the control and evaluation unit.

In a further particularly preferred embodiment of the signal transmission system according to the invention, the signal value generator is designed in such a way that it can detect and evaluate a serial data stream which is transmitted by the control and evaluation unit via the line and is superimposed on the constant current. For this purpose, the signal value generator has a comparator, wherein an input of the comparator is connected to the line. In this way, a serial data stream can be generated in a simple manner from the voltage changes applied to the line, which are caused by the clocked switching of the switches in the control and evaluation unit, which serial data stream corresponds to the serial data stream for the clocked switching of the switches in the control and evaluation unit. A schmitt trigger may be connected downstream of the comparator, which then is connected to the output of the comparator and generates a hysteresis in order to generate an unambiguous binary signal from the analog input signal.

The above-described clock-wise switching of the switches in the control and evaluation unit and the above-described design of the measured value generator with the comparator therefore provide the possibility of transmitting information in the form of serial data from the control and evaluation unit to the measured value generator and receiving the serial data from the measured value generator without the need here to replace already laid lines (for example two-wire or three-wire lines) with dedicated bus lines.

In the above-described design of the measured value generator, the measured value generator preferably has a microprocessor which is connected to the output of the comparator or to the output of the schmitt trigger if a schmitt trigger is connected downstream of the comparator. By means of the microprocessor, the detected serial data stream can be checked for plausibility and transmission errors can thus be recognized. Furthermore, the received data stream may be evaluated in the microprocessor.

According to a further particularly preferred embodiment of the signal transmission system, the signal value generator is additionally designed in such a way that, in addition to the actual measured values, serial data can also be transmitted by the signal value generator to the control and evaluation unit via the line. For this purpose, the signal value generator preferably has a mixer in which a clock signal is superimposed on the measured values to be transmitted, the clock signal representing the serial data stream. A modulated current is then fed into the line by the signal value generator, the amplitude of which current is modulated. The carrier amplitude can also be proportional to the measured value to be transmitted, that is to say preferably between 4mA and 20 mA. By superimposing the clock signal, the carrier amplitude is slightly changed in the clock of the serial data stream, so that the transmission of the measured values via the line is influenced only during data transmission by amplitude modulation.

In order to detect and evaluate the data transmitted by the signal value generator via the line, according to a further advantageous embodiment of the signal transmission system according to the invention, the control and evaluation unit likewise has a comparator, wherein an input of the comparator is connected to the line. The one input of the comparator is connected here in particular to a connection point between the load and the electrical component. By using a comparator, the data signal modulated onto the constant current can be detected and evaluated again in a simple manner. The serial data stream modulated by the signal value generator onto the current can thus be recovered from the voltage dropped across the load, which voltage is pulsed with the clock of the data stream, by means of the comparator.

In this case, a schmitt trigger can again be connected downstream of the comparator, whereby the schmitt trigger is connected to the output of the comparator. In this case, a microprocessor is also preferably connected to the output of the comparator or the schmitt trigger in order to check the plausibility of the evaluated data stream and thus to detect possible transmission errors.

The above-described signal transmission system according to the invention makes it possible to modernize existing signal transmission systems in a simple and flexible manner, wherein existing lines can continue to be used. In a first step, the existing control and evaluation unit can be replaced and modified according to the invention. Furthermore, the control and evaluation unit can be connected to an already existing analog transmission measured value generator or alternatively to a continuous new measured value generator which can transmit and receive additional data.

As mentioned at the outset, the invention also relates to a measured value generator and a control and evaluation unit for such a signal transmission system, in addition to the signal transmission system described in detail above. With regard to the design of the measured value generator according to the invention, reference is made to claim 11. According to the invention, the measured value generator has, in addition to the at least one sensor, the electronics unit and the current source, a comparator, wherein an input of the comparator is connected to an output connection, at which an applied current is available. So that the serial data stream can be determined from the pulsating voltage applied to the output terminal.

According to a preferred embodiment of the measured value generator, the measured value generator has a mixer by means of which a clock signal representing the serial data stream is superimposed on the measured value detected by the sensor, so that an amplitude-modulated current is available at the output connection of the measured value generator. In addition to the actual measured values, the measured value generator can thus also transmit further information via the line to the control and evaluation unit, so that, for example, information about the remaining service life of the wear-prone sensor of the measured value generator can be evaluated. Therefore, the impending defects can be identified in time, and corresponding recovery measures can be taken. Overall, the disturbance times can thus be minimized and the operating time and productivity of the installation can thereby be increased.

The control and evaluation unit according to the invention is characterized in that it has at least one electrical component and a switch in addition to a microprocessor and a load connected to the signal input, wherein a resistor is connected in series with the load and can optionally be bridged by means of the switch. In this case, the microprocessor determines the voltage drop across the load both for the case in which the electrical component is connected in series with the load and for the case in which the electrical component is bridged. If the comparison of the two voltages shows that the difference between the two voltages exceeds a threshold value, the control and evaluation unit identifies an error, which can then be output correspondingly. Such error notification may be performed optically, acoustically and/or electrically.

According to a preferred embodiment of the control and evaluation unit, the control and evaluation unit also has a comparator, and preferably also a schmitt trigger, wherein an input of the comparator is connected to a connection point between the load and the electrical component, and the schmitt trigger is connected to an output of the comparator. Furthermore, the microprocessor is preferably connected to the output of the schmitt trigger.

Drawings

In particular, there are a large number of possibilities for designing and expanding the signal transmission system, the measured value generator and the control and evaluation unit according to the invention. For this reason, reference is made to the claims as well as to the following description of preferred embodiments in connection with the accompanying drawings. In the drawings:

FIG. 1 shows a simplified block diagram of a signal transmission system with a control and evaluation unit and two measured value generators, an

Fig. 2 shows a simplified circuit diagram of a signal transmission system according to the invention.

Detailed Description

Fig. 1 shows a schematic view of how a signal transmission system 1 can be constructed. Two measured value generators 2 are shown, which, like a plurality of other measured value generators not shown, are each connected via a line 3 to a control and evaluation unit 4. The individual measured value generators 2 each have at least one sensor 5, which, depending on the design, can measure different physical variables.

If the signal transmission system 1 is, for example, a gas alarm installation, the respective measured value generator 2 has a sensor 5, which sensor 5 can measure the concentration of flammable, toxic gases or the concentration of oxygen. In the case of such a gas alarm installation, the control and evaluation unit 4 is also often referred to as a gas alarm centre. The connection of the individual measured value generators 2 to the control and evaluation unit 4 is usually carried out via a three-wire line 3, two wires being used to supply the measured value generators 2 with energy and the third wire being used to transmit the measured values. The measured values detected by the sensor 5 of the measured value generator 2, i.e. for example a specific gas concentration, are converted by means of the current source 7 and the electronic unit 6 arranged in the measured value generator 2 into an applied direct current in the range of 4mA to 20mA, wherein typically 4mA corresponds to the minimum value of a specific adjustable measuring range and 20mA corresponds to the maximum value of said specific adjustable measuring range. The reverse assignment is also conceivable, whereby 4mA corresponds to the maximum value of a particular adjustable measuring range and 20mA corresponds to the minimum value of said particular adjustable measuring range.

As can be seen from the simplified circuit diagram of the signal transmission system 1 shown in fig. 2, the current source 7 in the measured value generator 2 is connected via the output connection 8 to a core of the line 3, i.e. the third core mentioned above, via which the applied direct current representing the measured value is transmitted. At the other end of the line 3, the core is connected to a signal input 9 of the control and evaluation unit 4. The control and evaluation unit 4 has a load 10, which is connected to the signal input 9, so that the current flowing via the third core of the line 3 flows through the load 10. The voltage drop across the load 10 can be detected and evaluated by means of an a/D converter in the microprocessor 11.

In the signal transmission system 1 according to the invention or in the control and evaluation unit 4 according to the invention, in addition to the load 10, electrical components, in particular a further resistor 12, are provided, which is connected in series with the load 10, wherein the further resistor is arranged between the signal input 9 and the load 10. Furthermore, a switch 13 is provided, by means of which the further resistor 12 can be bridged. For this purpose, the switch 13 is arranged in a bridge branch 14 which is arranged in parallel with the other resistors 12. Instead of only one resistor 12 as an electrical component, a series circuit of a resistor and a diode (not shown here) may also be connected in series with the load 10, for example.

Since, in the case of the signal transmission system 1, a current is fed into the line 3 by the measured value generator 2, the current flowing through the load 10 is kept constant by the current source 7, irrespective of whether the further resistor 12 is bridged when the switch 13 is closed or is connected in series with the load 10 when the switch 13 is open. By switching or bridging the further resistor 12, an active load change takes place in the control and evaluation unit 4, which however should not normally lead to a change in the current flowing through the line 3 and thus also through the load 10, since this load change is compensated for by the current source 7.

Now, if the current flowing through the load 10 or the voltage dropping across the load 10 is measured by the microprocessor 11 of the control and evaluation unit 4 both when the switch 13 is open and when the switch 13 is closed, an error in the signal transmission system 1 can be detected from a comparison of these two voltages or currents. For example, the error may be: the measured value generator 2 or its current source 7 is defective or there is a defect in the wrong wiring or on the line 3. By the above-described measures for selectively bridging the further resistor 12 or connecting the further resistor 12 in series with the load 10, errors in the control and evaluation unit 4 can be detected with very little circuit complexity.

In the preferred embodiment of the signal transmission system 1 shown in fig. 2, the switch 13 is not only opened or closed in order to carry out the above-described error recognition, but the switch 13 is also used for transmitting data from the control and evaluation unit 4 to the measured value generator 2. For this purpose, the switch 13 is switched in a clocked manner, the clock representing a predetermined serial data stream. This superimposes a voltage change on the constant current fed into the line 3, which voltage change can also be detected in the measured value generator 2. For this purpose, the measured value generator 2 has a comparator 15 and a schmitt trigger 16, wherein one input 17 of the comparator 15 is connected to the line 3 and the other input 18 is connected to a reference potential 19. The output 20 of the comparator 15 is connected to the schmitt trigger 16, so that the measured value generator 2 can receive the serial data stream from the voltage change applied to the line 3, which is caused by the clocked switching of the switch 13 in the control and evaluation unit 4, and thus the information sent by the control and evaluation unit 4. For evaluating the received serial data stream, the measured value generator 2 also has a microprocessor 21, which can also be used to check the plausibility of the received serial data stream. For this purpose, the microprocessor 21 is connected to the output 22 of the schmitt trigger 16.

In order to be able to transmit additional information in addition to the measured values from the measured value generator 2 to the control and evaluation unit 4, the measured value generator 2 has a mixer 23, by means of which mixer 23 a clock signal is superimposed on the measured values detected by the sensor 5, said clock signal representing the serial data stream, so that a modulated current is fed into the line 3. The current fed into the line 3 is then amplitude modulated, the carrier amplitude being determined by the measured values measured by the sensor 5, i.e. preferably between 4mA and 20 mA.

By virtue of the fact that the control and evaluation unit has a comparator 24 and a schmitt trigger 25, which is connected downstream of the comparator in the present case, information of the measured value generator, which is transmitted in this way via the line 3 in addition to the actual measured value, which information may contain conclusions about the remaining service life of the sensor 5, for example, is detected in the control and evaluation unit 4. One input 26 of the comparator 24 is connected to a connection 27 between the load 10 and the further resistor 12, while the other input 28 is connected to a reference potential 29. An output 30 of the comparator 24 is connected to a schmitt trigger 25, the schmitt trigger 25 being connected to an input of the microprocessor 11.

By using the comparator 24 and the above-mentioned interconnections, the data signal modulated onto the constant current can be detected and evaluated in a simple manner. Additional information modulated by the measured value generator 2 onto the actual measured value can thus also be received on the control and evaluation unit 4 side.

Fig. 1 also shows a schematic representation of the measured value generator 2 and the control and evaluation unit 4. In the case of the measured value generator 2, an electronics unit 6 and a microprocessor 21 are shown in addition to the sensor 5. It is also shown that the measured value generator 2 has a display 31, via which the measured values can also be displayed on site. The other previously described main electronic components of the measured value generator 2, such as the current source 7, the comparator 15 and the mixer 23, are shown in the simplified circuit diagram according to fig. 2.

Correspondingly, the individual electronics in the case of the control and evaluation unit 4 are also only shown in fig. 2, but not in fig. 1. However, a microprocessor 11, which is not shown in fig. 2, is shown in the block diagram according to fig. 1, with which microprocessor 11 switch 13 and evaluate the serial data signal received via comparator 24 and schmitt trigger 25. The control and evaluation unit 4 also has a display 32 and a plurality of operating buttons 33, with which the individual measured values of the individual measured value generators 2 can be selected.

If the control and evaluation unit 4 is designed to be connected to a plurality of measured value generators 2, as shown in fig. 1, the circuit shown in fig. 2 is equipped with a load 10, a further resistor 12 and a switch 13, respectively, for each signal input 9. A comparator 24 may also be provided for each signal input 9 correspondingly. Alternatively, however, only one comparator may be provided, which may optionally be connected to the signal input 9 or the connection point 27, respectively, via a corresponding switch.

Claims

1. A signal transmission system (1) has at least one signal value generator (2), a line (3) and a control and evaluation unit (4),

wherein the signal value generator (2) has an electronic unit (6) and a current source (7),

wherein the control and evaluation unit (4) has a load (10) and a microprocessor (11),

wherein the signal value generator (2) feeds an applied current in the line (3) between a predetermined first value and a predetermined second value, in particular in a range between 4mA and 20mA, and

wherein a voltage drop across the load (10) or a current flowing through the load (10) is detected and evaluated in the control and evaluation unit (4),

it is characterized in that the preparation method is characterized in that,

the load (10) is connected in series with at least one electrical component (12) in the control and evaluation unit (4), a direct voltage drop occurring at the electrical component (12) when a current flows through the electrical component (12), wherein the at least one electrical component (12) can be selectively bridged by means of a switch (13), and

the microprocessor (11) compares the voltage U1 dropped across the load (10) or the current flowing through the load (10) when the electrical component (12) is bridged with the voltage U2 dropped across the load (10) or the current flowing through the load (10) when the electrical component (12) is connected in series with the load (10), and identifies an error when a threshold value is exceeded or fallen below.

2. The signal transmission system (1) according to claim 1, characterized in that the signal value generator (2) is configured as a measured value generator and the signal value generator (2) additionally has at least one sensor (5) for detecting measured values, wherein the current fed into the line (3) is proportional to the detected measured values.

3. The signal transmission system (1) according to claim 1 or 2, characterized in that the electrical component (12) is constructed as a resistor, a diode or a series circuit of a resistor and a diode.

4. Signal transmission system (1) according to one of claims 1 to 3, characterized in that the switching of the switch (13) in the control and evaluation unit (4) takes place in a clocked manner, wherein the clock represents a predefined serial data stream.

5. Signal transmission system (1) according to claim 4, characterized in that said switch (13) is controlled by said microprocessor (11).

6. Signal transmission system (1) according to claim 4 or 5, characterised in that the signal value generator (2) has a comparator (15), wherein one input (17) of the comparator (15) is connected to the line (3) in such a way that a serial data stream is detected from a voltage change applied to the line (3), which is caused by the clocked switching of the switch (13) in the control and evaluation unit (4).

7. Signal transmission system (1) according to claim 6, characterized in that the signal value generator (2) has a microprocessor (21) which is connected to the output (20) of the comparator (15) or to the output (22) of a Schmitt trigger (16) which is connected downstream of the comparator (15).

8. Signal transmission system (1) according to one of claims 1 to 7, characterized in that the signal value generator (2) has a mixer (23), by means of which mixer (23) a clock signal representing a serial data stream is superimposed on the applied current, so that a modulated current is fed in the line (3).

9. Signal transmission system (1) according to claim 8, characterized in that the control and evaluation unit (4) has a comparator (24), wherein one input (26) of the comparator (24) is connected to a connection point (27) between the load (10) and the at least one electrical component (12) such that a serial data stream is detected from a voltage change occurring at the load (10) as a result of the modulated current.

10. The signal transmission system (1) according to claim 9, characterized in that the microprocessor (11) of the control and evaluation unit (4) is connected to the output (30) of the comparator (24) or to the output of a schmitt trigger (25) which is connected downstream of the comparator (24).

11. A measured value generator (2) for a signal transmission system (1), in particular for a signal transmission system (1) according to one of claims 1 to 10, having a sensor (5) for detecting a measured value, an electronic unit (6) and a current source (7), wherein the measured value generator (2) provides an applied current on an output connection (8) which is proportional to the detected measured value in a range between a predetermined first value and a predetermined second value, in particular between 4mA and 20mA,

it is characterized in that the preparation method is characterized in that,

a comparator (15) is provided, wherein an input (17) of the comparator (15) is connected to the output connection (8) in such a way that a serial data stream can be determined from the ripple voltage applied to the output connection (8).

12. Measured value generator (2) according to claim 11, characterized in that a schmitt trigger (16) is provided, the input of which is connected to the output (20) of the comparator (15).

13. Measured value generator (2) according to claim 11 or 12, characterized in that a microprocessor (21) is provided, which is connected to the output (20) of the comparator (15) or to the output (22) of the schmitt trigger (16) connected downstream of the comparator (15).

14. Measured value generator (2) according to one of claims 11 to 13, characterized in that a mixer (23) is provided, by means of which mixer (23) a clock signal representing a serial data stream is superimposed on the measured value detected by the sensor (5), so that an amplitude-modulated current is provided on the output connection (8).

15. A control and evaluation unit (4) for a signal transmission system (1), in particular for a signal transmission system (1) according to one of claims 1 to 10, the control and evaluation unit (4) having a load (10) which is connected to a signal input (9) and a microprocessor (11) which detects and evaluates a voltage drop across the load (10) or a current flowing through the load (10),

it is characterized in that the preparation method is characterized in that,

the load (10) is connected in series with at least one electrical component (12), a direct voltage drop occurring across the electrical component (12) when a current flows through the electrical component (12), wherein the at least one electrical component can be selectively bridged by means of a switch (13), and

the microprocessor (11) compares the voltage dropped across the load (10) or the current flowing through the load (10) when the electrical component (12) is bridged with the voltage dropped across the load (10) or the current flowing through the load (10) when the electrical component (12) is connected in series with the load (10), and identifies an error when a threshold value is exceeded or fallen below.

16. The control and evaluation unit (4) according to claim 15, characterized in that the switch (13) is controlled by the microprocessor (11) and the switching of the switch (13) takes place in a clocked manner, wherein the clock represents a predefined serial data stream.

17. Control and evaluation unit (4) according to claim 15 or 16, characterized in that a comparator (24) and preferably a schmitt trigger (25) connected downstream of the comparator (24) are provided, that one input (26) of the comparator (24) is connected to a connection point (27) between the load (10) and the at least one electrical component (12), and that the microprocessor (11) is connected to an output (30) of the comparator (24) or to an output of the schmitt trigger (25).