WO2012139640A1

WO2012139640A1 - Transmission system, central unit or field device in such a transmission system, and method for operating the system

Info

Publication number: WO2012139640A1
Application number: PCT/EP2011/055830
Authority: WO
Inventors: Gerhard Hammer; Thomas Hauschulz
Original assignee: Siemens Aktiengesellschaft
Priority date: 2011-04-13
Filing date: 2011-04-13
Publication date: 2012-10-18

Abstract

The invention relates in particular to a transmission system (10), comprising at least one central unit (12) and at least one field device (14), wherein a connection for supplying energy to the field device (14) and a communication connection from and to the field device (14) are present simultaneously by means of a two-wire cable (16). The transmission system (10) is characterized by a central unit (12) at least having a direct current source (UK, UT), either in the form of two independent direct current sources - first and second direct current sources (UK, UT) - or in the form of a controlled direct current source, and having a communication module - first communication module (UART) -, at least one field device (14) at least having a first and a second direct current sink (IK, IT) and a communication module - second communication module (UART) -, and a direct superimposition of an output signal (TX) of the communication module (UART) and a bus voltage applied to the two-wire cable (16) by the direct current source, the two-wire cable (16) being connected to the direct current source (UK, UT) of the central unit (12) and to the first and second direct current sinks (IK, IT) of the field device (14).

Description

description

Transmission system, central processing unit or field device in such a transmission system and method of operation of the system

The invention relates to a transmission system, a central processing unit and / or a field device in such a transmission ^¬ system or for such a transmission system and a method for operating the system and / or the included or usable in such a system units, ie in particular central unit and field device. For the Übertra ^¬ supply system it is provided that an outbound connection from the central unit to supply energy to the or each field device and a communication link to and from the field device ^¬ is simultaneously via a two-wire line be.

In the automation of technical processes (automation technology), individual devices, eg. As sensors, actuators and the like, often distributed and decentralized entspre ^¬ accordingly be referred to as field devices. They usually need to be powered up. They also have the jeweili ^¬ gen automation system, so at least engineering equipment is a automation or a plurality of networked automation ^¬ devices to communicate. For the field devices thus acts at least one automation device as the central unit.

If both tasks, namely power supply and data transmission, are solved independently of one another, there is the necessity of connecting the field devices to their environment via four connection lines. The associated effort, z. For example, for cable, connection technology, planning, installation, and the inherent error possibilities are not desirable in many applications. Especially in process engineering, explosion protection is added as a further requirement. For these reasons, a solution that allows power and data transmission over only a pair of wires, strong prefers. However, it can be assumed that for an acceptance of a solution with only one pair of wires, a two-wire line, it is necessary that the costs necessary to solve the problem are not higher than the savings that can be achieved during handling.

Transmission systems based on a two-wire line with a central unit and at least one field device are known per se. By way of example, in this respect reference may be made to transmission systems according to the standard known under the designations PROFIBUS PA or PROFIBUS FF.

Common to known two-wire solutions is the property that on the transmitter side the data present as NRZ signal with information-dependent DC component is coded so that a DC-free signal is produced, which is superimposed on one of the power supply of the field devices serving DC voltage. On the receiver side, this signal, also referred to as AC signal, is split off and decoded by a high-pass filter. Only then is there again an NRZ signal, which z. B. can be processed by a UART as Kommu ^¬ nikationsbaustein. Such known and comparatively fast two-wire transmission systems, however, do not always come without special additional modules, eg. As encoder / decoder and / or a special, so-called medium attachment unit (MAU) from. This is shown by way of illustration in FIG. Such construction ^¬ stones are moreover expensive and thus determine to a large extent the overall costs of an interface in the above well-known and further similar transfer systems.

Accordingly, it is an object of the present invention to provide a transmission system which is suitable for a variety of applications and at the same time inexpensive.

This object is achieved with the features of claim 1. An ^¬ . For this purpose, in a transmission system having at least one central unit and at least one field device, frequently a plurality of field devices, wherein a connection to the power supply of the or each field device and a communicative connection from and to the field device simultaneously consists of a two-wire line, the following is provided:

The central processing unit comprises at least one DC source ^¬ and a communication module. As a communication module is a so-called UART (Universal Asynchronous Receiver Transmitter) into consideration.

The field device comprises at least a first and a second DC sink and also a communication ^¬ block. Again, a so-called UART is considered as a communication module.

An output signal of the communication module, in particular ^¬ sondere a transmission signal as an output signal is superimposed directly one of the two-wire line impressed by the DC bus voltage source. The direct storage is there ^¬ for example, without any additional coding, as her otherwise intended to ^¬ for transmission over two-wire lines to avoid an information-dependent DC component. The direct superposition furthermore takes place, for example, as a unipolar, so-called NRZ signal.

The two-wire line is connected for data and energy transmission in the transmission system to the DC power source of the central unit and to the first and second DC sink of the or each field device.

Since both the central unit and the or each field device each have a communication module, the communication module of the central unit as the first communications module and the communications module of a field device are referred to as a second communication module ^¬ below.

The central unit has the DC power source either in the form of two independent DC power sources - first and second DC power sources - or in the form of a controlled DC power source. With two independent DC sources ^¬ is one of the direct current sources controlled. Only one controlled DC source is basically to a summary of an independent first and second DC power source, one of which is controllable.

The advantage of the invention is that via a two-wire line, so two cable wires, a transmission of

Energy and data succeed in addition under the following boundary conditions:

The transmission is transparent to the respective proto ^¬ kollschicht and thus basically suitable for all common bus protocols.

The data and energy transfer is scalable in terms of data rate, cable length and energy requirements.

The transmission is independent of a specific topology and therefore suitable for star, tree and bus structures as well as end-to-end links.

The costs for the device interface are no higher than for a standard RS485 interface today.

Under the names PROFIBUS DP and DeviceNet well- known transmission systems allow unlike the above two wire-compatible transmission systems scalability in terms of data rate and line length, as no VELOCITY ^¬ keitsabhängigen blocks are required. Between the UART parameterizable in its baud rate and the line there is only one RS485 transceiver, which is speed-transparent within certain limits. Hence the ^¬ represents Asked in FIG 1 System structure. It is characterized by protocol and speed transparency and cost-effective implementation, due to the use of a UART included in most microcontrollers and an RS485 transceiver, which is an inexpensive standard component.

An advantage of the invention is thus also that a transmission system similar to that shown in FIG 2 is possible, consisting of a the automation system associated central unit, one or more field devices and the required connection cables. The field devices are from powered by the central unit and can communicate with it. In particular, as in the case of the known four-wire systems (eg, FIG. 1), this communication takes place via the communication component integrated in almost every microcontroller and known by the name UART. By suitable design of transmitter and receiver, ie central unit and field device, in the proposed transmission ^¬ system but only a two-wire line is needed. As in the known two-wire systems, a constant supply voltage UK is applied to the transfer ^¬ transmission cable via the central unit, the field equipment can be found for its power supply a constant current IK and generate a voltage UV to supply the electronics and of the AN closed sensor or actuator. When sending the field devices take an additional power IT.

The central unit has at least one port, often via a plurality of ports, to each of which one or more field devices can be connected. The number of ports and the number of field devices that can be connected to each port depend on the respective boundary conditions. As energy needs, necessary data rate, cable length, etc. So z. For example, for higher data rates, one field device per port can be connected if certain line lengths are exceeded.

The proposed transmission system combines the cost ^¬ advantages of a system with two lines for power and data transmission (four-wire system) with known two-wire systems, because the output of the communication module directly, ie without further coding or the like, one of the two-wire line impressed by the DC power source bus voltage is superimposed , This saves costs (omission of the encoder / decoder) and the full transparency of the physical layer is guaranteed. Advantageous embodiments of the invention are the subject of the dependent claims. The references used in this case point to the further development of the subject of the Hauptanspru ^¬ ches by the features of the respective dependent claim; they should not be construed as a waiver of obtaining independent, objective protection for the feature combinations of the dependent claims. Furthermore, with a view to an interpretation of the claims in a closer specification of a feature in a subordinate claim, it is to be assumed that such a restriction does not exist in the respective preceding claims.

The advantage of one or more embodiments of the invention described below is that in addition to the energy and data transmission over a two-wire line and in addition to the boundary conditions already mentioned above, the fulfillment of one or more of the following boundary conditions is possible:

The transmission system and the field devices provided and designed for use in such a transmission system allow the fulfillment of all relevant EMC standards for the respective field devices.

The transmission system is generally eligible for certification according to EN 62061 (Functional Safety, Safety Integrity Level, SIL).

The transmission system and it included devices or units, so at least the central unit and ^¬ least one field device, suitable to meet the requirements for receiving the explosion protection Ex-i, so that a relevant certification can be considered.

The transmission system allows the simple connection (plug & play) of other field devices as communication participants by avoiding false line terminations and / or polarity errors, especially in special embodiments.

The lowest possible power requirement of the field devices is not higher than the standard 4 to 20mA standard communication technology used today. The transmission system with standard processors, in particular UART (see above), without proprietary Zusatzbau ^¬ stones, such. B. special encoder / decoder, realized. By the central unit in a DC power source with two independent DC sources one of the DC sources as activatable DC source or a controlled DC power ^source , the controlled DC ^source as ge ^¬ controlled activatable DC power source and means for controlled activation of the activatable DC power source summarizes can be with each activation of the activatable DC power source of the two-wire line information, so an information in the form of a 1-bit data signal memorize. For simpler conditions - without renouncing the broader general validity - hereinafter goes ^¬ gone out that the central unit has the DC power source in the form of two independent DC power sources - first and second DC power source - and that one of the DC power sources, eg. B. the second DC power source, which is ^¬ controlled activatable DC power ^source . Activation of the activatable DC power source can, for. B. consist in that the activatable DC power ^source briefly short ^¬ closed. The two-wire line impressed by the DC power source bus voltage results in this

Situation only because of the first, not influenced by the activation DC power source. The change in the impressed bus voltage or the impressed bus voltage which arises as a result of the change represents the information transmitted in the communication system. Once the activation, ie z. B. the short circuit is canceled again, the bus voltage impressed on the two-wire line by the DC power source again results due to the ers ^¬ th and the second DC power source.

The second DC power source can also be called and understood as a communication source against this background. The communication source is z. Short-circuited, in particular sondere to be transmitted by activation of a dedicated switching element when a technique called SPACE UART characters (ent ^¬ speaks logic "0"), ie, if the communication block a corresponding output signal, in particular transmit signal (TX) emits.

In an embodiment of the field device, it may be provided that it comprises a terminating resistor and means for the controlled activation of the terminating resistor. Such a terminator is z. B. on end-to-end commu- nication links switched. As a means for the controlled activation of the termination resistor ^¬ a processing unit comes to the type of a microcontroller or a micro controller into account. The processing unit may comprise the communication module.

By the field device with its first and second DC sink, the second DC sink as activatable

Current sink and means for controlled activation of the second DC sink, a field device of the two ^¬ wire line in addition to the current drawn from the first DC power source for power supply of the field device targeted additional power draw. This additional current drain affects the two-wire line embossed bus voltage and thus is one of the two-wire line embossed in ^¬ formation, that is, a information in the form of a 1-bit data signal. The additional current draw by controllably activating the second DC current sink especially comes into consideration, if by the field device is to be transmitted the UART character designated SPACE (see above), ie if the communication module of the respective field device outputs a corresponding output signal, in particular a transmission signal (TX). To activate the second DC sink is a control of a designated switching element into consideration, in a parallel circuit of first and second DC sink so z. B. a switched in series with the second DC sink switching element, the Parallel branch with the second DC sink either akti ^¬ fourth or disabled.

If in the transmission system, the central unit and / or the or each field device comprises or have a signal conditioning unit and a decision unit, wherein the decision unit is upstream of the respective communication module and from a supplied signal based on a decision logic implemented in the decision logic and a predetermined or predeterminable decision threshold converts the supplied signal into a binary ^¬ signal and wherein the signal conditioning unit of the decision unit is preceded and causes a processing of the decision unit supplied signal, a splitting of the current transmitted via the two-wire line into a part for supplying power to the or each field device and one transmitted data possible part. Based on a decision unit feed ^¬ cash size of the decision threshold you can specify when a respective signal value is evaluated as a logical "0" or logic "1". The decision unit thus makes a 0/1 decision for the generation of the binary signal for the respectively received signal. The term 0/1 decision will also be used below.

The combination of signal conditioning unit and decision-making ^¬ unit will be referred to as a receiver. The receiver is connected upstream in a central unit or a field device to the local communication module. The recipient effects an analysis of the two-wire line via the respective received signal and its treatment, so that an output signal of the receiver by the respective Kommu ^¬ nikationsbaustein is immediately processable. As signal conditioning unit comes z. As a high pass filter into consideration. However, because the output of the communi ^¬ cation block one of the two-wire line impressed by the DC bus voltage source directly, without spe- is encoded, the signal comprises an information-dependent DC component. If such a signal with a high pass in a known per se filtered, created at the entrance of a covered by the high-pass filter comparator initially an exponentially extending a ^¬ oscillating operation. Due to the low-frequency components, the signal also fluctuates sharply by a fixed value.

Since the information-dependent DC component is determined by the respective data pattern, there is a normalized and idle-mode average signal of a UART character with a start character, eight data characters, one parity character and one stop character, in total eleven characters, depending on Data content and parity mode between 9% ("one of eleven") and 91% ("ten of eleven"). Therefore, as an alternative to high-pass filtering or any other type of filtering, the following approach may be considered: In the data transmission over the two-wire line, a group of e.g. B. summarized eight bits of data and the combined data bits, ie z. As a data byte, are transferred to ^¬ next in its original form and then inverted (or vice versa) to the communication block. This approach results in a fixed or largely fixed signal average. This is an averaging not at bit level, but based on the combined data bits, so z. B. at the byte level. This averaging works without additional hardware components or the like. For the decision unit can be defined in ^ for the 0/1-decision with the decision ^¬ threshold.

If, during operation of the transmission system on the part of the central unit based on the respective output signal of the communication module, a compensation voltage is determined, which is impressed on the two-wire line in addition to the bus voltage generated by the DC power source, and / or on the part of the field device based on the field device via the two-wire line due to impressed bus voltage, a compensation current for additional When the first DC sink is triggered, the terminating resistor fault that is unavoidable with unipolar signals can be compensated. The object mentioned above is achieved by a central processing unit for a single central unit which can be used in a transmission system as described here and below, having means making it suitable for use in the transmission system or in a transmission system according to one or more of the further embodiments described herein. As a means for rendering a central processing unit in a here be ^¬ signed transmission system used, are in particular: A DC power source, either in the form of two independent DC power sources - first and second DC source - or in the form of a controlled DC power source, and a communications module - first communication module ( UART) -, wherein the central unit comprises the ge ^¬ controlled DC power source at a DC power source having two independent DC power supplies one of the DC sources as activatable direct current source or a controlled DC power source as a controlled activatable DC ^¬ current source and means for controllably activating the acti ^{¬-activatable} DC power source. The same applies to a usable in a transmission system of the type described here ^¬ field device. As means of such a field device, which make this usable in such a transmission system, are in particular: At least a first and a second DC sink and a communication module - second communication module -, wherein the field device, the second DC sink as an activatable current sink and means for the controlled activation of second DC sink includes. For the controlled activation of the activatable DC power source and the second direct-current sink is a proces ^¬ processing unit of a microcontroller in the manner contemplated. The means comprising a central processing unit or a field device in one make it usable for implementation as software or firmware, so that the invention in as far as a computer program with executable by a computer program code instructions and on the other hand, a storage medium with such a computer program and finally also a central processing unit or a field device on or on which such a computer program is loaded or in whose memory such a computer program is loadable.

An embodiment of the invention will be explained in more detail with reference to the drawing. Corresponding objects or elements are provided in all figures with the same reference numerals.

The or each embodiment is not to be understood as limita ^¬ effect of the invention. Rather, numerous variations and modifications are possible within the scope of the present disclosure, especially such variants and combination ^¬ nations, the z. B. by combination or modification of individual in conjunction with the described in the general or specific part of the description and included in the claims and / or the drawing features or elements or method steps for the skilled in the art with a view to solving the problem and combinable features lead to a new subject or to new process steps or process step ^sequences .

The drawings show a transmission system according to the prior art with a four-wire line as the communication medium and for the power supply of connected field devices, a transmission system according to the prior art with a two-wire line as the communication medium and for the power supply of connected field devices. an embodiment of a transmission system according to the invention with a two-wire line as a communication medium and power supply connected field devices, a graph comparing a superposition of data transmitted in the transmission system in the context of data transmission and a provided for power supply field devices DC voltage, a high and low pass filter as an example Means for separating a signal with transmitted data provided by the power supply for connected field devices DC voltage, and

a signal waveform for received data in a high-pass filtering of a signal with direct current component, a schematically simplified block diagram of a provided in a central unit and / or a field device of the transmission system according to FIG 3 receiver, a comparison of several data patterns to illustrate an information-dependent DC component, a dependency detected pulse lengths of a decision threshold in a decision unit of the receiver, a representation for clarification of the aspect of the unlike in the prior art not on bit level, but executed at the byte level mean ^¬ compensation, FIG. 12 and FIG

13 shows two circuits for implementing a current sink and

FIG. 14 and FIG

FIG 15 two circuits showing an embodiment of a current sink with a Arbeitspunktstabili ^¬ tion by means of compensation.

FIG. 1 shows a transmission system 10 with a central unit 12 and a field device 14. The transmission system 10 comprises a four-wire connection line as the line 16 for energy and data transmission, which connects the central unit 12 and the field device 14.

The illustrated transmission system 10 is a system as known under designations such as PROFIBUS DP or DeviceNet. Such transmission systems 10 are characterized by a scalability in terms of data rate and line length, since no speed-dependent blocks are necessary. Between a communication component UART which can be parameterized in its baud rate and the line 16, there is only one speed limit within limits

RS485 transceiver arranged. This results in the system structure shown in FIG. It is characterized by protocol and speed transparency and cost-effective implementation due to the use of a UART included in most micro-controllers (μθ) and an RS485 transceiver.

In contrast, all other physical designs require additional components. FIG 2 shows in so far a transmission system 10 with a central processing unit 12 and a field device 14. The transmission system 10 comprises as line 16 for energy and data transmission a two-wire ^¬ line. The CODEC (coder / decoder) and MAU (Medium Attachment Unit) modules shown in FIG. 2 considerably increase the cost of an interface. In addition, all two-wire solutions have in common the property that on the transmitting side the data present as an NRZ signal with an information-dependent DC component is coded in such a way that a DC-free signal is produced, which is superimposed on a DC voltage serving for the energy supply. On the reception ^¬ side of this AC signal is split off and decoded by a high pass. Only then is there again an NRZ signal, which z. B. can be processed by a UART. Scalability over long distances and / or high data rates is achieved by using a line ^{termination resistor} 18. In the case of a two-wire solution with

Energy transmission is instead of the line termination resistor 18, however, an RC series circuit necessary so that the DC power supply is not consumed by the line ^{terminator ¬} final resistance 18 and on the other hand, the line 16 is completed with its wave resistance for the data AC voltages.

3 shows an embodiment of a transmission system 10 according to the invention. The transmission system 10 comprises a central processing unit 12 and at least one field device 14 assigned to an automation system or automation device, not shown. Four field devices 14 are shown here. The central unit 12 and the field devices 14 are connected via a two-wire line as line 16 for power and data transmission. In that regard, the structure of the transmission system 10 is similar to the transmission system 10 shown in FIG. 2. For a transmission system 10 of the type illustrated in FIG. 3, ie for transmission systems 10 with a two-wire line as the communication medium and for the power supply of the connected field devices 14, the terms "line" will be used 16 and two-wire line 16 used synonymously. The field devices 14 are supplied from the CPU 12 with Ener ^¬ energy and can communicate therewith. This communication takes place, as in the known four-wire systems (cf., FIG. 1), via a communications module. In the illustrated embodiment, is of a nearly in each microcontroller (μθ) integrated UART so-called assumed as a communication module ^¬ (Universal Asynchronous Receiver Transmitter). By suitable design of the transmitter and receiver, that is central unit 12 and field device 14, in the transmission system 10 in FIG. 3, however, only one two-core cable is required as line 16.

In the transmission system 10 shown in FIG. 3, a constant supply voltage UK is applied to the line 16 via the central unit 12. Each field device 14 draws a constant current IK for its power supply and generates a voltage UV for supplying the electronics and the connected sensor S or actuator. When sending the field devices 14 take an additional power IT.

The central unit has at least one port PI, P2 ... PN. Each port PI, P2, PN may be or may be connected to one or more field devices 14. The number of ports PI, P2, PN and the number of connected or connectable field devices 14 depends on the respective boundary conditions, such. B. Energy requirements, required data rate, cable length, etc.

For the power supply of the field devices 14, the central unit 12 comprises two voltage sources, namely a first voltage source UK and a second voltage source UT. The first voltage source UK supplies the supply voltage for the power supply of connected field devices 14 and a value of the supply voltage UK can either be controlled or fixed via the microcontroller (μθ). The second clamping ^¬ voltage source UT can also be regarded as a communication source and referred to, and is short-circuited by the microcontroller when a UART character "SPACE" (corresponding lo- gisch "0") is to be transmitted. Then the voltage applied to the field devices 14 voltage UB decreases accordingly. The first and the second voltage source UK, UT can be combined as an alternative to a correspondingly controlled voltage source (not shown).

Between the first and second voltage source UK, UT on the one hand and the ports PI, P2, PN on the other hand, a measuring ^resistor RM and for each port PI, P2, PN each have a limiting resistor R are provided. Each limiting resistor R assumes a plurality of functions, namely first a one-sided termination of the line 16, then a limit current in use in a hazardous area, and finally ensuring suitability of the over- tragungssystems 10 and / or the central unit 12 to the fulfillment ^¬ development of the conditions referred to the so-called Fieldbus Intrinsically Safe Concept (FISCO) and / or for the preservation of explosion protection Ex-i. Further causes of Be ^¬ limiting resistor R, a generation of a voltage signal from the current signal in the event that, in a

Transmission system 10, the field devices 14 must communicate with each other, as shown in the illustration in FIG 3 for the connected to the port PN two field devices 14. In individual cases, in which the illustrated aspects play no role, the limiting resistance R can ent ^¬ fall.

By means of a measuring unit MI can be measured via the measuring resistor RM, both the static supply current IK, which is taken from the connected field devices 14, as well as the transmission current IT, which is additionally taken from a currently transmitting field device 14. The separation of the dynamic part of the transferred via the line 16 stream from the static part takes place in a recipient ger E. There, the digital Emp ^¬ catch signal (AC signal; RX) is obtained from the dynamic part is extracted, which is supplied to the communica ^¬ tion module UART becomes. Alternatively, the central unit 12 may also contain a combination of DC / DC voltage source UK, limiting ^resistor R and a transmitting / receiving unit / current sink, as described below as part of a field device 14. A separate measuring resistor RM is omitted. The current sink IT replaces the second clamping voltage source ^¬ UT.

The field devices 14 contain constant current sinks, which each remove the current IK from the line 16. By a current-voltage converter (I / U;., For example, a zener diode) is generated Ver ^¬ supply voltage UV for the field device 14.. A second current source draws an additional current IT if a UART character "SPACE" (corresponds to logical "0") is to be transmitted. The necessary connection lines are a receiving line RX and a transmission line TX. The illustrated further lines are optional and can be used for automatic parameterization of the interface by the microcontroller (μθ).

In the illustrated embodiment mean:

BT: Bus Terminator, for connecting a terminating resistor (eg optionally switchable for end-to-end routes)

IK: Constant current, for setting the size of the constant current (eg changeable, if the field device 14 temporarily has an increased power requirement)

UB: bus voltage, to determine the voltage applied to the field device 14 (eg for diagnostics or parameterization).

Further auxiliary signals are possible.

In contrast to a transmission system 10 as shown in Figure 2, wherein the voltage applied to the power supply DC voltage an encoded, DC-free signal is superimposed over ^¬, in which in FIG Übertra ^¬ supply system 10 provided for the power supply DC voltage shown in Figure 3 a the respective output signal of communication tion blocks (UART output signal, TX) correspond directly ^¬ the signal superimposed without further coding. The case of four-wire systems ^¬ (FIG 1) possible, inexpensive transmission ^¬ principle of NRZ-signals is therefore transmitted to a transmission system 10 with a line 16 with only one pair of wires for power and data transmission. The direct overlay costs are saved because z. B. components such as an encoder / decoder, which were required in a transmission system according to FIG 2, omitted.

The illustration in FIG. 4 shows, for clarification in the upper part designated by a), a superimposition of a DC voltage provided for the purpose of energy supply with a DC-free data signal, as results in a transmission system 10 according to FIG. A direct superposition of a provided for the power supply DC voltage below is shown with a UART output signal for discriminating in a designated b) part and an overlay ^¬ tion as shown in FIG arises in a transmission system 10. 3

In the case of a superimposed signal as shown in the upper part a) of the illustration in FIG. 4, a separation of the transmitted data - hereinafter referred to as signal (AC signal) - from the supply voltage on the one hand and the signal detection on the other hand with a known high-pass filter or a likewise known per se low-pass filter. Circuit examples for a simple embodiment ^¬ form of a high and low pass filter, each with a

Resistor R, a capacitor C and a comparator K are shown in FIG. 5 (left the low-pass filter, right the high-pass filter). The high or low pass filter acts as a kind of "conventional" signal or 0/1 decision maker. In the embodiment of the transmission system 10 with a more ^¬ ter superposition of the energy supply is provided for

DC voltage with the UART output signal, however, contains the transmitted data - ie according to the above terminology the signal - an information-dependent DC component. If such a signal is filtered with a high-pass filter (FIG. 5, right), an exponential transient process initially occurs at the input of the comparator K. In addition, the signal fluctuates greatly because of the low-frequency components by a fixed value. This is (9-bit pattern) shown in FIG 6 the example ei ^¬ nes quasi-random signal. The effects of such high-pass filtering can be presented even more clearly with a fixed, repeating bit pattern. In the pattern "1000 0000" shown in FIG. 7, the signal 20 after the high-pass filtering is shifted significantly upwards relative to the mean value 22, but clearly downwards in the case of the inverse pattern 24. For the transmission system 10, it is therefore provided in a particular embodiment that the receiver E (FIG. 3) of a central unit 12 and / or of a field device 14 comprises a signal conditioning unit AB and a decision unit EE, as shown in FIG. EE the communication module UART is connected upstream, wherein by means of the decision unit EE an input and supplied in operation AC signal S is ^¬ based on a decision implemented in the decision unit EE decision logic and based on a predetermined or specifiable decision threshold UV in a binary signal BS ^¬ wherein the signal conditioning unit AB of the decision unit EE is connected upstream and a Aufberei ^¬ tion of the decision unit EE signal supplied be ^¬ acts.

For some data patterns and the parity modes "EVEN" and "ODD", the relationships in FIG. 9 are shown. In the examples it is assumed that the UART output signal consists of a start character ST, eight data characters DO... D7, one parity character P and one stop character SP, thus a total of eleven characters. Depending on the data content and parity mode is an in an idle state (Idle state) of the Übertra ^¬ supply system 10 related, normalized signal averaging, so an information-dependent DC component, between 9% (1/11) and 91% (10/11).

FIG. 10 shows a dependence of detected pulse lengths on a decision threshold in the decision unit EE for different signal mean values MW. Therefore EE for the decision unit is provided that there implemen ^¬ oriented decision logic a predetermined or predeterminable threshold decision UV is fed. The decision threshold UV can be suitably adjusted depending on different data and boundary conditions.

A particular embodiment is characterized in that it can be assumed that a fixed decision threshold UV. For this, a mean value compensation is initially provided and 9 showed the dependency of a signal ^¬ average MW from the respective data contents. However, it was recognized that an averaging can be achieved in a simple manner, when in the transmission of data over the two-wire line 16, a group of z. B. eight data bits combined into one byte of data and the data ^¬ byte first in its original form and then inver ^¬ tiert (or vice versa) to the communication module (UART) is passed. FIG 11 shows as an example the then resulting conditions. Regardless of the information ^¬ content of the respective data bytes (byte 1, byte 2) is always obtained, along with the inverted data byte a signal average in the range of "0.5". The signal mean values MW resulting for the concretely represented situations are "12/22" (= "6/11") and "10/22" (= "5/11"). The middle ^¬ worth balancing is therefore performed not at the bit level, but at the byte level ^¬. Since the parity bit in the UART is generated as a function of the parity of the data byte (or possibly also of the data word) and the parity does not change for an even number of combined bits due to inversion, the mean value still fluctuates between the above-mentioned values 5/11 and 6/11, depending on the distribution of the EVEN and ODD bytes. When setting the decision threshold UV to 0.5 the remaining systematic error is limited to a maximum of ± 4.5%, whereby the resulting pulse length distortion and the loss of signal-to-noise ratio can be negligible. However, it must be taken into account that if the high-pass frequency is selected too high, the signal distortions due to pulse flattening ("sloping roof") will worsen the result again if there is no signal change. However, a low cutoff frequency causes a long transient process (see FIG. 6), so that the use of an adaptive decision threshold UV is expedient.

Such a mean value compensation, as shown by way of example in FIG. 11, avoids, unlike, for example, FIG. As a so-called Man ^¬ chester encoding, otherwise necessary, additional hardware components. In Manchester coding, a fixed average is generated by inversely repeating the data content of one bit within a bit period. This link is made in a so-called Manchester encoder. Although it is also conceivable to process coding and decoding with a corresponding computer program in the microcontroller, this would then have to be able to perform this task in parallel to the actual application, which usually requires a higher-performance microcontroller and a higher power requirement per se draws.

3 shows the output current limit for each strand by a respective limiting resistor R. The current limitation by means of the limiting resistor R is especially required for operation in potentially explosive areas.

Each limiting resistor R also serves as a line terminating resistor. In an interpretation of Com ^¬ munikationssystems 10 for suitability for explosive atmospheres ^¬ the operating parameters are limited to UDCBUS <17.5V and IDCBUS <200mA. The maximum transmission bandwidth should be 2 MBit to 6 MBit and the own energy demand of the communication electronics in a field device 14 should be so small that previous field devices 14 with analogous 4 to 2 OmA interface can only Job adaptation can continue to be used with 4mA operating current in this new digital communications system 10. This results in the following requirement profile for the communication hardware in a field device 14:

Maximum operating voltage 17,5V

Maximum current 200mA

"Output Swing @ 3MHz" lVpp

Own electricity consumption 550μΑ *

* 4mA - 3,55mA = 550μΑ for all functions of the communication interface, not only for the generation of communica ^¬ tion flow

A search in the amplifier offer of the large offerers of

Semiconductor devices quickly shows that the market does not offer components with the required profile. Even with USAGE ^¬ dung an additional power component, transistor, or FET, the market supply fails again and the current requirement for a "slew rate" of 12ν / μ3 and a "Output Swing" of LVPP at 3MHz. The best approach is provided by a few operational amplifiers. A discrete circuit of single standard small signal transistors and a power transistor could meet these requirements. Their -3dB bandwidth is about 1.5Mhz and they are suitable up to 60V, 1A. 12 shows the simplest conceivable controlled current sink for use as current sink IK and second current sink IT in a field device 14 (FIG. 3) according to this design principle. The embodiment of a current sink shown in FIG. 12 is known.

An increase of the controlling input voltage UE leads to increasing collector current of the small signal transistor TRK and the power transistor TRL. The voltage at the current measuring resistor Rmess gets more negative regarding GND. The more negative voltage across the feedback resistor RR counteracts the A ^¬ input voltage UE, so that the base-emitter voltage remains constant UBE and the through current IK + iT + ikomp proportional to the input voltage UE in accordance with the gain ^¬ factor IK + iT + ikomp = RR / RE / Rmess * UE increases. By means of

Zener diode, the operating voltage UV is generated.

The transistors TRL, TRK and the Zener diode may also be components of other types, e.g. B. FETs or reference diodes or Vierpolschaltungen. In the general case, the base-emitter voltage U BE is to be understood as any desired operating-point voltage. The operating point voltage of transistors and FETs, however, depends on the one hand on an ambient temperature and on the other hand on the respective copy strongly. Therefore, as a supplement to the circuit in FIG. 12, a stabilizing amplifier may also be considered, as shown in FIG. The embodiment of a current sink with stabilizing amplifier shown in FIG. 13 is also known.

However, the power requirement of such, for stabilization purposes before seen amplifier high bandwidth prevents the fulfillment of the requirements profile.

The ideal transfer function

the current sink shown in FIG 13 shows the strong dependence of the penetrating current i of the temperature-dependent working ^¬ point voltage UBE.

If the circuit is extended by an additional resistance RE2 at the summation point, the transfer function is

and shows how the influence of the working point voltage ÜBE can be eliminated: with the dimensioning

the transfer function becomes independent of the operating point voltage UBE and thus independent of temperature and copy:

1 R R

U

R R

measure E

Since the bias voltage UBE is a DC voltage, the compensation voltage can Ukomp as shown in the overall ^¬ circuit in Fig 14 by means of a "micropower" operational amplifier smaller bandwidth, are generated.

The relationship on the basis of which a dimensioning of the resistors can take place is

This circuit is functional, but does not fully meet the requirements profile set up at the beginning. The fairly high base current of the power transistor TRL flows via TRK and the sensor electronics are lost. The TRK input impedance is low and would require a very low-current current-drooping sizing of RR, RE and RE2. The expansion of the circuit of FIG 14 by two emitter followers TR2K, TR3K, Rl, R2 and R3 eliminated These problems and the resulting circuit is shown in FIG.

The circuit has a high input resistance and the own power demand not provided by the sensor electronics is approx. 120μΑ at 3MHz bandwidth. By

suitable dimensioning of Rl, R2, R3 are possible with increasing power demand even higher bandwidths. The reduction of the bandwidth reduces the power requirement. The material cost of a current sink without Zener diode, as shown in FIG 15, are negligible and amounted at current component ^¬ only cost about EUR 0.16. The current sink shown in FIG. 15 is thus outstanding as a controlled current sink IK

Use in a field device 14 suitable.

The series connection of the voltage-controlled voltage source (operational amplifier with Ukomp = V * Urbeitspunkt) and at the summation point connected RE2 can be replaced by a voltage-controlled current source

Ikomp = VI * UApoint,

whose output current is fed directly into the summation point.

The compensation method described above is effective for each operating point to be stabilized, irrespective of the nature of the components used or four poles instead of the active building parts ^¬.

The circuit in the illustrations in Figures 14 and 15 is an example of a current sink with a working ^¬ stabilizer compensation.

Individual standing in the foreground aspects of here is rich ^¬ th description can thus be briefly summarized as follows: As defined on the basis of the claims

Invention and an embodiment according to FIG. 3 of a transmission system 10, the approach presented here combines the advantages of previous two and four-wire solutions, which are not combinable without further ado. The approach enables the transparent communication between UARTs and the power supply of field devices 14 via a two-wire connection 16 with only a small amount of hardware.

As a result, the costs of a two-wire communication ^¬ interface over, for example, a PROFIBUS PA interface can be reduced by orders of magnitude and are below the level of an RS485 interface. This savings the ability to greatly increased transmission rates of PROFIBUS PA is the same at 31.25 kBaud to min ^¬ least 6 Mbps over. In addition, despite the improvement in the order of magnitude of these two essential parameters, the internal energy requirement of the communication electronics drops and makes field devices 14 with an operating current of 4 mA instead of at least 10 mA (practically 12 mA) possible with PROFIBUS PA. The number of a central unit 12 as a master to ^¬ closable subscriber (field devices 14) for applications in the hazardous area increases per strand (see FIG 3) by a factor of three. Theoretically, the number of strands is unlimited, technically still easy to control are three strands per master.

The much higher transmission bandwidth can, for. This can be used, for example, for software services that are not possible with any of the current two-wire solutions due to a lack of bandwidth.

The solutions described can be applied to both process field devices, as well as sensors and actuators in the manufacturing ^¬ automation. By using a corresponding 2-wire / 4-wire adapter 2-wire devices ^(¬ PROFI bus PA, DeviceNet, IO-Link, etc.) can be integrated into a conventional network with RS485 interface.

Claims

claims

1. Transmission system with at least one central unit (12) and at least one field device (14),

wherein a connection to the power supply of the field device (14) and a communicative connection from and to the field device (14) simultaneously via a two-wire line (16),

marked by

a central unit (12) at least with:

a DC power source (UK, UT), either in the form of two independent DC power sources - first and second DC power sources (UK, UT) - or in the form of a controlled DC power source, and a communications device - first communications device (UART) -;

at least one field device (14) at least with:

a first and a second DC sink (IK, IT) and a communication block - second communication block (UART) -;

a direct superposition of an output signal (TX) of the

Communication module (UART) and one of the two-wire line (16) impressed by the DC power bus voltage, wherein the two-wire line (16) to the DC power source (UK, UT) of the central unit (12) and to the first and second DC sink (IK, IT) of the field device (14) is connected.

2. Transmission system according to claim 1, wherein the central unit (12) at a DC power source (UK, UT) with two independent DC sources (UK, UT) one of the DC sources (UT) as an activatable DC power source or in a controlled DC power source, the controlled DC ^¬ power source as controlled activatable DC power source and means for the controlled activation of the activatable DC power source comprises.

3. Transmission system according to claim 1 or 2, wherein the central unit (12) has a plurality of ports (PI, P2 ... PN) and wherein the central unit (12) comprises a limiting resistor (R) for each port (PI, P2, PN).

4. Transmission system according to claim 1, 2 or 3, wherein the central unit (12) comprises a measuring unit (MI) and a measuring ^¬ resistance (RM), wherein by means of the measuring unit (MI) above the measuring resistor (RM) through the first and / or second DC sink (IK, IT) of the line (16) ent ^¬ taken current is measurable.

A transmission system according to any one of claims 1, 2, 3 or 4, wherein the field device (14) comprises a terminating resistor and means for controlled activation of the terminating resistor.

6. Transmission system according to one of claims 1, 2, 3, 4 or 5, wherein the field device (14) comprises the second DC sink (IT) as an activatable current sink and means for the controlled activation of the second DC sink (IT).

7. Transmission system according to one of the preceding claims, comprising a signal conditioning unit (AB) and a decision unit (EE),

wherein the decision unit (EE) precedes the communication component (UART),

wherein by means of the decision unit (EE), a feed ^¬ bares and supplied in the operation signal on the basis of a in the decision unit (EE) implemented decision logic and on the basis of a predetermined or predeterminable threshold decision (UV) into a binary signal and is convertible

wherein the signal conditioning unit (AB) of the decision ^¬ tion unit (EE) is connected upstream and causes a processing of the decision unit (EE) signal supplied.

8. Transmission system according to one of the preceding claims, with a current sink with a Arbeitspunktstabili ^¬ tion by means of compensation.

9. Central unit for use in a transmission ^¬ system (10) according to one of claims 1 to 8, with a

DC source (UK, UT), either in the form of two independent DC sources - first and second DC source (UK, UT) - or in the form of a controlled DC source, and a communication module - first

Communication module (UART) -, wherein the central unit (12) at a direct current source (UK, UT) with two inde ^¬ dependent direct current sources (UK, UT) is one of direct current sources (UT) as activatable direct current source or a controlled DC power source, the controlled DC ^¬ power source as a controllable activated DC power source and means for controlled activation of the activatable DC power source comprises.

10. Field device for use in a transmission system (10) according to one of claims 1 to 8, with at least a first and a second DC sink (IK, IT) and a communication module - second communication module

(UART) -, wherein the field device (14) comprises the second DC ^¬ sink (IT) as an activatable current sink and means for ge ^¬ controlled activation of the second DC sink (IT).

11. A method for operating a transmission system (10) according to any one of claims 1 to 8 or method for operating a central processing unit (12) according to claim 9 in such a transmission system (10), wherein an output signal of a

Communication module (UART) is superimposed on the central unit (12) of the two-wire line (16) impressed DC voltage directly by in response to the output of the communication module (UART) which can be activated DC ^¬ source is activated, in particular short-circuited.

12. A method for operating a transmission system (10) according to one of claims 1 to 8 or a method for operating a field device (14) according to claim 10 in such a transmission system (10), wherein an output signal of a Communication module (UART) of the field device (14) of the two-wire line (16) impressed DC voltage is superimposed directly by the second DC sink (IT) is activated in response to the output signal of the communication module (UART).

13. A method for operating a transmission system (10) according to any one of claims 1 to 8 or the method of claim 11 or 12, wherein in the transmission of data over the two-wire line (16) summarized a group of eight data bits to a data byte and the data byte first in its original form and is then passed to the inverted Kommu ^¬ nikationsbaustein (UART).