DE102013220155A1 - Uncoupling unit for two-conductor field bus for reading bus telegram in e.g. bus monitor, has signal processing unit processing partial signals, where bus telegram is output based on signal processing unit when partial signals are evaluated - Google Patents

Abstract

The unit has determined frequency channels attached to individual bus participants, which serve for transmission of bus signals. A signal level of the bus signals is varied along two-conductor bus line based on missing line terminals by reflections of the bus signals. Two separate directional couplers (R0, R1) output the bus signals as direction-specific and/or conductor-specific partial signals. A signal processing unit (S) processes the partial signals and a bus telegram is output based on the signal processing unit when the partial signals are evaluated.

Description

The invention relates to a decoupling unit for a two-wire fieldbus for reading bus telegrams.

Two-wire fieldbuses are frequently used in automation technology for communication between several bus users (central controllers PLC, sensors, actuators). Both the communication as well as the power supply of the slaves is carried out via a two-wire fieldbus cable.

Many fieldbus systems often work according to the master slave principle (eg Profibus), whereby bus telegrams (data packets) are exchanged between the master and the individual slaves. In addition to slaves as communicative bus users, bus monitors are also used, which act as purely passive bus participants, because they only read the bus telegrams as "listeners" and do not generate any response telegrams. Bus monitors are mainly used for general bus monitoring or as a safety device to switch off system components in critical situations.

The number of slaves is due to various restrictions such. B. the bus specifications or bus physics limited to a few bus subscribers (Z. B. 32 or 64).

In conventional fieldbus systems, the slaves are galvanically connected to the two conductors of the two-wire bus cable via a bus interface.

On the one hand, the bus interface serves as a decoupling unit for reading bus telegrams. On the other hand, it also serves to write bus telegrams. Bus monitors only need the bus interface in the function as decoupling unit.

When reading the bus telegrams, the bus interface converts the analog bus signals that are transmitted on the bus cable into digital data.

As a rule, the bus interface is followed by a communication controller, which is connected e.g. contains the bus-specific protocol stack in which the bus telegrams are evaluated and their telegram content is extracted. The corresponding information can e.g. be further processed in a downstream microprocessor.

For various bus systems bus interfaces and communication controllers are commercially available, such. B. the SIM1 block and the SPC4 communication controller from. Siemens.

In order to increase the number of slaves and data throughput, fieldbuses can also use multi-frequency data transmission techniques.

The master-slave communication then takes place on different data channels, wherein the channel assignment can also be flexible.

From the DE102004001474 a fieldbus system is known which employs a second bus communication technique according to the DSL standard in addition to a conventional first bus communication technique. Both techniques use the same two-wire fieldbus cable as the transmission medium.

In such data transmission techniques, the following problems can not be ruled out.

Due to missing terminations, the high-frequency bus signals (eg in the range of 1-10 MHz) of individual bus subscribers can be canceled out by reflections in spatially extended bus segments (up to, for example, 200 m), in particular in bus topologies with a tree structure. This means that there may be specific locations on the bus line at which the signal level of the bus signals of a single or even several bus users is so small that the bus signals can no longer be reliably read or evaluated with a conventional bus interface.

A bus subscriber connected to such a location would no longer be able to read all bus telegrams.

The object of the invention is therefore to provide a decoupling unit for a two-wire fieldbus for reading bus telegrams, which allows the reading of all relevant data packets on the bus, regardless of the location of the decoupling point.

This object is achieved by the features specified in claim 1.

The essential idea of the invention is to use two directional couplers in a decoupling unit for a two-wire fieldbus for reading bus telegrams in order to be able to decouple the bus signals in a direction-dependent or line-specific manner and to evaluate them accordingly.

Advantageous developments of the invention are specified in the subclaims.

The invention is explained in more detail with reference to an embodiment shown in the drawing.

Show it:

1 schematic representation of a decoupling unit according to the invention with two separate directional couplers

2 schematic representation of a decoupling unit according to the invention with a symmetrical directional coupler

3 schematic representation of a decoupling unit according to the invention with two unbalanced directional couplers

1 schematically shows a decoupling unit A according to the invention, which is galvanically connected to a two-wire fieldbus. The two-wire fieldbus is shown schematically by the two conductors L1 and L2. With the fieldbus (not shown in the drawing) typically still a master and other slaves are connected.

Directional couplers R0 and R1 are connected to the two conductors L1 and L2. At the inputs A0, A1, A2, A3 of a signal processing unit S, the direction-specific and line-specific partial signals, which are obtained from the actual bus signal, are applied in each case. These are combined in pairs in a linking unit V0 or V1, amplified and checked according to the bus specifications, filtered and evaluated and then decoded into two decoding units D0 and D1. In a merge unit Z, the decoded data is merged and z. B. provided as a digital bus telegram for further processing. The further processing of the bus telegram can be carried out in accordance with known conventional slaves, bus monitors or repeaters.

In the illustrated simpler variant, the analog bus signals are combined before they are decoded. This only requires two decoder units. Under favorable signal conditions, both decoder units provide identical bus telegrams, of which only one is needed for further processing. This selection takes place in the assembly unit.

In a more complex variant, not shown, all four bus signals applied to the inputs A0-A3 are processed separately. Therefore, the two linking units V0, V1 can be omitted. In the case of unfavorable signal conditions, at least one of the four decoding units required in this case should output a valid bus telegram.

Under favorable conditions, all four decoder units also output matching bus telegrams.

The signal propagation unit functions more or less like two or four known bus interfaces (bus ICs) arranged in parallel.

2 shows a schematic representation of a decoupling unit according to the invention with a symmetrical directional coupler. The balanced coupler operates as a symmetrical coupler with contacted on the two conductors L1 and L2 capacitances. Here, a 50 ohm adaptation to the two inputs A0 and A2 is required. This variant is particularly suitable for slaves and repeaters fed via the bus. The necessary chokes must be connected to opposite points on the bus line.

3 shows a schematic representation of another decoupling unit according to the invention with two asymmetrical directional coupler. The unbalanced coupler (Bruene bridge) has no galvanic connection to the two-wire fieldbus. This variant is particularly suitable for bus monitors, as they are normally not bus-powered.

By means of the directional coupler R0 or R1, the bus signals can be coupled out direction-specific or line-specific as partial signals. The directional coupler can separate the bus signals according to their propagation direction. In the event of cancellation, the bus coupler supplies two correspondingly separate partial signals.

The sub-signals can be separated in the signal processing unit S or partially also linked further processed.

In principle, only one evaluable signal must be present so that the signal processing unit S can output a digital bus telegram for further processing. This evaluable signal can z. B. be one of the sub-signals.

The decoupling units according to the invention have a multiplicity of advantages. By appropriate evaluation in the signal processing unit S, the propagation direction of the associated bus signals can also be determined for individual bus telegrams.

In addition, the position of other bus subscribers, such. B. other slaves are determined relative to the decoupling unit. In addition, the asymmetry of the bus signals can be determined.

A significant advantage of the invention is that the decoupling unit can be clamped at any point along the two-wire fieldbus cable without the risk that not all bus telegrams are read can, because at the decoupling an extinction of the bus signals takes place.

In a bus monitor equipped with the decoupling unit according to the invention, all the information of the slaves connected to the fieldbus can thus be made visible to the user independently of the point of connection on the monitor. In bus analysis, it is important for the user that all bus signals are read and all bus information is visible.

In a slave having a decoupling unit according to the invention, all data channels are available independently of the connection location. This is especially important if the bus system has many slaves and the available data channels are largely occupied.

The decoupling unit according to the invention can also be used in so-called repeaters, which serve as galvanic isolation between two bus segments. With repeaters equipped in this way, all bus signals are reliably detected and the data telegrams considered valid are forwarded reliably to the other bus segment.

With the decoupling unit according to the invention it is also possible to localize slaves on the bus. About the signal strength, the ratios of the signal level and the delay times of individual bus telegrams can be determined whether the corresponding slave is seen from the decoupling point closer or farther away from the master.

The coupling unit according to the invention normally has two directional couplers. In principle, variants with only one directional coupler or even distributed structures with several directional couplers are conceivable.

The invention is preferably suitable for a two-wire field bus with symmetrical signal transmission in a frequency range 1-10 MHz. Symmetrical signal transmission is common with unshielded bus cables.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102004001474 [0012]

Claims (4)

Decoupling unit for a two-wire fieldbus for reading bus telegrams, wherein the bus telegrams are transmitted as high-frequency bus signals, wherein the individual bus participants certain frequency channels are assigned, which serve to transmit the bus signals, wherein the signal level of the bus signals along the two-wire bus z. B. due to lack of line terminations by reflections is characterized in that the decoupling unit has two directional coupler R0, R1, which output the bus signals as direction-specific or conductor-specific sub-signals, that a signal processing unit S is provided, in which the sub-signals are processed and at the output Bus telegram is output if at least one partial signal can be evaluated. Decoupling unit according to one of the preceding claims, characterized in that the sub-signals before they are supplied to a provided in the signal processing unit S decoding D0, D1, are linked together in a linking unit V0, V1 analog. Decoupling unit according to one of the preceding claims, characterized in that the decoupling unit is used in a slave, a repeater or in a bus monitor. Decoupling unit according to one of the preceding claims, characterized in that the high-frequency bus signals in the frequency range between 1 and 10 MHz.

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