DE102013220155B4 - Decoupling unit for a two-wire fieldbus for reading bus telegrams - Google Patents

Abstract

Decoupling unit for a two-wire fieldbus for reading bus telegrams, the bus telegrams being transmitted as high-frequency bus signals, with the individual bus participants being assigned certain frequency channels that are used to transmit the bus signals, the signal level of the bus signals along the two-wire bus line, e.g. B. due to missing line terminations by reflections, characterized in that the decoupling unit has two directional couplers R0, R1, which output the bus signals as direction-specific and / or conductor-specific partial signals, that a signal processing unit S is provided in which the partial signals are processed and at its output Bus telegram is output if at least one partial signal can be evaluated.

Description

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

Two-wire fieldbuses are often used in automation technology for communication between several bus participants (central controls PLC, sensors, actuators). Both the communication and the energy supply to the slaves take place via a two-wire fieldbus cable.

Many fieldbus systems often work according to the master-slave principle (e.g. Profibus), in which case bus telegrams (data packets) are exchanged between the master and the individual slaves. In addition to slaves as communicative bus participants, bus monitors are also used that act as purely passive bus participants because, as “listeners”, they only read the bus telegrams 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 parts in critical situations.

The number of slaves is due to various restrictions such as B. the bus specifications or the bus physics are limited to a few bus participants (e.g. 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.

The bus interface serves on the one hand as a decoupling unit for reading bus telegrams. On the other hand, it is also used to write bus telegrams. Bus monitors only require the bus interface to function as a 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 that contains, for example, the bus-specific protocol stack in which the bus telegrams are evaluated and their telegram content is extracted. The corresponding information can be processed further in a downstream microprocessor, for example.

Bus interfaces and communication controllers are available for purchase for various bus systems, e.g. B. the SIM1 module and the SPC4 communication controller from Siemens.

In order to increase the number of slaves and data throughput, multi-frequency data transmission technologies can also be used with fieldbuses.

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

From the pamphlets US 7 260 185 B2 , US 2003/0 012 365 A1 , U.S. 5,504,736 A and WO 2011/116 928 A1 various monitors for bus signal are known.

A field bus system is known from DE 10 2004 001 474 A1 which, in addition to a conventional first bus communication technique, uses a second bus communication technique in accordance with the DSL standard. Both technologies use the same two-wire fieldbus cable as the transmission medium.

With such data transmission techniques, the following problems cannot be ruled out.

The high-frequency bus signals (e.g. in the range of 1-10 MHz) from individual bus participants can be canceled out due to missing terminations, especially in bus topologies with a tree structure, due to reflections in spatially extended bus segments (up to e.g. 200 m). This means that there can be certain points on the bus line at which the signal level of the bus signals of a single or multiple bus subscribers is so low that the bus signals can no longer be reliably read or evaluated with a conventional bus interface.

A bus participant connected at such a point would no longer be able to read all bus telegrams.

The object of the invention is therefore to specify a decoupling unit for a two-wire field bus for reading bus telegrams, which enables all relevant data packets to be read on the bus regardless of the location of the decoupling point.

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

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

Advantageous further developments of the invention are given in the subclaims.

The invention is explained in more detail below on the basis of an exemplary embodiment shown in the drawing.

• 1 schematische Darstellung einer erfindungsgemäßen Auskoppeleinheit mit zwei separaten Richtkopplern
• 2 schematische Darstellung einer erfindungsgemäßen Auskoppeleinheit mit einem symmetrischen Richtkoppler
• 3 schematische Darstellung einer erfindungsgemäßen Auskoppeleinheit mit zwei unsymmetrischen Richtkopplern

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 asymmetrical directional couplers

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

To the two ladders L1 and L2 are directional couplers R0 and R1 connected. At the entrances A0 , A1 , A2 , A3 a signal processing unit S. the direction-specific and line-specific partial signals that are obtained from the actual bus signal are present. These are each paired in a linking unit V0 or. VI summarized, amplified and checked, filtered and evaluated according to the bus specifications and then in two decoding units D0 and D1 decoded. In a merging unit Z the decoded data are 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 the same way as is known from conventional slaves, bus monitors or repeaters.

In the simpler variant shown, the analog bus signals are linked to one another before they are decoded. This means that only two decoding units are required. If the signal conditions are favorable, both decoding units deliver identical bus telegrams, of which only one is required for further processing. This selection takes place in the merge unit.

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

If the conditions are favorable, all four decoding units also output matching bus telegrams here.

The signal processing unit functions 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 symmetrical coupler works as a symmetrical coupler with the two conductors L1 and L2 contacted capacities. Here is a 50 ohm match on the two inputs A0 and A2 required. This variant is particularly suitable for slaves and repeaters that are fed via the bus. The necessary chokes must be connected to opposite points on the bus line.

3 shows a schematic representation of a further decoupling unit according to the invention with two asymmetrical directional couplers. The asymmetrical coupler (Bruene bridge) does not have a galvanic connection with the two-wire fieldbus. This variant is particularly suitable for bus monitors, as these are normally not bus-powered.

Using the directional coupler R0 or. R1 the bus signals can be decoupled as partial signals in a direction-specific and / or line-specific manner. The directional coupler can separate the bus signals according to their direction of propagation. In the event of a cancellation, the bus coupler delivers two correspondingly separated partial signals.

The partial signals can be in the signal processing unit S. can be processed further separately or partially linked.

In principle, only one evaluable signal has to be present for the signal processing unit S. can output a digital bus telegram for further processing. This evaluable signal can, for. B. be one of the partial signals.

The decoupling units according to the invention have a number of advantages.

By appropriate evaluation in the signal processing unit S. the direction of propagation of the associated bus signals can also be determined for individual bus telegrams.

In addition, the position of other bus participants, such as. B. other slaves can be 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 can be read because the bus signals are deleted at the decoupling point.

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

In the case of a slave that has a decoupling unit according to the invention, all data channels are available regardless 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 what are known as repeaters, which serve as electrical isolation between two bus segments. With repeaters equipped in this way, all bus signals are reliably recorded and the data telegrams considered to be valid are reliably forwarded to the other bus segment.

With the decoupling unit according to the invention, it is also possible to locate slaves on the bus. The signal strength, the relationships between the signal levels and the delay times of individual bus telegrams can be used to determine whether the corresponding slave is closer to or further away from the master when viewed from the decoupling point.

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

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

Claims (4)

Decoupling unit for a two-wire fieldbus for reading bus telegrams, the bus telegrams being transmitted as high-frequency bus signals, with the individual bus participants being assigned certain frequency channels that are used to transmit the bus signals, the signal level of the bus signals along the two-wire bus line, e.g. B. due to missing line terminations by reflections, characterized in that the decoupling unit has two directional couplers R0, R1, which output the bus signals as direction-specific and / or conductor-specific partial signals, that a signal processing unit S is provided in which the partial signals are processed and at its output a bus telegram is output if at least one partial signal can be evaluated. Decoupling unit after Claim 1 , characterized in that the partial signals before they are fed to a decoding unit D0, D1 provided in the signal processing unit S, are linked to one another analogously in a linking unit V0, VI. Decoupling unit according to one of the preceding claims, characterized in that the decoupling unit is suitable for use 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 are in the frequency range between 1 and 10 MHz.

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