DE102007024803B4 - Telecommunication terminal with automatic SO / Up switching - Google Patents

Abstract

Telecommunication terminal with an interface that can be used either for an ISDN S0 or for an ISDN Up interface, characterized in that the circuit arrangement is constructed in such a way that the type (S0, Up) of the connected interface is recognized and a corresponding automatic switchover the operating mode of the circuit arrangement between the two interface types (S0, Up) and that the circuit arrangement recognizes the interface type (S0, Up) based on the supply voltage at the Up connection or the supply voltage at the S0 connection.

Description

The invention relates to a telecommunication terminal with an interface which can be used either for an ISDN S ₀ or for an ISDN U _p interface. The detection and the switching takes place automatically.

Telecommunication terminals are used for operation in a telecommunications system, which consists of a more or less large number of other components and devices. Nowadays usually those telecommunication terminals are used, which correspond to the ISDN standard regulations. Two different types of interfaces are used in ISDN technology, namely 4-wire so-called S ₀ interfaces with separate wire pairs for the transmission and reception direction and 2-wire so-called U _p interfaces. As a rule, ISDN terminals therefore have either an S ₀ interface or an U _p interface. Both interfaces are often provided in the practice of telecommunications equipment.

However, this means that a buyer of an ISDN terminal must make sure prior to the purchase of his device, which interface type is provided in his telecommunications system as a connection for the telecommunications terminal equipment. In this one of the two possible modes must then namely the ISDN terminal to be operated.

This means, of course, that the trade must hold two different types of telecommunication terminal equipment, since both can be in demand and needed. This leads at factory fixed to S ₀ or U _p set ISDN terminals to high storage costs and to a higher variety of products than would actually be required for terminals with the same performance options.

In addition, a later change of operating mode is completely eliminated. It is therefore not possible to continue to use the purchased and existing telecommunications terminal when switching to a new telecommunications system that provides interfaces of the other type.

As an improvement, devices have already been developed in which the operating mode can be selected differently by means of plug-in cards, so that a specific telecommunications terminal only has to be rebuilt by means of these plug-in cards in order to operate either in operating mode U _p or in operating mode S ₀ . However, this requires unchanged expertise in the conversion and a corresponding time and technical effort.

Another proposal will be in the DE 10 2004 005 089 A1 already described a way that allows a combined interface in an ISDN terminal. In a device of this type, however, the operator must set before connecting the appropriate device by means of jumpers and / or switches, in which mode he wants to use the device. So while only one type of device is needed, it is very cumbersome to make use of this advantageous option.

Such embodiments with switching options usually lead to higher equipment costs. In addition, a user must either become familiar with the technology or switch on appropriately trained specialists who can switch over for him / her. This applies not only to the initial purchase of the system, but also to a possible change or change of parts of the telecommunications system as a whole. For the dealer, this means that he must hold either this specialized personnel, which is often not possible or not desirable, or that he already has to keep previously switched devices and thus 2 types.

The prior art is from the patent US 5,629,926 A It is known that an ISDN interface can automatically switch between an S / T interface and a U-interface. Here, however, a complex method of ISDN signal detection is used to determine the type of interface.

From the WO 00/73 806 A1 For example, an interface switch between two different ISDN U standards for different transmission distances is known. The switchover is accomplished by first attempting to establish a connection with the line code for short distances, and if this fails, it will attempt to establish the connection with the line code for longer distances.

Furthermore, from the patent US 5,956,385 a circuit arrangement is known, which can switch for testing purposes between different ISDN interface standards.

The object of the invention, in contrast, to provide a simplified way for such telecommunication terminals here.

This object is achieved in a generic telecommunication terminal thereby that the circuit _arrangement is constructed so that a detection of the type (S ₀ , U _pO ) of the connected interface and a corresponding automatic switching of the operating mode of the circuit _arrangement between the two interface _types (S ₀ , U _p ) takes place in which the supply voltage from the interface is evaluated.

The invention makes it possible to obtain ISDN terminals which automatically adapt to the respective operating mode of the telecommunications system. Nevertheless, the resulting additional costs compared to conventional ISDN terminals are comparatively low. However, with these low additional costs for the somewhat more complex circuit arrangement, the advantage arises that the trade only has to keep one and not two types of a specific type of device in stock, ie it can significantly reduce its stock. The customer does not have to specify in advance which ISDN terminal he wants to use now. In addition, the customer has the opportunity to continue to use the ISDN terminal even when changing the telecommunications system, without having to worry about the interface type.

Also conversion work is completely eliminated. Neither plug-in cards nor jumpers nor other components must be changed, removed, plugged in or kept in stock as additional components to the device. Similarly, eliminates the need for associated professionals and the corresponding amount of time.

There is also often the need or at least the desire for subsequent changes in the type of interface provided.

For example, 4-wire S ₀ lines are used to connect conventional ISDN devices to an S ₀ bus. However, it may happen that you want to operate on just these 4 wires more devices. It then makes sense in the telecommunications system (often abbreviated as a PBX) to exchange the S ₀ module for a U _p module. In this case you can then use the 4 existing wires for 2 complete U _p connections. On the terminal side, the same devices can still be used in circuit arrangements according to the invention, which was not possible or at least only with great effort.

In a preferred embodiment of the invention, the telecommunications terminal is equipped so that the circuit arrangement is constructed so that automatically the supply voltage from the respective supply voltage from the provided interface in both modes (S ₀ , U _p ) is obtained.

Thus, the supply voltage itself is used to directly derive the supply voltage of the telecommunication terminal. In the case of an interface of the U _p type, the supply voltage is preferably between two RX terminals, wherein it is still rectified and then decoupled. In the case of an S ₀ type interface, the supply is between TX and RX lines. It can then be tapped there.

The switching on of a preferably provided transistor T1 takes place directly from the S ₀ supply voltage.

This low-resistance semiconductor switch (preferably a transistor) T ₁ can also serve a further purpose in preferred embodiments. It then not only switches on the supply voltage in the S ₀ mode, but it ensures in case of operation at a U _p interface that no direct current flows through the RX transformer (TR2). A preferred embodiment of the invention is characterized in that two analog switches (IC1, IC2) are provided, which are connected so that the transmission lines of the ISDN chip are selectively switched to the one (TR1) or other (TR2) transformer, and a diode (D7) is provided in the supply voltage line so as to prevent the signal alternating current being limited via the protective diodes in the analog switch.

With this configuration, the case can be particularly cleverly and reliably taken into account that telecommunication terminals are also switched off, but then may not or should not burden these with multiple terminals on a bus. This is especially true in S ₀ operation. However, there is now a certain AC voltage of, for example, 0.7 V _pp or 1.4 V _SS . The transformers have a transmission ratio of 2: 1 (2 on the chip side, 1 on the line side). This AC voltage is therefore twice as high on the analog switch side (on the chip side). Analog switches without a power supply, however, divert their input signal to the supply connections via the protective diodes. These usually conduct from a voltage of about 0.7 volts. However, this limits the peaks of the AC signal (wanted signal), which should not be the case. By incorporating said decoupling diode in the supply voltage of the analog switch, however, this undesirable limiting effect can be reliably prevented.

The current flow through the mentioned diode D7 is blocked in the reverse direction. This eliminates the need for other, less favorable ways to avoid this undesirable limiting behavior, such as by supplying the analog switch with a supply voltage.

In the following, the basic requirements for the invention and beyond some embodiments will be described with reference to the drawings. Show it:

1 an S ₀ basic connection;

2 a U _p base connection;

3 an alternative shading of a U _p basic connection;

4 a subcircuit with the interconnection of a S ₀ port;

5 a subcircuit with the interconnection of a U _p -Ports;

6 a circuit with a switching of U _p and S ₀ , composed of the sub-circuits 4 and 5 , here with galvanic isolation; and

7 a circuit similar 6 , but without galvanic isolation.

Telecommunication (ISDN) terminals are usually connected via a 4-wire so-called S ₀ connection to the type or a telecommunications system. As a rule, these four lines also supply the power supply for the connected terminals. Such S ₀ base connection is in the 1 shown.

Two lines each are used for a transmission direction (TX +, TX-) and a reception direction (RX +, RX-). On the transmitting and receiving side usually transformer (TR1, TR2) are used with center tap (I, II). The TX side supplies one pole of the power supply (to pin 2 and pin 3 of TR1), the RX side to the other (to pin 2 and pin 3 of TR2). If the transformers TR1, TR2 are connected in such a way that both TX lines are connected to the ends of the transformer winding (TR1, pin 1 and pin 4), the supply voltage (UB) can be taken from the middle connection (pin 2, pin 3). Accordingly, the RX transformer (TR2) is interconnected. Since the DC current is thus conducted through the transformers (TR1, TR2) in both directions, the DC components subtract and the transformers remain DC-free. They can thus be made quite small and relatively inexpensive. A capacitor C2 is usually used to screen the supply voltage UB thus generated, which is applied to the terminals + UB and -UB.

The U _p port is often used in ISDN to save lines. Often, old lines of analogue telephone technology can continue to be used, which saves enormous costs in the modernization of the telecommunications infrastructure. The U _p interface is a 2-wire connector that is normally connected to a telecommunications system. As a rule, the terminals are also supplied with power via these two lines. Such a U _p basic connection is in the 2 shown.

Unlike the S ₀ connection, both the transmission direction (TX) and the reception direction (RX) are transmitted here via only two lines (RX +, RX-). In the ping-pong (time division multiplex) method, the line is used alternately for the transmission and reception direction.

Again, a transformer (TR2) with two identical windings (I, II) is usually used, but in the middle (pin 2 and pin 3) are not directly connected together. Here, in the middle, there is a capacitor (C1) on which the supply voltage (UB) at the terminals + UB and -UB is tapped in parallel. Since direct currents do not subtract in the transformer, the transformer must be designed for the very high DC component, which makes it quite large and expensive.

When U _p port both lines (RX +, RX) may be reversed. Therefore, the polarity of the output voltage (UB) is not defined, which is why usually a rectifier is connected downstream.

The resistors R1 and R2 together form the terminating resistor for the line. Since only one terminal can be connected, the terminating resistor is permanently switched on.

In 3 an alternative wiring of the U _p connection is shown.

One can avoid the DC superposition in the transformer (TR2), in which one does not pick up the supply in the middle of the transmitter, but directly from the lines (RX +, RX-). This can be used inexpensive commercial S ₀ transmitter. Here, however, attention must be paid to an AC decoupling, which can be done for example via inductors (L1, L2) or a correspondingly connected current sink (not shown).

When U _p port both lines (RX +, RX) may be reversed. Therefore, the polarity of the output voltage (UB) is not defined, which is why usually a rectifier is connected downstream.

The resistors R1 and R2 together form the terminating resistor for the line. Since only one terminal can be connected, the terminating resistor is permanently switched on.

The 4 . 5 and 6 show in connection now the circuit according to the invention. Here is the overall circuit in the 6 shown in the 4 and 5 For better clarification of the functioning of each part of these circuits are specially pulled out that perform a very specific function.

There are in the 4 those elements that are relevant for a DC voltage supply in S ₀ operation are summarized.

In the 5 those components are summarized that are important for a DC supply or DC decoupling in U _p operation.

In 6 For example, a circuit is shown which automatically switches between the operating modes U _p and S ₀ .

The 4-wire S ₀ signal with the lines TX +, TX-, RX + and RX- or the 2-wire U _p signal with the lines RX + and RX- is applied to the connector (BU1).

In S ₀ OPERATION the two lines are TX + and TX- at the same DC potential. The same applies to the RX + and RX- lines. With the S ₀ -Bus there is both the operating mode with supply, as well as without. In powered mode, the supply voltage is between the TX (TX +, TX-) and RX (RX + and RX-) lines to power the telecommunications terminal:
In U _p operation is a power supply of the terminals usual. Therefore, it is reliable to use the DC voltage between the two terminals (RX + and RX-) as a switching signal for the operating mode in the terminal.

In U _p operation, a DC voltage is present between terminals RX + and RX- of socket BU1 for feeding the terminals. It passes through the winding I and II from the transformer TR2 to the resistors R3, R4, which form a voltage divider. Since the supply voltage polarity is arbitrary during U _p operation, the detection must work irrespective of the polarity. These resistors feed the LEDs from the optocoupler OK1. In the example, it is an optocoupler with two anti-parallel LEDs. Alternatively, a bridge rectifier with a simple optocoupler can be used. In a further alternative, two simple optocouplers can be used, which are connected in parallel on the input side and in parallel on the output side.

The DC voltage can alternatively be tapped on the input side of the bridge rectifier GR1 (pin 1 and pin 2).

In this example ( 6 ), the terminal is galvanically isolated from the U _p connection. This makes sense if the terminal has further interfaces to the outside, as in the underlying application.

It is also the case without this galvanic separation conceivable. In that case, the voltage at pin 2 and pin 3 from the transformer TR2 may be used for switching immediately after a level conversion (not shown).

If the terminal is to be fed from the ISDN line both in S ₀ mode and in U _p mode, then a feed is available in both operating modes. In that case it is easier to make the switch on the basis of catering in the S ₀ OPERATION.

This is exemplified in 7 via the voltage divider R8, R9 and the voltage-limiting zener diode D6.

In the overall circuit ( 6 ), but also in the extracted S ₀ circuit ( 4 ), the DC output is listed in S ₀ mode.

The positive supply voltage at pin TX + and TX- is fed via the transformer TR1, winding I and II and via a diode D5 directly to the filter capacitor C2.

The negative supply voltage (DC, negative pole) at pin RX + and RX- leads to two diodes D1 and D2 via transformer TR2, winding I and II when operating at an S ₀ interface. These are preferably Shottky diodes D1 and D2. These have a forward voltage of about 0.3 volts. Shottky diodes have a smaller and thus less scattering forward voltage than other diodes and are therefore preferred here. When operating at an S ₀ interface, it makes sense if the current is distributed as evenly as possible to the two diodes D _{1 and} D ₂ . In the case of an asymmetry, a little more direct current would flow through one of the two diodes D1 or D2 than through the other of the two diodes. The However, this would mean that the two DC components differ in the transformer TR2. They would not subtract completely from each other. The transformer TR2 could thereby fall into the range of saturation and thus limit the superimposed AC voltage, which in turn could lead to undesirable distortions.

From there it goes via the switching transistor T1 on to the filter capacitor C2, where the supply can be tapped. The transistor T1 is turned on via the voltage divider R5 and R6 when a supply voltage is applied to the S ₀ bus. The capacitor C1 is required so that the useful AC voltage passes from pin 2 to pin 3 of the transformer TR2.

While the DC current flows through the diodes D1 and D2, an AC voltage of about 0.7 volts _pp is applied to a rectifier GR1. This can just rectify the rectifier GR1. Thus, the signal peaks are guided and limited to the filter capacitor C2. By a series connection with a further diode D4 another 0.7 volts can be obtained, which then no longer comes to a limit.

In the overall circuit ( 6 ), but also in the extracted U _p circuit ( 5 ), the DC decoupling is listed in U _p mode.

In U _p mode, the supply voltage is obtained directly from the two connecting lines RX + and RX- with the aid of the two rather inexpensive inductors L1 and L2. The bridge rectifier GR1 rectifies them and makes them independent of their polarity. C2 serves as a filter capacitor for the supply voltage (UB).

In U _p operation, the transistor T1 is turned off and thus prevents a direct current flowing through the transformer TR2. The negative pole arrives in U _p OPERATION of RX + and RX- via a diode in the rectifier GR1 and the diode D4 to the filter capacitor C2. Both diodes together have a forward voltage of approximately 1.4 volts. Without the blocking transistor T1 of the DC current of RX + and RX- via the transformer TR2 (windings I and II) and one of the diodes D1 and D2 would _(p depending on the polarity of the U OPERATION) for a filter capacitor (C2) reach. Since these diodes have a forward voltage of only 0.3 volts, the direct current would seek its preferred path here, the transformer TR2 would be traversed by dc, so get into saturation and thus limit the ac voltage, which leads to undesirable distortions.

In U _p mode current flows through the rectifier GR1. As a result, the diodes contained in it conduct. They are thus very low-impedance and thus lay the two-part terminating resistor R1 and R2 low resistance to the filter capacitor C2. Thus they form the required AC terminating resistor of 100 ohms.

Also, the diode D5, the mentioned at S ₀ OPERATION above the positive supply voltage leads to the smoothing capacitor C2, when operating at a Up interface in a preferred embodiment receives an additional function. Namely, it prevents the positive supply voltage from the pin 3 of the rectifier GR1 from reaching the connection lines TX + and TX- via the transformer TR1. Although this is usually harmless, as nothing is connected here, it is an additional safeguard in the event that building wiring is misplaced.

In S ₀ mode, this is a real bus connection so that normally no terminating resistors are switched on at the terminals. You are currently in the spring (hardwired in the last junction boxes on the S ₀ bus.

An automatic switching of the reception path is not necessary. By clever choice of transformer TR2 for the receive direction in both modes, a common signal path is possible.

The received signal is fed from the socket BU1 (line RX +, RX-) to the transformer TR2 (pin 1, pin 4). Pin 2 and pin 3 are AC-connected by the capacitor C1. By the series connection of the windings III and IV from the transformer TR2, the received signal passes via the input circuit C3 / R10 and C4 / R11 on the ISDN chip to the inputs INT-RX + and INT-RX-.

If the S ₀ supply is used in S ₀ operation, then Pin 2 and Pin 3 of TR2 are additionally connected to each other in an alternating voltage by the diodes D1 and D2 operating in the forward direction.

In the transmission direction, the transmission signal of the ISDN chip IC3 (pin INT-TX +, INT-TX-) must be switched to the transformer TR1 (S ₀ operation) or the transformer TR2 (U _p operation), depending on the operating mode. In S ₀ operation, the optocoupler OK1 is not activated. The switching signal SELECT controls the two analog switches IC1 and IC2 so that the transmission signal from the ISDN chip IC (pin INT-TX +, INT-TX-) on the transformer TR1 (winding III, IV) passes. Via winding I, II the transmission signal reaches the socket BU1, pin TX +, TX-.

In U _p operation, the optocoupler OK1 is driven. The switching signal SELECT controls the analog switch IC1 and IC2 so that the transmission signal from the ISDN chip IC3 (pin INT-TX +, INT-TX-) on the transformer TR2 (winding III, IV) passes. Via winding I, II, the transmission signal reaches the socket BU1 (pin RX +, RX-).

For example ( 6 ) outlines a circuit that provides a galvanic isolation between the ISDN interface and the internal circuit of the tail gas.

In 7 There is a simpler circuit that does not allow galvanic isolation. However, the elements are largely similar to those discussed above 6 discussed.

LIST OF REFERENCE NUMBERS

• BU

  ISDN connection socket to which an S ₀ connection is optionally applied;

  IC1

  simple analogue switch for the transmission signal from the ISDN chip to the transmitters;

  IC2

  simple analogue switch for the transmission signal from the ISDN chip to the transmitters;

  IC3

  ISDN chip that controls the operating modes U _p - and S ₀

  R1

  one half of the line terminating resistor in U _p mode;

  R2

  one half of the line terminating resistor in U _p mode;

  R3

  Series resistor of the voltage divider in the operating mode switch;

  R4

  Resistance in the voltage divider of the mode selector;

  R5

  Series resistor for driving the power switch T1;

  R6

  Gate resistance to safely disable the power switch T1;

  R7

  Pull-up resistor in mode switching;

  R8

  Series resistor of the voltage divider in the operating mode changeover at S ₀ supply;

  R9

  Resistance in the voltage divider in the operating mode changeover at S ₀ supply;

  R10

  Coupling resistance on the INT-RX + input from the ISDN chip;

  R11

  Coupling resistance at the INT-RX input of the ISDN chip;

  R12

  Alternative terminating resistor in U _p mode;

  C1

  Capacitor connecting alternating voltage with the RX transformer pin 2 and pin 3;

  C2

  Filter capacitor for the supply voltage;

  C3

  Coupling capacitor on the INT-RX + input from the ISDN chip;

  C4

  Coupling capacitor at the INT-RX input of the ISDN chip;

  D1

  Shottky diode for decoupling the supply voltage in S ₀ mode;

  D2

  Shottky diode for decoupling the supply voltage in S ₀ mode;

  D3

  eliminated;

  D4

  Diode in series with rectifier GR1 to increase the forward voltage;

  D5

  Diode for coupling out the S ₀ supply voltage and for blocking in U _p mode;

  D6

  Limiting diode in 7 upon obtaining the switch from the S ₀ supply;

  D7

  Blocking diode on the analogue switch to prevent current drain via V _cc ;

  OK1

  Optocouplers with anti-parallel diodes in mode switching;

  T1

  Circuit breaker for the S ₀ supply, for example power FET, bipolar transistor or the like;

  TR1

  Simple S ₀ transmitter for the transmission direction in S ₀ operation;

  TR2

  Simple S ₀ transmitter for the transmitting and receiving direction in U _p operation and the receiving direction in S ₀ operation;

  W1

  DC / DC converter for galvanic isolation and voltage stabilization of the supply.

Used terms

• U _p

  ISDN 2-wire interface, basic connection, is widely used in telecommunications equipment; _Terms such as U _pn or U _p0 are common

  S ₀

  ISDN 4-wire interface, basic connection, standard connection at the office and in telecommunications system;

  PBX

  Telecommunications equipment;

  RX

  Receiver, receiving line;

  TX

  Transmitter, transmission line;

  ISDN chip

  Integrated circuit with ISDN functionality

Claims (17)

Telecommunication terminal with an interface that either for an ISDN S ₀ - or for a ISDN U _p interface is usable, characterized in that the circuit arrangement is constructed so that a detection of the type (S ₀ , U _p ) of the connected interface and a corresponding automatic switching of the operating mode of the circuit arrangement between the two interface types (S ₀ , U _p ) takes place and that the circuit arrangement recognizes the interface type (S ₀ , U _p ) on the basis of the supply voltage at the U _p connection or the supply voltage at the S ₀ connection. Telecommunication terminal with a circuit arrangement according to claim 1, characterized in that the circuit arrangement comprises two S ₀ transducers (TR1, TR2). Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that the circuit arrangement is constructed so that automatically the supply voltage from the respective supply voltage from the provided interface in both modes (S ₀ , U _p ) is obtained. Telecommunication terminal with a circuit arrangement according to claim 3, characterized in that the circuit arrangement is constructed so that in U _p operation even when the two lines (RX +, RX-) the supply voltage from the supply voltage from the interface by rectification (GR1) wins. Telecommunication terminal with a circuit arrangement according to claim 3 or 4, characterized in that the circuit arrangement is constructed so that by a galvanically coupled switching (R8, R9), the mode switching is enabled by the supply voltage in S ₀ operation. Telecommunication terminal with a circuit arrangement according to one of claims 3 or 4, characterized in that it comprises a galvanically isolated voltage converter (W1) and an optocoupler (OK1) and they are connected so that they allow a galvanic isolation of the telecommunications terminal to be supplied from the interface , Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that the circuit arrangement comprises an optocoupler (OK1) and the diode side antiparallel LEDs, which are connected so that a switch in permutation (polarity reversal) of the lines (RX +, RX-) in U _p operation is enabled. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that two optocouplers are provided, which are connected in parallel on the diode side and output side. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that the circuit arrangement comprises a low-resistance semiconductor switch, in particular a transistor (T1), which is connected so that it takes over the switching of the power supply. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that the circuit arrangement is constructed so that at the U _p interface automatically a terminating resistor (R1, R2) is turned on. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that inductors (L1, L2) are provided, which are connected so that when connected to a U _p interface, a higher load current is allowed, than by the terminating resistors ( R1, R2) would be possible. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that the circuit arrangement is constructed so that in S ₀ operation no terminating resistor is turned on. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that connection pins are provided, and in that for the U _p interface, the same connection pins are used on a socket (Bu1), as for the S ₀ reception line (RX +, RX-). ) or as for the S ₀ transmission line (TX +, TX-). Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that two analog switches (IC1, IC2) are provided which are connected so that the transmission lines of the ISDN chip selectively switched to one (TR1) or other (TR2) transformer and that a diode (D7) is provided in the supply voltage line so as to prevent the signal alternating current from being limited via the protective diodes in the analog switch. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that two diodes (D1, D2) are provided, which are connected so that the connection of the two center terminals of the ISDN transformer (TR2) is automatically carried out during S ₀ operation. Telecommunication terminal with a circuit arrangement according to claim 17, characterized in that the two diodes (D1, D2) are Shottky diodes. Telecommunication terminal with a circuit arrangement according to one of the preceding claims, characterized in that a diode (D4) is provided in series with a rectifier GR1 for increasing the forward voltage.

Images