JPH0342836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a current loop forming circuit used in a central office line trunk interface of a switching device.

(Prior art and its problems) In recent years, the subscriber line interface of PBX and key telephone equipment has been computerized, and SLIC is generally used.
LSIs and HICs called As computerization progresses in this way, the parts of the system components that have been computerized and the parts that have not been computerized will be extremely unevenly mounted, and the mounting unit dimensions will inevitably become uneven. In PBX and key telephone equipment, while the extension side interface is becoming more electronic, the central office line side interface still uses large transformers,
Relays and large capacitors are used. This is because the applied DC voltage and incoming signal are excessive, and the current capacity also requires over 100 mA, and is also a measure against lightning.

A conventional circuit is shown in FIG. Here, L ₁ , L ₂
is the subscriber line (office line) input terminal, RA is the incoming call detection circuit, T is the audio coupling transformer, C is the spark absorption capacitor, R is the spark absorption resistor, RL is the dial pulse that operates based on the dial signal The sending relay, SW is a communication path switch, and TEL ₁ to TEL _o are telephones. Conventionally, terminal telephone TEL (e.g.
Call path switch SW based on the calling operation of TEL ₁ )
A dial pulse was sent out by capturing the station lines L ₁ and L ₂ via the terminal, and then operating the dial to open and close the contacts RL of the relay RL. This relay RL required a current of approximately 120 mA to flow, was large in size, and was expensive. Furthermore, a capacitor C and a resistor R are required to prevent sparks.
Capacitor C is large because it requires sufficient withstand voltage. Next, a current of approximately 120 mA is required to flow through the transformer T as well. This requires an expensive and large-sized transformer. Furthermore, since the inductance cannot be made large and coupling due to leakage magnetic flux occurs between the transformers, it is difficult to obtain crosstalk characteristics with a low crosstalk level below a predetermined value.

Recently, computerization has been progressing with the goal of eliminating large transformers and relays. An example of this electronic circuit is shown in FIG. Here, T ₁ is an audio coupling transformer, C ₁ is a direct current blocking capacitor of the transformer, 1 is a diode bridge, and 2 is a resistance of about 50 to 300 Ω in terms of DC, but a value close to infinity in terms of AC. In this electronic sink circuit, Q is a transistor, C ₂ is a capacitor, and R ₁ , R ₂ , and R ₃ are resistors. Further, 3 is a dial pulse sending circuit. The DC current flowing into the transformer T ₁ is blocked by the capacitor C ₁ , and the DC current that should flow into the transformer T ₁ is made to flow into the electronic sink circuit 2 . For this reason, the transformer T ₁ can be made small, and furthermore, the cost of the components can be reduced. On the other hand, electronic sink circuit 2
is required to pass direct current but not alternating current. For this reason, this sink circuit is driven by an input obtained by smoothing the voltage between A and B in the figure. Point D in the figure is the above-mentioned smoothing voltage.

The operation when sending out a dial signal is as follows. When making a dial, the TTL level or C/MOS level dial signal input corresponding to the dial operation becomes LOW, capacitor _C2 is charged, transistor Q is then turned on, and electronic sink circuit 2 operates. . At dial break, the dial signal is HIGH,
The charge stored in capacitor _C2 is mainly discharged through the base of transistor Q and resistor _R3 .
Furthermore, a portion is discharged through resistor R ₂ , but this current is small compared to the former.

Note that when making dials, it is necessary to drive the electronic sink circuit 2 immediately after making the dials to form a DC loop for L ₁ and L ₂ . but,
Since the charge on capacitor C ₂ is decreasing, charging takes place first. Thereafter, the transistor Q is turned on, and the electronic sink circuit 2 performs normal operation. As explained above, when viewed from L ₁ and L ₂ ,
When transmitting dial pulses, there is a drawback that the transmitted dial pulse waveform is deformed because time is required to charge the capacitor _C2 when moving from break to make.

Furthermore, in order to eliminate these drawbacks, an electronic sink circuit as shown in FIG.
Proposed by No. 48693. In this electronic sink circuit 2a, _the capacitor C ₂
and relay contact _K2 , which cuts off the discharge path, are connected in series. However, such a configuration
This results in the need for a relay to digitize the current loop forming circuit, which goes against the original purpose of digitization and has virtually no practical effect. Moreover, the relay contact
The path of K ₂ serves as a charging/discharging path for capacitor C ₂ and cannot be replaced with a unidirectional electronic circuit element. Furthermore, even if the relay contact _K2 is inserted into the base circuit of the transistor Q as shown in the electronic sink circuit 2b of FIG. 3b,
This alone may cause the charge in capacitor C ₂ to be discharged through resistor R ₁ . In this example circuit,
Although only a path including only the resistor _R1 is described, in reality, this circuit configuration does not provide sufficient characteristics as the current sink circuit 2. Particularly when the distance from the station is small, there is a drawback that the transistor Q is saturated by direct current. Therefore, in practice, a resistor _R2 is also required as shown in FIG. 2, which is also an essential drawback of the circuits shown in FIGS. 3a and 3b.

(Object of the Invention) The present invention provides a current loop forming circuit that configures an electronic sink circuit without using these large transformers or relay contacts, and eliminates the purification of dial pulse signals when sending out dial pulses. It provides:

(Structure of the Invention) In order to achieve this object, the current loop forming circuit of the present invention includes a diode bridge for converting a DC current from a central office line into a DC output with a constant polarity, and a a dial pulse sending circuit disposed on the output side, and at the output of the dial pulse sending circuit, a series circuit of a first resistor and a second resistor and a collector-emitter path of a transistor forming a communication current loop are provided. A diode is connected in parallel and is arranged between a connection point between the first resistor and the second resistor and the base of the transistor in a forward direction with respect to the base current of the transistor, and a cathode side of the diode. A capacitor for smoothing an alternating current signal appearing at the base terminal of the transistor is connected to the capacitor, and the capacitor is configured to be turned off when the dial breaks in the forward direction between the cathode of the diode and the base of the transistor or the emitter path of the transistor. It is equipped with a light-receiving side element of a photocoupler.

(Example) Examples of the present invention will be described in detail below.

FIG. 4 shows one embodiment of the present invention. here,
T ₁ is the audio coupling transformer, C ₁ is the transformer's DC current blocking capacitor, 1 is the diode bridge,
21 is an electronic sink circuit which exhibits a resistance value of about 50 to 300 Ω in direct current and a value close to infinity in alternating current, and 31 is an electronic dial pulse sending circuit. Now, based on the calling operation of the terminal phone TEL (for example TEL ₁ ), the central office line is connected via the call path switch SW.
A case in which L ₁ and L ₂ are captured and then a dial operation is performed will be sequentially explained. Note that both R ₁ and R ₂ have a resistance value of about 10 kΩ, and the resistor R ₃ is, for example,
It has a resistance value of about 20Ω. The electronic sink circuit 21 is used as a DC loop forming circuit,
Direct current is required to flow, but alternating current is not required to flow. For this reason, the voltage between AB in the diagram is
The charging current divided by R ₁ and R ₂ and passed through the forward diode D ₁ is stored in the capacitor C ₂ and smoothed, and the input is used to drive the transistor Q ₁ of the sink circuit. Point D in the figure is the above-mentioned smoothing voltage. PC ₁ is a photocoupler that constitutes an optical coupling switch. When the dial signal (HIGH) is input, transistor _Q2 is turned on and the photocoupler is turned on.
PC ₁ works.

Point D shows a predetermined smoothed voltage during dial make, but when dial breaks, current i ₃ in the figure attempts to discharge through the transistor side of photocoupler PC ₁ and resistor R ₃ . When a discharge occurs, the capacitor
The charge on C ₂ disappears. In this state, when the next dial make occurs, the capacitor _C2 is charged first, so the electronic sink circuit 21 cannot be immediately driven to form a DC loop. Therefore, when the dial breaks, the photocoupler PC ₁
Turn off the transistor side so that current i ₃ does not flow. Furthermore, for stabilization, a diode _D1 is inserted to prevent current _i2 from flowing. Due to the above,
At dial break, the charge on capacitor _C2 remains charged. When the next dial make occurs, the electronic sink circuit 21 is immediately driven to form a DC loop.

Next, the electronic dial pulse sending circuit 31 will be explained. When the dial signal (HIGH) is input during dial sending, it is inverted by the inverter (INV) and the inverted output becomes LOW.

First, during dial make, when the TTL level or C/MOS level dial signal input corresponding to the dial operation is inverted by the inverter INV and becomes LOW, [Q ₃ OFF → PC ₂ OFF → Q ₄ ON → Q ₅ ON] The operation is [L ₁ → diode bridge 1 → electronic sink circuit 21 → Q ₅ → diode bridge 1 →
L ₂ ] loop is created and output as a make signal. Next, at the time of diode bridge, when the dial signal input mentioned above is inverted by the inverter INV and becomes HIGH, the operation becomes [Q ₃ ON → PC ₂ ON → Q ₄ OFF → Q ₅ OFF], and [L ₁ → electronic sink Circuit 21→Q ₅
(OFF)→L ₂ ] loop is opened and output as a break signal. In addition, the path of [collector emitter of PC ₂ → R ₄ ] enters in parallel to the loop that has become "open". If the resistance R ₄ is set to a value sufficiently higher than 100 kΩ (for example, 200 kΩ), the condition of 100 kΩ or more required for the station to detect a break can be satisfied. Note that the resistance R ₅
When is too large to drive transistor _Q5 ,
Transistor _Q5 can be made into a Darlington circuit.

In this way, by controlling the photocouplers PC ₁ and PC ₂ and the dial pulse signal simultaneously, the dial pulse signal can be sent out.

FIG. 5 shows another embodiment of the invention. This uses an electronic sink circuit 22 in which the photocoupler is moved from the base side of the transistor _Q1 as shown in the embodiment in FIG. 4 to the emitter side, and the operating principle is as shown in FIG. This is the same as in the embodiment.

(Effects of the Invention) As explained in detail above, the present invention inserts a photocoupler in the charge discharge path of the smoothing capacitor to prevent the charge from discharging, so the waveform has good rise and fall characteristics and less distortion. A dial pulse can be sent out. Therefore, it is possible to achieve the effect of eliminating large relays and large coils and realizing a current loop forming circuit using a small and inexpensive electronic circuit.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a conventional current loop forming circuit, FIGS. 2 and 3 a and 3 b are circuit diagrams showing other examples of the conventional current loop forming circuit, and FIG. FIG. 5 is a circuit diagram showing another embodiment of the present invention. 1...Diode bridge, 2, 21, 22...
...Electronic sink circuit (DC loop formation circuit), 3,
31...Dial pulse sending circuit, _L1 , _L2 ...
Office line, C ₂ ... Smoothing capacitor, PC ₁ , PC ₂ ... Photocoupler (optical coupling switch circuit), Q, Q ₁ ,
Q ₂ , Q ₃ , Q ₄ , Q ₅ ... Transistor, RA ... Incoming call detection circuit, SW ... Call path switch, TEL ₁ ...
TEL _o ...terminal telephone.

Claims (1)

[Claims] 1.Equipped with a diode bridge for converting DC current from a central office line into a DC output with a constant polarity, and a dial pulse sending circuit placed on the output side of the diode bridge, the output of the dial pulse sending circuit is A series circuit of a first resistor and a second resistor and a collector-emitter path of a transistor forming a passing current loop are connected in parallel to the connecting point of the first resistor and the second resistor. A diode is disposed between the base of the transistor in the forward direction with respect to the base current of the transistor, and a capacitor for smoothing the alternating current signal appearing at the base terminal of the transistor is connected to the cathode side of the diode. A current loop forming circuit comprising a light-receiving side element of a photocoupler configured to be controlled to be turned off at the time of a dial break in the forward direction between a cathode and a base of the transistor or an emitter path of the transistor.