JPH07121029B2 - Cable broadcasting phone - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a cable broadcasting telephone commonly connected to one line.

2. Description of the Related Art FIG. 3 shows conventional cable broadcasting telephones 21a and 21b commonly connected to the lines L ₁ and L ₂ from the cable broadcasting exchange 1.

The telephones 21a and 21b respectively have a secret-talk circuit (T) 22 for preventing crosstalk to other telephones with respect to the lines L ₁ and L _2.
a, 22b, a call circuit (NET) 23a, 23b, which is a hybrid circuit connecting a transmitter and a receiver (not shown), and a hook switch (HS) 24a, 24b are connected in parallel, and a frequency selection circuit is connected. The receiving circuits (SEL) 25a and 25b including the hook switches 24a and 24b are connected in parallel.

In the above configuration, for example, when the handset of the telephone 21a is hooked off and the hook switch 24a is turned on, the fourth
As shown in the figure, the loop current from the exchange 1 flows through the line L ₁ to the confidential circuit 22a of the telephone 21a, the communication circuit 23a, the hook switch 24a, and the line L ₂ to form a loop.

When the exchange 1 detects the voltage drop due to this loop formation, it reverses the polarities of the lines L ₁ and L ₂ as shown in FIG.
It supplies a call current to the telephone 21a and sends a dial tone.

In this polarity state, if the other telephone 21b is hooked off and the hook switch 24b is turned on, the line
Since the voltage of L ₁ and L ₂ has dropped, the handset of the telephone 21b becomes silent due to the confidential circuit 22b.

Incidentally, the exchange 1 sends a dial tone after reversing the polarities of the lines L ₁ and L ₂ and then, when the handset of the telephone 21a is continuously removed, a call end signal is given as shown in FIG. After being sent, the busy tone is superimposed on the loop current and sent.

The polarities of the lines L ₁ and L _{2 in} this discussion are as shown in FIG.
Since the polarity is the same as that of the telephone 21a before the call is made, the loop between the telephone 21a and the exchange 1 is not released.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in the above-mentioned conventional cable broadcasting telephone, since there is no function of detecting the handset disconnection, it is configured to rely on the line cutout function on the extension line of a normal single telephone. However, other telephones have a problem that the line cannot be used even when the telephone is off-hook.

In view of the above conventional problems, it is an object of the present invention to provide a cable broadcasting telephone in which other telephones can use the line even if there is a telephone without a handset.

Means for Solving the Problems In order to achieve the above-mentioned object, the present invention comprises a current control circuit for increasing the loop resistance with an exchange, and this current control is performed when a call end signal from the exchange is detected. The circuit is designed to open the exchange.

With the above configuration, the present invention allows the exchange to be opened even if there is a telephone with the handset removed, so that another telephone can use the line.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. First
1 is a block diagram showing an embodiment of a cable broadcasting telephone according to the present invention, and FIG. 2 is a waveform diagram showing main signals in FIG.

In FIG. 1, reference numeral 4 is a cable broadcasting exchange to which the two-line lines L ₁ and L ₂ are connected, and 51a and 51b are telephones connected in parallel to the lines L ₁ and L ₂ , respectively.

The telephones 51a and 51b respectively have a secret communication circuit (T) 52 for preventing crosstalk to other telephones with respect to the lines L ₁ and L _2.
a, 52b and a current control circuit (I) 53a, 53b for making the loop resistance of the exchange 4 high, and a communication circuit (NE) which is a hybrid circuit connecting a transmitter and a receiver (not shown).
T) 54a, 54b and hook switches (HS) 55a, 55b are connected in parallel, and incoming circuits (SEL) 56a, 56b including frequency selection circuits and hook switches 55a, 55b are connected in parallel. To be done.

Further, the telephones 51a and 51b are respectively provided with current control circuits 53a and 53b.
Busy tone sending circuits (BT) 56a, 56b for sending a busy tone to its own receiver via b, confidential talk circuits 52a, 52b, current control circuits 53a, 53b, call circuits 54a, 54b, busy Sound transmission circuit
It has microcomputers 58a and 58b for controlling 56a and 56b as described later.

Next, the operation of the above embodiment will be described with reference to FIG.

For example, when the handset of the telephone 51 is hooked off and the hook switch 55a is turned on, the microcomputer 58a
Receives the electric power from the exchange 4 from the lines L ₁ and L ₂ and puts the confidential circuit 52a and the current control circuit 53a into the through state with respect to the communication path 54a.

Therefore, the loop current from the exchange 1 flows through the line L ₁ to the confidential circuit 52a of the telephone 51a, the current control circuit 53a, the communication circuit 54a, the hook switch 55a, and the line L ₂ to form a loop.

In this case, the confidential circuit 52a, the current control circuit 53a, and the communication circuit 54a are configured so that the loop resistance is about 200 to 500Ω.

When the exchange 4 detects the voltage drop due to this loop formation, it reverses the polarities of the lines L ₁ and L ₂ as shown in FIG.
It supplies a call current to the telephone set 51a and sends a dial tone.

In this case, the microcomputer 58a controls the confidential circuit 52a so that the loop can be maintained even if the polarities of the lines L ₁ and L ₂ are reversed, and the other telephone 51b is off-hooked and hook-switched. If 55b is turned on,
The microcomputer 58b detects that the polarity of the line L _1, L ₂ is inverted, by activating the busy tone transmission circuit 57b without the scrambler circuit 52b through state, interrupts the call telephone 51a Prevent that.

In this polarity state, when the handset of the telephone 51a is continuously off-hook for a predetermined time, the exchange 4 sends a call end signal (time t) as shown in FIG.

Upon detecting the time t of the non-polarized call end signal, the microcomputer 58a controls the current control circuit 53a to become a high resistance circuit so that a minute current of about 0.8 mA flows and the busy current is sent by this minute current. The circuit 57a is activated. Therefore, the busy tone is sent to the receiver via the current control circuit 53a and the communication path 54a.

When the loop current becomes very small, the exchange 4 restores the polarities of the lines L ₁ and L ₂ to the original state and restores them.

Therefore, according to the above-described embodiment, even if the handset of the telephone 51a is removed, the loop is set to a high resistance to open the exchange 4, so that the other telephone 51b can use the line and effectively use the line. can do.

EFFECTS OF THE INVENTION As is apparent from the above-described embodiment, the present invention forms a loop between the telephone circuit of the exchange and the telephone when the hook switch is on, and forms a loop when the hook switch is off. Since the switch is not formed, turning on the hook switch directly closes the communication circuit to the line of the exchange, so that even if there is a telephone with the handset removed, it is possible to make the circuit configuration compact so that other telephones can use the line.

It is possible to prevent the other telephone from interrupting the call of the other telephone when the telephone is in a call without removing the handset.

Has the effect.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a cable broadcasting telephone according to the present invention, FIG. 2 is a waveform diagram showing main signals in FIG. 1, and FIG. 3 is a conventional cable broadcasting telephone. The block diagram shown in FIG. 4 and FIG. 4 are waveform diagrams showing the main signals in FIG. 4 ... Switch, 51a, 51b ... Telephone, 53a, 53b ... Current control circuit (I), 57a, 57b ... Busy tone sending circuit (BT), 58a, 58b ... Microcomputer.

Claims (2)

[Claims] 1. A switching system connected to a plurality of telephones by a line, a hook switch provided on the telephone, a call circuit connected to a handset of the telephone, and the switch and the calling circuit when the hook switch is turned on. A first loop is formed, and when the hook switch is turned on, a high current is detected when a call end signal is detected from the exchange, and a current control circuit that opens the exchange and the hook switch is turned off together with the exchange. A cable broadcasting telephone provided with a call circuit and an incoming circuit forming a second loop which does not include the current control circuit. 2. A busy tone sending means for sending a busy tone to its own handset and the same circuit as the call circuit are connected, the hook switch of another telephone is turned on, and the exchange is not opened. 2. The cable broadcasting telephone according to claim 1, further comprising: a secret communication circuit for preventing the loop current from flowing through the loop and activating the busy tone sending means when the detection is made.