Direct inward-outward dialing trunk circuit

Details Direct inward-outward dialing trunk circuit Direct inward-outward dialing trunk circuit

1998-01-20

2002-11-12

Isen, Forester W. (Department: 2644)

Telephonic communications

Plural exchange network or interconnection

Interexchange signalling

C379S240000, C379S379000, C379S198000, C379S233000, C379S244000, C379S142030, C379S252000, C379S196000, C379S211020, C379S216010

Reexamination Certificate

active

06480596

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a trunk circuit for use in a private automatic branch exchange (PABX), and more particularly to a Direct Inward-Outward Dialing (DIOD) trunk circuit for bidirectional interfacing between a private automatic branch exchange (PABX) and a central office exchange (CX).
2. Related Art
Private automatic branch exchange (PBX) systems as described, for example, in U.S. Pat. No. 4,028,498 for Private Automatic Branch Exchange System And Apparatus issued to Mehaffey et al, U.S. Pat. No. 4,941,171 for Electronic Private Branch Exchange issued to Hasegawa, and U.S. Pat. No. 5,291,549 for Private Branch Exchange And Line Exchange Method issued to Izumi, are well known to utilize the step-by-step and crossbar switching techniques for providing switched communications between a plurality of extension subscribers and a central office exchange (CX). Trunk circuits are required to interface a number of trunk lines from the central office exchange (CX) to a number of telephone lines within the private automatic branch exchange (PBX). Typical trunk circuit can be application specific such as direct inward dialing (DID) trunk, direct outward dialing (DOD) trunk, signaling trunk and operator access trunk, or universal in nature to provide all of the necessary features as disclosed, for example, in U.S. Pat. No. 3,748,396 for Direct Inward Dialing Trunk Circuit issued to Hestad et aL, U.S. Pat. No. 3,806,660 for Trunk Circuit Having Selective Interface Combinations issued to Gueldenpfennig et al., U.S. Pat. No. 4,049,919 for Communication System Trunk Circuit issued to Young, U.S. Pat. No. 4,310,725 for Interface For Private Branch Exchange issued to Mehaffey, U.S. Pat. No. 4,776,007 for Solid State Trunk Circuit issued to Styrna et al., U.S. Pat. No. 4,894,860 for Trunk Interface Circuitry For Telephone Systems issued to Korsky et al., and U.S. Pat. No. 4,914,690 for Universal PBX Interface issued to Hagedorn.
For example, a DID trunk such as disclosed in U.S. Pat. No. 3,748,396 is used to allow a calling subscriber from a distant office to call a predetermined extension of the PABX through the central office exchange (CX) directly without an operator. When a calling subscriber dials the extension of the called subscriber of the private automatic branch exchange (PABX), the central office exchange (CX) downloads the extension of the called subscriber on a DID trunk and the PABX receives the extension number to route the call directly to the called subscriber. However, the DID trunk cannot be seized for direct outgoing calls from extension subscribers of the PABX to a distant office through the central office exchange (CX). A loop start trunk can be used for outgoing calls from extension subscribers of the PABX to a distant office through the central office exchange (CX). Both the DID trunk and loop start trunk can be implemented for handling incoming calls and outgoing calls. However, the combined trunk circuit requires construction of large inductance coils exhibiting high impedance and capacitors exhibiting high capacitance which are bulky and cost. A special trunk circuit, known as a direct inward-outward dialing (DIOD) trunk is circuit, can also be constructed for automatic operation on both incoming and outgoing calls such as disclosed in the IBM Technical Disclosure Bulletin, Vol. 11, No. 1, Jun. 1968.
Conventional DIOD trunk circuit which is commonly incorporated into a private automatic branch exchange (PABX), is illustrated in FIG.
1
. This private automatic branch exchange (PABX) is exemplary of an European PTT ALS-70 system which generally includes a central processing unit (CPU)
10
, a memory
20
, an interface circuit
30
, at least an extension line subscriber circuit
40
, a switching circuit
50
, a tone signal generator
60
, a dual-tone multi-frequency (DTMF) receiver
70
, a DTMF transmitter
80
, a DIOD trunk circuit
90
, a tie line circuit
100
, an extension subscriber telephone
120
and a ring generator
130
connected to the extension line subscriber circuit
40
. The CPU
10
controls call switching operations of incoming and outgoing calls via the DIOD trunk circuit
90
connected to a central office line and the tie line circuit
100
, and provides a variety of services to a system user. Memory
20
stores a program that enables the CPU
10
to perform call processing and a variety of other system functions. Switching circuit
50
switches various tone signals and voice signals under the control of the CPU
10
. Subscriber circuit
40
transmits and receives a variety of tone signals and voice signals to and from an extension telephone
120
of an extension subscriber, selectively connects the extension telephone
120
to a central office C.O. line through the DIOD trunk circuit
90
, supplies power to the extension telephone
120
, and also acts as an interface between the extension telephone
120
and the switching circuit
50
. Ring generator
130
generates a ring signal that is supplied to the extension telephone
120
through the subscriber circuit
40
. Tone signal generator
60
generates various tone signals that are supplied to the switching circuit
50
under the control of the CPU
10
. DTMF receiver
70
is connected to the switching circuit
50
for analyzing a dual-tone multi-frequency (DTMF) signal generated from the extension telephone
120
. DTMF transmitter
80
generates and supplies the DTMF signal to the switching circuit
50
under the control of the CPU
10
. The interface circuit
30
serves to interface the subscriber circuit
40
, the tone signal generator
60
, the DTMF receiver
70
, the DTMF transmitter
80
, the DIOD trunk circuit
90
and the tie line circuit
100
to the switching circuit
50
.
FIG. 2
illustrates the conventional DIOD trunk circuit
90
which is connected to a central office C.O. line from a central office exchange (CX) and serves as an interface to the private automatic branch exchange (PABX). As shown in
FIG. 2
, the conventional DIOD trunk circuit
90
is constructed of an overvoltage protection circuit
91
, a high impedance generation circuit
92
, a backward loop sensing circuit
93
, a forward loop sensing circuit
94
, a large core transformer
95
, and a series of large size coils L
1
and L
2
with high impedance and large size capacitors which are comparably expensive and add to the cost of the unit. For example, in order to adjust the balance of a transmission signal on the telephone line, capacitors C
3
and C
4
must have capacitance values of at least 1 &mgr;F/100V. The requirement of large inductance coils and high capacitors, as I have observed, is cost prohibitive and bulky. Accordingly, an improved DIOD trunk circuit for interface between a private automatic branch exchange (PABX) and a central office exchange (CX) can still be contemplated.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved trunk circuit for interface between a private automatic branch exchange (PABX) and a central office exchange (CX).
It is also an object to provide a direct inward-outward dialing (DIOD) trunk circuit for a private automatic branch exchange (PABX) designed to minimize the use of coils and capacitors with high capacitance in order to reduce cost and increase production.
It is another object of the present invention to provide a direct inward-outward dialing (DIOD) trunk circuit for a private automatic branch exchange capable of handling incoming direct inward dialed (DID) telephone calls and outgoing direct outward dialed (DOD) telephone calls without an operator.
These and other objects of the present invention can be achieved by a direct inward-output dialing trunk circuit which comprises a tip T and ring R terminals which are connected to a central office line with a battery source of −48 volts D.C. potential; and first and second transformers T
1
and T
2
connected in series and having a primary coil respectively connected to the tip T and ring R terminals and a secondary

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