Digital key telephone connecting apparatus and method in a digital key telephone system

In connecting a first digital key telephone to a second digital key telephone in a digital key telephone system, an additional connection module provides data transmission for a call between the first digital key telephone and the second digital key telephone. Synchronizing signal terminals and a B clock terminal of a transmitter of the connection module are connected to a transmitter of the first digital key telephone. Data transmission and receiving terminals, a D clock terminal and an input/output control port of a microprocessor of the first digital key telephone are also connected to the transmitter of the connection module. A transformer is connected to the second digital key telephone through a transmission line. The transformer is connected to transmission input and output terminals of the transmitter of the connection module through capacitors and diodes.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.119 from an application 
for Digital Key Telephone Connecting Apparatus And Method In A Digital Key 
Telephone System earlier filed in the Korean Industrial Property Office on 
8 Oct., 1994 and there assigned Ser. No. 25803/1994. 
BACKGROUND OF THE INVENTION 
The present invention relates to a method for connecting digital key 
telephones in a digital key telephone system, and more particularly to a 
method for connecting a digital key telephone to another digital key 
telephone by installing an additional connection module in a digital key 
telephone system. 
With Integrated Services Digital Network (ISDN) communication techniques, B 
and D data channels can be used to effectively transmit data. One prior 
art reference exemplary of this technique is disclosed in U.S. Pat. No. 
5,150,402 entitled ISDN Terminal Having Diagnostic Function issued to 
Yamada on 22 Sep., 1992. In Yamada '402, a terminal adapter section has a 
data rate conversion function for communication with a terminal of a 
distant station connected to the integrated services digital network. 
While Yamada '402 has merit in that its terminal adapter is capable of 
performing a diagnostic function to, for example, read out and change or 
set optional functions, I believe that it can be further improved to 
provide the user with a simplier and more efficient means by which to add 
new users to the system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved method and apparatus for connecting digital key telephones in a 
digital key telephone system. 
It is another object to provide a method and apparatus for connecting 
digital key telephones by simply installing an additional module in a 
digital key telephone system. 
It is still another object to provide a method and apparatus for extending 
transmission lines in a digital key telephone system without requiring 
additional line work, to thereby operate a more economical system. 
These and other objects can be provided according to the principles of the 
present invention with an apparatus for connecting a first digital key 
telephone to a second digital key telephone in a digital key telephone 
system using an additional module for providing data transmission between 
the first digital key telephone from the second digital key telephone. 
Synchronizing signal terminals and a B clock terminal of a transmitter of 
the additional connection module are connected to a transmitter of the 
first digital key telephone. Data transmission and receiving terminals, a 
D clock terminal and an input/output control port of a microprocessor of 
the first digital key telephone are also connected to the transmitter of 
the connection module. A transformer is connected to the second digital 
key telephone through a transmission line, and the transformer is 
connected to transmission input and output terminals of the transmitter of 
the connection module through capacitors and diodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the drawings and referring to FIG. 1, a conventional digital 
key telephone system connecting a standard telephone is shown. A 
conventional telephone subscriber interface circuit 101 includes a codec 
112 that performs a digital-to-analog (D/A) and an analog-to-digital (A/D) 
conversion process and a subscriber line interface circuit (SLIC) 111 
connected between codec 112 and a telephone handset. A time switch 115, 
which transmits and receives electrical signals to and from a main central 
processing unit (CPU) 114, is connected to codec 112 via a pulse code 
modulation (PCM) highway. Subscriber line interface circuit (SLIC) 111 
receives signals from main central processing unit (CPU) 114 through a sub 
central processing unit (CPU) 113. 
Recently, a time switch using a time compressed multiplexing (TCM) system 
has been widely used, instead of a cross point, to connect a call in a 
private branch exchange (PBX) or a digital key telephone system, such as 
the one shown in FIG. 1. The time switch has a direct effect on the line 
according to its capacity. In the past, a voice signal exchange function 
was largely utilized. With increased user demand for a variety of 
services, however, the necessity for data exchange is being increasingly 
expressed. Referring now to FIG. 2, a digital transmission system using a 
2B+D structure is shown. This structure is a basic system widely used in 
Integrated Services Digital Network (ISDN) communication techniques and 
applied techniques therefor have been published. 
With the 2B+D digital transmission protocol, a voice signal and data can be 
simultaneously transmitted and transmission speed is remarkably fast as 
compared to an analog system. The 2B+D designation refers to the fact that 
the structure is constructed with B1, B2 and D channels. In the digital 
key telephone system, since a data highway is allocated to the B1 and B2 
channels, the B1 and B2 channels can be used without being restricted to 
the voice signal or data. If either the B1 or B2 channel is used, one is 
used for voice transmission and the other is reserved. If both the B1 and 
B2 channels are used, they are typically used for voice/data transmission, 
but they may also be used for data/data transmission or voice/voice 
transmission. In FIG. 2, a microprocessor 201 is connected to a key pad 
and light emitting diode (LED) 202 and a liquid crystal display (LCD) 203. 
Microprocessor 201 is further connected to a transmitter 206 via the D 
channel, and voice codec of B1 channel 205 and voice codec of B2 channel 
204. Transmitter 206 is connected between lines L1 and L2 and voice codec 
of B1 channel 205 and voice codec of B2 channel 204. A power source 207 
provides power to lines L1 and L2. 
Referring now to FIG. 3, the voice/voice signal transmission using the 2B+D 
digital transmission system is processed in a digital key telephone 300. A 
transmitter 313 is operated pursuant to the control of a one-chip 
microprocessor 315 within digital key telephone 300. For speech 
processing, a codec 312 for processing a voice signal of the B1 and B2 
channels allocates one B channel. Codec 312 is also connected to a 
telephone handset. 
Transmitter 313 of the digital key telephone system shown in FIG. 3 is 
connected to a subscriber interface circuit 301 through a transformer 314 
and two lines (2B+D) 316. A voice signal and data switched by a digital 
time switch 321 connected to a main central processing unit (CPU) 320 are 
transmitted to the digital key telephone 300 together with a power source 
through a transmitter 318, a transmission transformer 317 and the two 
lines (2B+D) 316. A sub central processing unit (CPU) 319 is connected to 
receive signals from transmitter 318 and main central processing unit 
(CPU) 320. Transmitter 313 of the digital key telephone 300 extracts a 
signal coded to alternate mark inversion (AMI) and separates the channel. 
A pulse code modulation (PCM) voice signal separated to the B1 channel is 
converted into an analog signal through the codec 312 within digital key 
telephone 300. 
In the conventional 2B+D digital transmission system, there is a 
disadvantage, however, in that the B2 channel is restricted only within 
the digital key telephone, although the channel may be used for the voice 
signal or data transmission. 
Referring now to FIG. 4, a circuit diagram showing a connection module 855 
between a digital key telephone 400 and another digital key telephone 857 
according to the principles of the present invention is illustrated. In 
FIG. 4, a transmitter 801 has the same function as transmitter 402 in a 
digital key telephone 400. Terminals B.sub.X and B.sub.R of the B channel 
of transmitter 801 are connected to the terminals B.sub.X and B.sub.R of 
transmitter 402 through 3-state buffers ST1-ST4, while respective BLCK 
terminals are connected to each other. Terminals FSA, FSB and FSC 
represent synchronization signal terminals. Terminals D.sub.X, D.sub.R and 
DCLK of the D channel of transmitter 801 and transmitter 402 are connected 
to a microprocessor 404. Transmitting and receiving terminals Lo and Li of 
transmitter 402 are connected to a main system through a resistor 614 and 
a capacitor 611, and a capacitor 612, respectively, and through diodes 
613, and a transformer 401. Transmitting and receiving terminals Lo and Li 
of transmitter 801 are connected to a transmitter 810 through a resistor 
803 and a capacitor 804, and a capacitor 805, respectively, and through 
diodes 806, a transformer 807, a transmission line 821, and a transformer 
808 of another digital key telephone 857. The internal configuration of 
digital key telephone 857 is the same as that of digital key telephone 
400. That is, both digital key telephones 400 and 857 respectively include 
components such as codecs 403 and 812, microprocessors 404 and 814, key 
pads and light emitting diodes (LEDs) 414 and 816, and liquid crystal 
displays 415 and 818. Thus, the digital key telephone 857 can be used 
through the affordable B channel of digital key telephone 400. If a 
digital data transmission device is connected instead of digital key 
telephone 857, however, data transmission between digital transmission 
devices can be performed through the B2 channel of digital key telephone 
400. 
FIGS. 5a-5e illustrate B channel interfacing of transmitter 402 of FIG. 4 
wherein pulse code modulation (PCM) data is conveyed to the B1 and B2 
channels. 
In FIG. 4, digital key telephone 400 is a master and digital key telephone 
857 is a slave. The master performs alternate mark inversion (AMI) coding 
of the pulse code modulation (PCM) data of the B2 channel by use of 
transmitter 801 of connection module 855, and the coded signal is powerfed 
through transformer 807. That is, to use the digital key telephone 857 as 
the slave, the transmitter 802 of connection module 855 functions as the 
master corresponding to voice or data transmission. In other words, two 
lines are connected between the digital key telephone system and the 
digital key telephone. Transmitters 402 and 810 of the digital key 
telephones 400 and 857, respectively, and the transmitter 801 of the 
connection module 855 are model "TP3403" manufactured by NATIONAL 
SEMICONDUCTOR Co., U.S., which is a subscriber loop digital adaptor 
(SLDA), widely used for 2B+D interface transmission equipment in the 
integrated services digital network (ISDN). 
In a connection between digital key telephone 400 (hereinafter, referred to 
as a "masterphone") and digital key telephone 857 (hereinafter, referred 
to as a "slavephone"), if the connection module 855 is installed to use 
the slavephone, transmitter 801 is initialized by a control signal 
generated from an input/output (I/O) control port of microprocessor 404 of 
the masterphone. If the slavephone is connected to the connection module 
855, an interrupt signal is generated from transmitter 810 of the 
slavephone to adjust synchronization for the transmission of voice data 
between transmitter 801 of the connection module 855 and transmitter 810 
of the slavephone. If the synchronization for mutual data transmission is 
performed, a physical link is set up from a first layer and a data link is 
set up from a second layer. If all the links are set up, the transmitter 
801 of the connection module 855 notifies the masterphone of information 
about the slavephone. 
Microprocessor 404 of the masterphone, which receives the information about 
the slavephone, informs the system through transmitter 402 that the 
slavephone is installed. The system transmits the information about the 
slavephone to the slavephone through the B channel, which is not used in 
the masterphone, and transmitter 801 of the connection module 855. In this 
case, a receiving signal of the system is not directly transmitted to the 
slavephone from the transmitter 402 of the masterphone. The transmitter 
402 of the masterphone receives the signal from the system and converts 
the signal into a format transmitted thereto from the system. The 
transmitter 801 of the connection module 855 receives the converted signal 
and converts the signal into a format transmitted thereto from the system. 
That is, the slavephone functions as if it is directly connected to the 
system. 
When the masterphone transmits data, since the signal of the B1 and B2 
channels is transmitted through the transmitter 402 from the 
microprocessor 404 of the masterphone, and is switched in the transmitter 
801 of the connection module 855, neither the slavephone nor the 
masterphone is altered in the function of the digital key telephone. 
Devices with a transmitter of the same form as the digital key telephone 
857 can be connected to the connection module 855. For example, liquid 
crystal display (LCD) telephones, basic telephones, add-on modules (AOMs 
which are DSS/BLF), door phone interface modules (DPIM), serial interface 
modules (SIMs which are modules capable of connecting a printer, a 
personal computer, etc.), and the like can be connected. 
As described above, if there is need to extend lines in a digital key 
telephone system, due to user demand or an increase in the number of 
users, it is economically favorable to use the additional connection 
module of the present invention instead of a digital telephone interface 
board. After installing the system, the lines can be simply extended 
without additional line work. Moreover, since digital key telephones, 
add-on modules, data connection equipment, door phone connection 
equipment, etc. can be selectively used, a wide variety of effective 
system operation can be achieved. 
While there have been illustrated and described what are considered to be 
preferred embodiments of the present invention, it will be understood by 
those skilled in the art that various changes and modifications may be 
made, and equivalents may be substituted for elements thereof without 
departing from the true scope of the present invention. In addition, many 
modifications may be made to adapt a particular situation to the teaching 
of the present invention without departing from the central scope thereof. 
Therefore, it is intended that the present invention not be limited to the 
particular embodiments disclosed as the best mode contemplated for 
carrying out the present invention, but that the present invention 
includes all embodiments falling within the scope of the appended claims.