KEY TELEPHONE SYSTEM

Abstract: A key telephone system includes a plurality of telephone sets, each having a hold button for temporarily holding a central office line, a plurality of call park lamps for temporari­ ly indicating that the central office line is being held, a plurality of call park buttons associated individually with the call park lamps, an outgoing call button for seizing an idle one of the central office lines when an outgoing call is to be initiated, and an answer button for seizing one of the central office lines over which an incoming call is being received so as to respond to said incoming call; at least one external indicator for indicating the states of all of the central office lines; and a central unit having hold control means for indicating with the call park lamps the central office line being held responsive to an operation of the hold button and controlling a response to that central office line in response to an operation of the call park button, central office line selecting means for selecting and seizing an idle one of the central office lines, in response to an operation of the outgoing call button and one of the central office lines over which an incoming call is being received in response to an operation of the answer button, and central office line indication control means for controlling the external indica­ tor to indicate the states of the central office lines.

Description: This invention relates to a key telephone system (hereunder briefly referred to as KT system). DESCRIPTION OF THE PRIOR ART A KT system is essential for a connection between one of a plurality of telephone sets and a selected one of a plurality of central office (CO) lines from a central office. A prior art KT system includes a plurality of telephone sets and a central unit adapted to selectively set up connection between the telephone sets and the CO lines. Each of the telephone sets is provided with a hold button and plural sets of CO lamps and CO buttons, each set corresponding one-to-one to the CO lines accommodated in the central unit. The CO lamp shows the user of the KT system a status of its associated CO line; it turns off while the CO line is idle, turns on while the CO line is in use, flashes slowly while the CO line is held, and flashes rapidly while an incoming call is received over the CO line. The CO buttons are available for selecting and seizing any of the CO lines. The central unit serves to detect an operation of the CO buttons which may occur at any telephone set so as to connect the telephone set to the selected CO line. For example, to make an outgoing call, the CO button is operated to seize the idle CO line, while, to answer a incoming call, the CO button is operated to seize the CO line over which the incoming call is being received. Further, while a certain CO line which has been connected to a certain telephone set is held by the hold button, it may be caught at another telephone set through the operation of the CO button thereof associated with that CO line. Stated another way, the CO line over which a incoming call is received may be transferred from one telephone set to another. An example of such a KT system of the type described is disclosed in U.S. Patent No. 4,132,860. The problem with such a prior art KT system is that the number of the CO lamps and that of the CO buttons on each station set have to be increased in proportion to the number of the CO lines, rendering the telephone set bulky and expensive. In the above-described prior art KT system, to catch an idle CO line for initiating an outgoing call or to seize a CO line for answering an incoming call which is received over that CO line, the user at the telephone set needs to check the CO lamps and then depress one of the CO buttons which is associated with the CO line. A KT system which for more efficient operation allows the telephone set to automatically seize a predetermined one of the CO lines as a prime line when a handset threrof is taken up is proposed in U.S Patent No. 4,196,316. This automatic acquisition scheme, however, still has a problem that while the prime line is in use, it is necessary to select and seize another CO line by awkward operation. Especially, concerning an incoming call which usually requires rapid response, discriminating the receiving CO line instantaneously is so difficult that the user is apt to interrupt a circuit in use due to an inadvertent-operation of the CO buttons. Such erroneous operation of the CO buttons in reception also occurs frequently while a circuit in a hold condition exists, as a result of misperception of flashing conditions of the CO lamps. In Systems Technology No. 21 of June 21 1975 there is described on pages 2 to 8 in an article by G.H. King of Plessey Telecommunications entitled "A new house telephone exchange system" an arrangement employing 16 buttons for intercom calls and other buttons and indicators for providing functions such as "answering", "seizing" and "holding" exchange-line calls and for supervising-calls. In Canadian Patent No. 1,146,245 in the name of Mitel Corporation, there is described a keyless and indicatorless local telephone switching system which seeks to dispense with key telephone sets. It is an object of this invention to eliminate the disadvantages particular to the prior art KT system described above and provide a KT system which promotes efficient operation. It is another object of this invention to provide a KT system equipped with telephone sets which are small size and inexpensive. The above objects are achieved by an arrangement according to Claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned objects and features of this invention can be more clearly understood from the following detailed description and attached drawings in which: FIG. 1 is a block diagram of an embodiment of this invention; FIGS. 2, 3 and 4 each shows a part of the embodiment of FIG. 1; FIGS. 5 and 6 respectively are a block diagram and an external view of a telephone set used in the embodiment of FIG. 1; FIG. 7 is a block diagram of the embodiment of FIG. 1 in detail; FIG. 8 is a block diagram representative of data exchange achieved in FIG. 7; FIG. 9 shows details of a data exchanging memory; FIG. 10 is a timing chart demonstrating details of the data exchange in FIG. 8; FIG. 11A shows details of a memory adapted to store states of CO lines; FIG. 11B shows correspondence between data stored in the memory of FIG. 11A and states of CO lines; FIG. 12A shows details of a memory adapted to store CO lines being held; FIG. 12B shows correspondence between data stored in the memory of FIG. 12A and CO line numbers; and FIGS. 13, 14, 15 and 16 are flowcharts demonstrating respectively an outgoing call initiating procedure, an incoming call receiving procedure, a holding procedure, and a hold cancelling procedure in the embodiment of FIG. 1. In these drawings, the same reference numerals denote the same structural elements. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an embodiment of this invention comprises a central unit 9 accommodating a plurality of CO lines L1 to L8, a plurality of telephone sets T1 to T24, and a plurality of external CO line indicators D1 to D4 adapted to display various states of the CO lines L1 to L8. The central unit 9 includes a control unit 10 which supervises the whole KT system, a CO line interface 15 accommodating the CO lines L1 to L8, a switching network 16, a telephone interface 17, and a CO line indication interface 18. Connected to a central office, the CO lines L1 to L8 are each provided with a particular subscriber's number. The interface 15 plays the roles of detection an incoming call (ring detection) on any of the CO lines L1 to L8, sending dial signals to any of the CO lines L1 to L8, and analog/digital converting of voice signals. As shown in FIG. 2, the interface 15 comprises interfaces 151 and 152 which accommodate respectively the CO lines L1 to L4 and the CO lines L5 to L8. The telephone interface 17 comprises a voice interface 171 connecting to voice lines of the respective telephone sets, and a data interface 172 connecting to data lines of the same telephone sets. The voice interface 171 and the data interface 172 are connected to the switching network 16 and the control unit 10, respectively. The interface 171 performs digital/analog conversion of voice signals. As shown in FIG. 3, the interface 172 comprises interfaces 1721, 1722, ..., 1726 respectively are connected to the telephone sets T1 to T4, T5 to T8, ..., T21 to T24. The switching network 16 serves to set up communication paths between the CO lines L1 to L8 and the telephone sets T1 to T24 on a time division basis responsive to a control signal applied from the control unit 10. A holding function of the CO lines is assigned to the network 16. The CO line indication interface 18 fetches display data from the unit 10 so as to display states of the respective CO lines on the CO line indicators D1 to D4. As shown in FIG. 4, each of the indicators D1 to D4 has CO lamps 180 respectively associated with the CO lines L1 to L8 and fixed to a panel 181. The CO lamps 180 show an "idle" state, a "use" state, a "hold" state and an "incoming" state of their associated CO lines by turning off, turning on, flashing slowly, and flashing rapidly, respectively. The indicators D1 to D4 may be installed one in each room in a location which is easy to see and the number thereof is open to choice. Analog signals are propagated over the CO lines L1 to L8, voice lines between the telephone sets T1 to T24 and the voice interface 171, and lines between the CO line indication interface 18 and the indicators D1 to D4. Digital signals are propagated over the other lines, i.e., lines installed in the control unit 10. Referring to FIG. 5, the telephone sets T1 to T24 each comprises a data transmitting/receiving circuit 30 connected to the data interface 172 to interchange data with the central unit 9, a button detector 31 for detecting the operation of a button unit 36, a button data converter 34 for converting detected button operation data to codes assigned one to each of buttons and sending the codes to the circuit 30, a lamp data converter 35 for transforming lamp data received by the circuit 30 to a particular code, a lamp controller 32 for controlling lamps in a lamp unit 37 responsive to the output code from the converter 35, and a voice circuit 33 connecting to the voice interface 171. Referring also to FIG. 6, the button unit 36 comprises call park buttons 22, a hold button 23, an outgoing call button 24, a CO line answer button 25, and dial buttons 28. The lamp unit 37 comprises call park lamps 21, an outgoing call lamp 26, and a CO line answer lamp 27. The voice circuit 33 includes a handset 20. The outgoing call button 24 is used to seize an idle CO line for initiating an outgoing call, while the outgoing call lamp 26 glows in green when an idle CO line is seized and in red when no idle CO line is available. The answer button 25 is used to answer a CO line over which an incoming call has come in. The answer lamp 27 paired with the button 25 glows in red when an incoming call has come in over a CO line and in green when a user has responded to a CO line which is receiving an incoming call or which is being held. The hold button 23 is operated to hold a CO line on which a call is in progress. The call park lamps 21 are adapted to show CO lines being held. Guided by the lamps 21, user may depress any of the call park buttons 22 which correspond one-to-one to the CO lines being held in order to answer a CO line being held. In this embodiment, the same number of CO lines as the call park buttons 22 can be held. Referring to FIGS. 1 and 7, the construction and operation of the control unit 10 will be described. The unit 10 includes a hold controller 12 adapted to control the hold of the CO lines and re-answer, a CO line selector 13 for selecting the CO line which is idle or receiving an incoming call and generating seizing data representative of the selected CO line, a CO line indication controller 14 for controlling the lamp indication on the external indicators D1 to D4 responsive to particular states of the CO lines, and a main controller 11 for performing various kinds of control which are assigned to the central unit 9. The main controller 11 comprises a central processing unit (CPU) 100, a read only memory (ROM) 101 which stores control programs, a random access memory (RAM) 102 having a work area 1021 therein, a data exchanging memory 103, a writing/reading I/O circuit 104, and a CO line state memory 105. Each of the memories 103 and 105 is implemented by a RAM. The controller 11 periodically sends data to and receive data from the interfaces 15 and 172 and periodically sends data to the interface 18. Such data sending and receiving are executed by the CPU 100 via the exchanging memory 103 at each interval of one frame (32 milliseconds). The data sent from the interface 15 to the memory 103 include incoming call data showing reception over the CO lines, while the data sent from the memory 103 to the interface 15 include CO line seize data and dial data. The data from the interface 172 to the memory 103 include button data outputted by the telephone sets T1 to T24, while the data fed in the opposite direction include lamp data applied to the telephone sets T1 to T24. For a period of time S1 (10 milliseconds) in the above-mentioned one frame, the CPU 100 reads all the data out of the memory 103 and writes processing data intended for the interfaces 15, 172 and 18 in the memory 103. For the remaining period of time S2 (22 milliseconds), the interfaces 15, 172 and 18 read the processing data out of the memory 103 and write data associated with the respective CO lines and the respective telephone sets in the memory 103. Referring to FIGS. 8, 9 and 10, the data exchange outlined above will be described in detail. The data exchanging memory 103 is constructed as shown in FIG. 8. The incoming call data from the interface 15 and the button data from the interface 172 are stored in areas 1033 and 1031 respectively, and each data is read out by the CPU 100. The CO line seizing data and the dial data from the CPU 100 are stored in an area 1034, the lamp data is stored in an area 1032, CO line indication data to be indicated by the indicators D1 to D4 is stored in an area 1035, and these data are read out by the interfaces 15, 172 and 18, respectively. As shown in FIG. 9, each of the discrete data (e.g. incoming call data 10331 associated with the CO line L1) has an 8 bits x 4 bytes format and an address particular thereto. The data constitute one data block (8 bits x 16 bytes) on a four CO lines or a four telephone sets basis; sixteen data blocks B1 to B16 are defined in total. For example, the data blocks B1 and B2 respectively are the incoming call data associated with the CO lines L1 to L4 and L5 to L8 which are to be sent from the interfaces 151 and 152 to the CPU 100. Likewise, the data blocks B3, ..., B8 respectively are the button data which are associated one with each four of the telephone sets, i.e. T1 to T4, ..., T21 to T24. The data blocks B9 and B10 respectively are the CO line seizing data and the dial data relating to the CO lines L1 to L4 and L5 to L8. The data blocks B11, ..., B16 are the lamp data relating one to each four of the telephone sets, i.e. T1 to T4, ...., T21 to T24. Further, a data block B17 is the CO indication data to be indicated by the CO line indicators D1 to D4 and has an 8 bits x 4 bytes format. As shown in FIG. 10, the sequence of data exchange is repeatedly performed at the period of one frame (32 milliseconds). As previously stated, one frame consists of the period of time S1 (10 milliseconds) and the period of time S2 (22 milliseconds). As shown in FIG. 10A, within the period of time S1, the CPU 100 performs a sequence of steps: writing the seizing data and the dial data (the data blocks B9 and B10) in the area 1034, writing the lamp data (the data blocks B11 to B16) in the area 1032, writing the CO indication data (the data block B17) in the area 1035, reading the incoming call data (the data blocks B1 and B2) out of the area 1033, and reading the button data (the data blocks B3 to B8) out of the area 1031. Concerning the period of time S2, 1 millisecond is assigned to each of the interfaces. As shown in FIGS. 10B and 10C, the interfaces 151 and 152 read respectively the seizing data and the dial data (the data blocks B9 and B10) associated with the CO lines L1 to L4 and L5 to L8 out of the area 1034 and write the incoming call data (the data blocks B1 and B2) in the area 1033. Next, as shown in FIGS. 10C to 10E, the interfaces 1721 to 1726 sequentially read the lamp data (the data blocks B11 to B16) relating to their associated telephone sets T1 to T24 out of the area 1032 and write the button data (the data blocks B3 to B8) in the area 1031. Finally, the interface 18 reads the CO indication data (the data block B17) out of the area 1035. Referring again to FIG. 7, the CPU 100 refers to the data stored in the exchanging memory 103 to determine the states of the CO lines L1 to L8 (idle, incoming, use, hold) and then writes state data representative of those states in the CO line state memory 105 via the I/O circuit 104. The memory 105 stores the states of the respective CO lines in separate areas, as shown in FIG. 11A. Specifically, an 8 bits x 1 byte area is assigned to each of the CO lines L1 to L8. As shown in FIG. 11B, such areas of the memory 105 individually store the state data particular to the respective CO lines in hexadecimal codes which are: "00H" representative of an "idle" state, "01H" representative of an "incoming" state, "02H" representative of a "use" state, and "03H" representative of a "hold" state. The CO line selector 13 refers to the state data stored in the memory 105 to select an idle CO line at the time of outgoing call initiation and a receiving CO line at the time of incoming call reception and, thereby, feeds the seizing data for seizing the idle CO line or the receiving CO line to the CPU 100. The seizing data is sent through the exchanging memory 103 to the interface 15 so as to seize the CO line. The CO line indication controller 14 periodically scans the state memory 105 to read out the states of the CO lines and sends them to the CPU 100 as the CO line indication data for causing the external indicators D1 to D4 to display the CO line states. The CPU 100 writes the indication data in the area 1035 of the exchanging memory 103, while as previously stated the contents of the area 1035 are read out and displayed at the interval of every 32 milliseconds. The hold controller 12 includes a CO line hold memory 121 having areas assigned to the call park buttons 22 and shared by all the telephone sets, and an I/O circuit 122 for controlling writing and reading operations of data in and out of the memory 121. As shown in FIG. 12A, in the hold memory 121, an 8 bits x 1 byte area is assigned to each of the call park buttons of the telephone set. These areas store hold data particular to their associated CO lines in hexadecimal codes such as shown in FIG. 12B, in which L1 is "10H", L2 "11H", ...., L8 "17H". So long as the call park is idle with no CO line held, data "00H" is written in the associated area. The operations for initiating an outgoing call, receiving an incoming call, and holding which may be effected on any of the telephone sets will be described together with the operation of the buttons of the telephone set and the changes in the states of the lamps. INITIATING AN OUTGOING CALL Assume a case wherein a user operates the telephone set T1 to place an outgoing call by seizing the idle CO line L7. As the user makes the telephone set T1 off-hook and then presses the outgoing call button 24, the CPU 100 of the main controller 11 discriminates reception of button data generated by the depression of the button 24, via the data interface 172 and the exchanging memory 103. This causes the CO line selector 13 to start on the control shown in FIG. 13. First, data "8" is written in the work area 1021 of the RAM 12 which is adapted to temporarily store a CO line number, a call park number and others (STEP 1301). This data "8" is used to see if the CO line L8 can be caught in the subsequent processing. If the CO line L8 cannot be caught, the decision will be sequentially repeated with the other CO lines L7, L6, ...., L1 in this order so that the content of the work area 1021, too, will be sequentially changed to "7", "6", ...., "1". Next, state data "02H" (FIG. 11A) is read out of the area of the state memory 105 which is associated with the CO line L8 (an area associated with the CO line whose number is represented by the data "8" stored in the work area 1021) (STEP 1302). Then, whether the state data "02H" is the idle state data "00H", meaning that the CO line L8 is available for initiating an outgoing call, is decided (STEP 1303). Since the CO line L8 is currently in use and, therefore, cannot be caught, the result of the decision at the STEP 1303 is "NO" so that the content of the work area 1021 is decreased by 1 (STEP 1304). This is followed by determining whether the content of the work area 1021 is "0" in order to see if all the CO lines down to L1 have been checked (STEP 1305). The result of the STEP 1305 is "NO" causing the operation to return to the STEP 1302. In the same manner, at the STEP 1302, the state data associated with the CO line L7 is read out. Since the CO line L7 is idle, "00H" (FIG. 11A), the decision at the STEP 1303 is "YES" with the result that the CO line L7 is selected as designated by the content of the work area 1021 (STEP 1307). The seizing data indicative of acquisition of the CO line L7 is fed to the CPU 100. The CPU 100 applies the seizing data to the interface 15 via the memory 103, whereby the CO line L7 is seized. At the same time, the CPU 100 controls the switching network 16 to set up a communication path between the telephone set T1 and the CO line L7. Further, the CPU 100 sends lamp data for the telephone set T1 to the exchanging memory 103 within the period of time S1, shown in FIG. 10. The data is read by the interface 172 and then discriminated by the lamp data converter 35. The lamp controller 32 causes the outgoing call lamp 26 to glow in green. Thereafter, state data "02H" representative of a use state is written in the area of the state memory 105 which is associated with the CO line L7 (STEP 1308). If all the CO lines L1 to L8 are not idle, "00H", the decision at the STEP 1305 is "YES" so that idle CO line unavailable processing inclusive of delivery of busy tone and glowing of the lamp 26 in red is performed (STEP 1306). Meanwhile, after the STEP 1308, the state memory 105 indicates that the CO line L7 is in use and, hence, the indication controller 14 generates indication data representative of the use state of the CO line L7. This indication data is written in the area 1305 of the exchanging memory 103 by the CPU 100, while the indication interface 18 reads it out. As a result, the lamps 180 of the indicators D1 to D4 which are associated with the CO line L7 are turned on to show that the CO line L7 is in use. Thereafter, as the user dials a desired subscriber's number by depressing the dial buttons 28 of the telephone set T1, these depressions are detected by the button detector 31 and then converted by the data converter 34 to codes (button data) particular to the depressed buttons. The output data of the converter 34 are applied to the interface 15 via the circuit 30, the interface 172, the area 1031 of the memory 103, the CPU 100, and the area 1034 of the memory 103. The interface 15 sends dial signals over the CO line L7. Initiation of an outgoing call on any of the CO lines is completed by the above procedure. It will be noted that the handset 20 may be picked up after pressing the button 24. It will also be noted that intercommunication between the telephone sets T1 to T24 may be set up by making the handset 20 off-hook and then entering a desired extension number. RECEIVING AN INCOMING CALL Assume that an incoming call has been received over the CO line L1 and is to be answered at the telephone set T1. When the incoming call is detected by the interface 15, the interface 15 sends incoming call data to the CPU 100 via the area 1033 of the exchanging memory 103. In response, the CPU 100 writes state data "01H" representative of an incoming state in the area of the state memory 105 which is assigned to the CO line L1. As the indication controller 14 reads out the data "01H" to generate incoming call indication data, the lamps 180 of the external indicators D1 to D4 associated with the CO line L1 are caused to flash rapidly to display the reception. Further, the CPU 100 generates lamp data which causes the answer lamps 27 of all the telephone sets T1 to T24 to flash in read. This lamp data is fed from the CPU 100 to the respective telephone sets T1 to T24 via the area 1032 of the memory 103 and the interface 172. When the answer button 25 of the telephone set T1 where the lamp 27 is flashing in red is pressed, the CPU 100 activates the CO line selector 13 responsive to the resulting button data. The selector 13 then performs the control shown in FIG. 14. First, data "1" is written in the work area 1021 of the RAM 102 (STEP 1401). This data "1" is used in the subsequent processing for determining whether or not the incoming call present on the CO line L1 to be answered. If it is unable to be answered, the decision will be sequentially made with the other CO lines L2, L3, ..., L8 in this order and, therefore, the content of the work area 1021 will be sequentially changed to "2", "3", ...., "8". Next, state data "01H" (FIG. 11A) is read out of the area of the state memory 105 which is assigned to the CO line L1 (an area associated with the CO line whose number is represented by the data "1" stored in the work area 1021) (STEP 1402). Then, whether or not the state data "01H" is the state data "01H", indicative of an incoming state which is able to be caught for answering, is decided (STEP 1403). At this instant, since the CO line L1 has received an incoming call and is ready to be caught, the result of the STEP 1403 is "YES" so that the CO line L1 represented by the content of the work area 1021 is selected (STEP 1407), and CO line seizing data meant for the CO line L1 is applied to the CPU 100. In response, the CPU 100 writes the seizing data in the exchanging memory 103 together with lamp data for the telephone sets. Based on the seizing data, the interface 15 seizes the CO line L1 over which the incoming call has been received. Responsive to the lamp data, the telephone set T1 which has responded to the incoming call turns on its answer lamp 27 in green, while the other telephone sets T2 to T24 turn off their lamps 27. The switching network 16 controlled by the CPU 100 sets up a communication path between the telephone set T1 and the CO line L1. Thereupon, state data "02H" representative of a use state is stored in the area of the state memory 105 which is associated with the CO line L1 (STEP 1408). Meanwhile, if the result of the STEP 1403 is "NO", the content of the work area 1021 is increased by 1 (STEP 1404) and, in this manner, the STEPs 1402 and 1403 are repeated with all the CO lines down to L8 (STEP 1405). If none of the CO lines receiving the incoming call has been found, answer CO line unavailable processing inclusive of sending busy tone is performed (STEP 1406). After the STEP 1408, the indication controller 14 reads the state data relating to the CO line L1 out of the state memory 105, and generates indication data indicative of the fact that the CO line L1 is in use. This indication data is written in the area 1035 of the exchanging memory 103 by the CPU 100. The indication interface 18 reads the indication data out of the area 1035 and, then, turns on the lamps 180 of the external indicators D1 to D4 which are associated with the CO line L1, thereby showing that the CO line L1 is being used. HOLDING THE CO LINES As the user using the telephone set T1 to communicate with the other party over the CO line L1 depresses the hold button 23 with the intention of holding the CO line L1, button data is applied to the CPU 100 as a hold instruction. In response, the CPU 100 activates the hold controller 12. The hold controller 12 performs a control as shown in FIG. 15. First, data "1" is written in the work area 1021 of the RAM 102 in order to temporarily store the call park number which is being processed (STEP 1501). This is to sequentially scan the call parks 1 to 4 to find an idle one. Next, CO line hold data "00H" (FIG. 12A) is read out of an area of the CO line hold memory 121 which is associated with the call park 1 (an area associated with a call park whose number is represented by the data "1" stored in the work area 1021) (STEP 1502). This is followed by a decision as to whether or not the data "00H" mentioned above is the data "00H" representative of an idle state (STEP 1503). In this case, since the result of the decision is "YES", the I/O circuit 122 sends a control signal to the CPU 100 in order to hold the CO line L1. In response to this signal, The CPU 100 controls the switching network 16 to hold the CO line L1 and send hold tone to the CO line L1. Further, responsive to the control signal from the I/O circuit 122, the CPU 100 writes state data "03H" indicative of the hold state in an area of the state memory 105 which is associated with the CO line L1 (STEP 1507). Hold data "10H" indicative of the held CO line L1 is written in an area of the hold memory 121 which is associated with the call park 1 (STEP 1508). If the result of the STEP 1503 is "NO", implying that the call park 1 is occupied by another CO line, the content of the work area 1021 is increased by 1 (STEP 1504) and, then, whether all the call parks have been searched is decided (STEP 1505). The STEPS 1502 to 1505 are repeated thereafter. If all the call parks are occupied, the result of the STEP 1505 is "NO" so that the operation advances to CO line hold unable processing (STEP 1506). The STEP 1506 may comprise sending an audible signal representative of the hold unable state or reconnecting the telephone set to the CO line on which the communication has been held. After the writing operation for the hold data in the hold memory 121 at the STEP 1508 has been completed, the CPU 100 generates lamp data for the respective telephone sets. In response to the lamp data, the first one (call park 1) of the call park lamps 21 at each of the telephone sets T1 to T24 flashes to show that the CO line L1 has been held at the call park 1. Further, the lamp 180 of each of the indicators D1 to D4 which is associated with the CO line L1 flashes slowly to indicate the hold state. When the CO line L1 being held as stated above is to be taken over by the telephone set T2, the first one (call park 1) of the call park buttons 22 at the telephone set T2 is depressed. The CPU 100 reads the call park button data out of the exchanging memory 103 and, then, activates the hold controller 12. The controller 12 performs a control as shown in FIG. 16. The hold data "10H" stored in the area of the hold memory 121 which is associated with the call park 1 is read out (STEP 1601) to see if the CO line is being held (STEP 1602). Since the CO line L1 is being held, i.e., "10H" "00H", the result of the decision is "NO" so that the CPU 100 controls the switching network 16 to cancel the hold state of the CO line L1 and connect the CO line L1 to the telephone set T2. In the state memory 105 (FIG. 11A), the state data of the CO line L1 turns from data "03H" (hold state) to data "02H" (use state). The lamps 180 of the indicators D1 to D4 associated with the CO line L1 turn on to show the use state. Since the telephone set T2 has responded to the CO line L1 to cancel the hold state due to the depression of the call park 1 button, the CO line answer lamp 27 grows in green responsive to the lamp data from the CPU 100. In the meantime, the call park 1 lamps of all the telephone sets T1 to T24 are turned off the indicate the cancellation of the hold state (STEP 1603). Then, data "00H" indicative of a no-hold state is written in the area of the hold memory 121 which is associated with the call park 1 (STEP 1604). In this manner, the incoming call received by the telephone set T1 at first is transferred to the telephone set T2. If the result of the STEP 1602 is "YES", meaning that no CO line is being held, hold cancellation unable processing is performed (STEP 1605). The STEP 1605 may comprise sending a predetermined audible signal or, alternatively, restoring the telephone set T2 to the off-hook state and then sending dial tone. Each of the telephone sets T1 to T24 is capable of simultaneously holding the same number of CO lines as the number of hold data (CO line numbers to be held) which may be stored in the hold memory 121, i.e. the number of call park buttons 21. Further, the hold state can be cancelled at any of the telephone sets. When the telephone set has been restored to the on-hook state after a communication, in response to hook data, the switching network 16 interrupts the communication path. The CPU 100, responsive to the hook data, writes idle state data "00H" in an area of the state memory 105 which is associated with the CO line that has completed a communication. Also, the use state indication on the indicators D1 to D4 and the answer lamp 27 of the telephone set which has glowed in green are turned off. In this embodiment, the various lamps have been shown and described as selectively glowing in green and in red, flashing and turning off to indicate various states, other suitable modes of indication may be used. The number of CO lines, that of telephone sets, that of call parks (i.e., the number of CO lines which may be held at the same time), and that of external indicators shown and described are not limitative. Especially, the number of call parks do not need to be equal to that of CO lines and may be determined in conformity to particular conditions of use of the KT system. A key number system may be adopted at the central office for the CO lines L1 to L8. Then, for more efficient use of the CO lines, a key number may be assigned to the CO line L1 such that while the line L1 is busy an incoming call is sequentially received over the other lines L2, L3, ...., L8, and an outgoing calls are sequentially initiated by the CO lines L1, L2, ..., L8. Further, the KT system of this invention may be furnished with a known function of allowing one to hold a communication while maintaining a hadset in an on-hook state. In summary, it will be seen that in accordance with this invention any desired number of external CO line indicators show states of respective CO lines to eliminate the need for CO lamps otherwise installed in each telephone set, thereby cutting down the dimensions and cost of the telephone set. The small size, low cost design of the telephone set is further enhanced and, simultaneously, easy operation is achieved because the use of an outgoing call button adapted for selection of an idle CO line and a CO line answer button adapted for response to an incoming call makes it needless to equip the each telephone set with CO buttons. In addition, while call park buttons and call park lamps are additionally installed for holding CO lines, they are smaller in number than the CO lines which are accommodated by the KT system and are exclusively used for holding purpose, promoting the ease of CO line transfer operation. Although this invention has been shown and described with reference to the preferred embodiment thereof, and in terms of the illustrative drawings, it should not be considered as limited thereby. Various possible modifications, omissions, and alternations to the embodiment could be conceived by one skilled in the art without departing from the scope of this invention defined by the appended claims.

Claims: A key telephone system including a plurality of telephone sets (T1 to T24), each set having a hold button (23), an outgoing call button (24) and an answer button (25), a central unit (9) which includes a switching network (16) for connecting the plurality of telephone sets either to central office lines (L1 to L8) or to intercom lines and a control unit (10) for controlling the switching network and being arranged to hold one of the central office lines (L1 to L8) in response to the operation of a hold button (23) and to select and seize an idle one of the central office lines (L1 to L8) in response to the operation of an outgoing call button (24) and one of the central office lines (L1 to L8) over which an incoming call is being received in response to the operation of an answer button (25), the key telephone system being characterised in that each of the telephone sets (T1 to T24) further includes a plurality of call park buttons (22) and respective call park lamps (21), the number of call park buttons (22) and lamps (21) being independent of the number of central office lines (L1 to L8), in that there is provided at least one indicator (D1 to D4) external to a telephone set (T1 to T24) for indicating the states of all of the central office lines (L1 to L8) and, in that the central unit (9) includes memory means (12) for storing number data representative of the central office line which has been held, in response to the operation of a hold button (23), in an unoccupied area of a hold store area which is respectively associated with a call park button (22), means (11) for generating lamp data in response to the storing operation of the number data to indicate a hold state in a call park lamp (21) associated with the hold store area storing the number data, and control means (11) responsive to the operation of a call park button (22) associated with a call park lamp (21) indicating a hold state, for deciding upon the held central office line from the stored number data in the memory means (12) and for controlling the switching network (16) to reset the hold state of the held central office line (L1 to L8) and to connect the central office line (L1 to L8) to the intercom line of the telephone set (T1 to T24) which initiated the operation of the call park button (22). A key telephone system as claimed in claim 1 characterised in that the control unit (10) includes first store means (105) for storing the states of the respective central office lines (L1 to L8). A key telephone system as claimed in claim 2 characterised in that the control unit (10) includes a central office line selecting means (13) which refers to the first store means (105) for selecting either the idle central office line or the central office line over which an incoming call is being received. A key telephone system as claimed in claim 2 characterised in that the control unit (10) includes a central office line indication control means (14) which refers to the first store means (105) to generate data for indictating the states of the respective central office lines. A key telephone system as claimed in claim 1 characterised in that, in response to the operation of the hold button, the control means (10) searches sequentially the hold store areas of the memory means (12) to store the number data assigned to the central office line which is to be held in the hold store area.

Inventors: HIENO, KENICHIRO, INOUE, MASAYOSHI, TANIHATA, RYOICHI
Applicants: NEC CORPORATION
Application year: 1986
Main classification: H04M9
IPC codes: H04M9, H04Q3