Abstract:
A line interface circuit having a dialer, a line interface unit and a DC masking circuit is adapted to execute an operation in either an emergency mode or a power-on mode. The line interface circuit unifies a voice processing part and an image processing part. The line interface circuit includes a single dialer, a communication port coupled to a non-voice terminal, and a line interface control unit for receiving dial information from a key matrix input at the non-voice terminal via the communication port in a power-on state of the non-voice terminal. The line interface control unit derives DC power converted from AC power supplied form a desired AC power supply, and transferring the dial information to the dialer. A method for controlling the line interface circuit is also provided to selectively couple a non-voice terminal and a voice terminal both equipped in a telephone communication system to a telephone line via the line interface circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a telephony communication device, and, in particular, to a line interface circuit for interfacing telephone systems. The line interface enables both voice and non-voice operations at a telephone terminal with a common design. The present invention further relates to a method for controlling the line interfacing circuit. 
     2. Description of the Related Art 
     Circuits have been developed to interface communications devices with a telephone exchange system, such as a transmitter/receiver circuit in a standard telephone. Some communications devices, such as a conventional facsimile machine, are designed for both voice and non-voice communications. In one mode, the facsimile machine transmits and receives images of documents or still pictures via a telephone line. Hence, the facsimile machine includes an interfacing circuit having an image processing unit for scanning and digitizing the documents and pictures for transmission via the telephone line. In a second mode, the facsimile machine can be used for voice communications for enabling a user to orally communicate with the intended recipient of the facsimile. Hence, the facsimile machine also includes a voice processing unit, generally within the interfacing circuit, for digitizing sound. 
     With reference to FIG. 1 there is shown a block diagram of a conventional facsimile machine. The facsimile machine includes a central processing unit or CPU FAX    101  which controls the operations of the facsimile machine, such as transmitting, receiving and copying. The CPU FAX    101  may be provided with an associated RAM memory and stored operating program for operating the facsimile machine or CPU FAX    101  may be connected to an external memory  109 . The CPU associated RAM may also store other data including data generated during the execution of the operating program. 
     An operating panel (OPE)  102  which includes a key matrix having a plurality of keys and a display unit is coupled to the CPU FAX    101 . The key matrix includes circuitry for generating at least one signal when a user depresses one of the keys to set a desired operational mode and/or to initiate an operation. Circuitry associated with the display unit of the OPE  102  receives the at least one signal from the control unit  101  and displays an appropriate display element such as LCD(s) indicative of the operational mode or operation being executed by the facsimile machine. 
     The facsimile machine further includes a sensor unit  103 , a scanner  104  and an image processing unit  105 . The sensor unit  103  senses the placement of a document in the facsimile machine and whether there is printing paper and generates appropriate signals which are transmitted to the CPU FAX    101 . The scanner  104  scans the document and converts the scanned image into digital data which are sent to the image processing unit  105 . The image processing unit  105  processes the digital data and transmits the data via a telephone line to a receiving facsimile machine or other communication device. 
     A printing unit  106  is also coupled to the CPU FAX    101  for printing image data during the receiving or copying operations. The facsimile machine further includes a modem  107  (a non-voice terminal) for performing data communications via a telephone line. In a transmitting mode, the modem  107  modulates the digital data received from the image processing unit  105  and transmits the modulated data under the control of the CPU FAX    101 . In a receiving mode, the modem  107  demodulates coded image signals received via the telephone line to re-create the original document data. 
     A line interface unit (LIU)  108  is coupled between the modem  107  and the telephone line. The LIU  108  establishes a channel between the modem  107  and the telephone line to facilitate: interfacing the telephone line with the modem  107 ; interfacing the telephone line with a telephone handset; processing of a ring signal received from an exchange system; and telephone dialing operations. The LIU  108  has a tip terminal T and a ring terminal R which are coupled to the telephone line. The above-mentioned functions of the LIU  108  will now be described with reference to FIG.  2 . 
     First, the function of interfacing the telephone line with the modem  107  will be described. 
     In facsimile data communications, a non-voice path (NVP) is selected by switch unit  110  in response to a switch control signal S 1  received from the CPU FAX    101  to trip a switch therein to couple the tip terminal T and ring terminal R to the modem  107  via a first dialer  120  and a first LIU  130  to establish a DC loop. The first LIU  130  serves to transmit signals such as modulated facsimile signals and dual tone multi-frequency (DTMF) dial signals generated from the modem  107  to the telephone line. The first dialer  120  generates the dial pulses. 
     In addition to selecting a non-voice path, the switch unit  110  can select a voice path (VP) along a second LIU  135 , a speech network  150  and a handset  180  to perform a telephone hook switching function. This is accomplished by transmitting a control signal S 2  from the CPU FAX    101  to the first dialer  120  to cut-off the DC loop and generate dial pulses. The first LIU  130  serves to keep an AC impedance matching with telephone exchange system EXH by carrying out primary/secondary separating functions, an AC signal coupling function, and a DC masking function. 
     The function of interfacing the telephone line with the telephone handset  180  to perform voice communications will now be described with respect to the CPU FAX    101  being in a power-on state and in a power-off state. When the user lifts the handset  180  the facsimile is thrown into a DTMF dial mode and the CPU FAX    101  is in the power-on state. In the DTMF dial mode, the speech network  150  can perform various functions such as an inter-conversion between speech signals and electrical signals, inter-conversion between two-wire signals and four-wire signals, AC impedance matching, a DC masking, and a speech transmitting and receiving function. The power-on state is sensed by a hook detector (not shown) which sends a hook-off status signal to the CPU FAX    101  to thereby prepare the modem  107  for a dialing operation via depression of keys on key matrix  250 . If the user does not lift the handset, the facsimile is in a dial pulse (DP) dial mode and the CPU FAX    101  controls the first dialer  120  to ready itself for a dialing operation. 
     In the power-off status of the CPU FAX    101 , i.e., when the facsimile machine is in a non-operational mode or emergency mode, a separate hook switch (not shown) connected to the switch unit  110  is mechanically tripped upon lifting of the handset  180  to couple the telephone line to the speech network  150 . A bridge diode may be utilized to supply regulated electric power to the telephone line despite variations in the polarity of the electric power. 
     The dialing operation is initiated by switching the second dialer  170  to the DTMF/DP dial mode by lifting the handset  180  as noted above. The second dialer  170  is supplied with electric power from the speech network  150  for converting dial data received from the key matrix  250  in a form corresponding to the DTMF/DP dial mode. The converted data from the second dialer  170  is then sent to the speech network  150  for the dialing operation. 
     The LIU  108  of FIG. 2 is uneconomical and inefficient in terms of its circuit configuration since it has an overlapping design. The LIU  108  uses two dialers  120  and  170 , two line interface units  130  and  135 , and overlapping DC masking circuits. In addition, there are problems regarding the various functions of the LIU  108 . 
     First, there is a problem regarding the function of interfacing the telephone line with the modem  107  and the telephone handset  180 . Since the modem  107  and the speech network  150  have AC impedance and DC masking circuits, respectively, the modem  107  exhibits AC impedance and the speech network  150  exhibits direct current resistance (DCR). Therefore, switching noise may be generated on the telephone line when switching occurs between the telephone handset  180  and the facsimile machine by switch unit  110  to switch from voice to non-voice communications. 
     Second, there is a problem regarding the dialing operation. Since one dialer  120  is used for the modem path and another dialer  170  for the telephone path, the dialers  120  and  170  are not appropriately designed to be able to execute both voice and non-voice communications. Therefore, one has to switch between dialers to change from voice to non-voice communications and vice versa. 
     Third, there is a problem with the LIU  108  in recognizing a confirm tone generated during the execution of dialing operation in the power-on mode for a voice communication if the switch unit is switched to the non-voice path. As a result, the user cannot initiate a dialing operation for a voice communication when switch unit  110  is switched to the non-voice path because one cannot hear dial tones through the handset  180 . The switch unit  110 , which operates under the control of the CPU FAX    101 , connects the telephone line to the telephone handset  180  only for a short period after the generation of a digit is ended, and before the next digit is generated. Accordingly, the speech network  150  does not operate throughout the dialing operation. As a result, an abnormal dialing operation occurs because the handset  180  intermittently outputs a dial tone whenever the switch unit  110  switches back to the speech network  150 . 
     Further, when a dialing operation is carried out for a voice communication using the handset  180  in a power-on state of the CPU FAX    101 , the second dialer  170  associated with the telephone handset  180  cannot be used for a non-voice communication. 
     Fourth, the last number redial feature may not work properly in the LIU  108  of FIG. 2, especially when the facsimile machine is in the emergency mode. The information about the last number dialed in the ON state is stored in the memory of the CPU FAX    101  while the last number dialed in the OFF state is stored in the second dialer  170 . In this regard, LNR information stored in the CPU FAX    101  and LNR information stored in the second dialer  170  may differ. Therefore, the user may dial an unintended number when executing the LNR feature. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a line interface circuit which includes a circuit configuration having a dialer, a line interface unit (LSU) and a DC masking circuit adapted to execute a dialing operation without switching between two different dialers and two different line interface units, one for voice communications and one for non-voice communications. The present invention also provides a method for controlling the line interface circuit. 
     It is another object of the present invention to provide a line interface circuit capable of allowing a user to continuously hear dial confirmation tones generated during a voice dialing operation. 
     Further, it is an object of the present invention to provide a line interface circuit which includes a single unit for storing LNR information, thereby preventing the dialing of an unintended number. 
     In one embodiment of the present invention, a line interface circuit for interfacing a voice terminal and a non-voice terminal with a telephone exchange via a telephone line, comprises: a single dialer for facilitating communication between the voice terminal and non-voice terminal with said telephone exchange; a communication port coupled to the non-voice terminal; and line interface control unit for receiving dial information from a key matrix input at the non-voice terminal via the communication port in a power-on state, for deriving power during said power-on state from said telephone exchange, for transferring the dial information to the dialer, for directly receiving dial information from the key matrix in a power-off state of the non-voice terminal, and for transferring the dial information to the dialer. 
     In a second embodiment of the present invention, a method for controlling a line interface circuit used to interface a telephone exchange system with a non-voice terminal and a voice terminal through a telephone line, including the steps of: sensing a hook-off status of a handset equipped to the voice terminal; transmitting a signal indicative of the sensed hook-off status to a main control unit from a line interface control unit of the line interface circuit; checking whether a response to the hook-off status signal is generated by the main control unit within a predetermined time from the time when the hook-off status signal is transmitted; setting an operational mode if it is determined that the response is generated within the predetermined time and coupling the voice terminal to said telephone line upon setting said operation mode; and checking receipt of dial information via the voice path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and various features and advantages of the present invention will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram schematically illustrating the circuit configuration of a conventional facsimile machine; 
     FIG. 2 is a block diagram illustrating the circuit configuration of an LIU of the conventional facsimile machine; 
     FIG. 3 is a block diagram of an LIU according to the present invention; 
     FIG. 4 is a schematic illustrating a line current detecting circuit which can be equipped in the LIU of FIG. 3; 
     FIG. 5 is a flow chart illustrating a line interfacing procedure in a power-on mode in accordance with the present invention; 
     FIG. 6 is a flow chart illustrating a line interfacing procedure in an emergency mode in accordance with the present invention; 
     FIG. 7 is a diagram illustrating protocols between a control unit of the LIU and a control unit of a facsimile machine in accordance with the present invention; 
     FIG. 8 is a diagram illustrating the procedure of switching internal paths of the LIU in accordance with commands received from the control unit of the facsimile machine; 
     FIG. 9 is a diagram illustrating an illustrative message format as transferred from the control unit of the facsimile machine to the control unit of the LIU; 
     FIGS. 10A and 10B are diagrams showing illustrative message formats as transferred from the control unit of the LIU to the control unit of the facsimile machine; and 
     FIG. 11 is a block diagram illustrating the circuit configuration of a LIU according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the telephone line interfacing circuit will now be described with reference to the drawings, in which like reference numerals designate identical or corresponding elements in each of the several views. It is noted that different terms may be used in the field to refer to objects of the present invention than the terms used herein. 
     A line interface circuit in accordance with the present invention is illustrated by FIG.  3 . In FIG. 3, block  108  denotes the part of a facsimile machine corresponding to the LIU which interfaces an exchange system EXH with a CPU FAX    101 , a modem  107 , and a telephone handset  180 . The LIU  108  uses DC power supplied from the exchange system EXH via line interface unit  370 . The LIU  108  may be incorporated onto a chip as an integrated circuit. 
     The LIU  108  includes a central processing unit or a digital logic circuit having functions similar to the central processing unit. The central processing unit is referred herein as “a CPU LIU ” and designated by numeral  320 . The CPU LIU    320  exchanges commands and status information between the exchange system EXH and the CPU FAX    101  within the facsimile machine via a communication port  340  for controlling line interfacing and telephone functions. A universal a synchronous receiver transmitter (UART) may be used for the communication port  340 . 
     The CPU LIU    320  receives dial digit information from the CPU FAX    101  via the communication port  340  and transfers the dial digit information to a dialer  360  to execute a dialing operation irrespective of the ON/OFF status of the main power. During the execution of the dialing operation, dial tones are generated and transferred by the CPU FAX    101  to a voice path in the LIU  108  in order for the user to confirm the dialing operation. 
     The dial digit information is received by the CPU FAX    101  from a key matrix  250 . The user depresses optional keys on the key matrix  250  and intrinsic key code values corresponding to the depressed keys are transmitted to the dialer  360  of the LIU  108  or to the CPU FAX    101  depending on whether the dialer  360  or CPU FAX    101  is designated by the CPU LIU    320 . For example, in an emergency mode, the CPU LIU    320  generates a key input enable signal so the key code values are transmitted to the dialer  360 . In a power-on mode, the CPU LIU    320  generates a key input disable signal so the key code values are transmitted to the CPU FAX    101 . 
     In addition, the CPU LIU    320  stores dial information regarding the last number dialed in the emergency mode and transfers the stored dial information to the CPU FAX    101  via the communication port  340 . The previous number dialed is sent from the CPU LIU    320  back to the CPU FAX    101  via the communication port  340  and to the dialer if there is a redial key input from the user for performing a LNR function. 
     As apparent from the above description, the voice processing portion and image processing portion of the facsimile machine commonly use certain circuits, such as the dialer  360  and line interface unit  370 . Accordingly, unlike the prior art, line noise is not generated during switching between the telephone and facsimile machine, since the same dialer is used for voice and non-voice communications. 
     FIG. 4 illustrates a line current detecting circuit which can be equipped to the line interface unit  370 . The line current detecting circuit includes resistors R 1  to R 4  and comparators COM 1  to COM 3 . The outputs LCV 0  to LCV 2  from the three comparators COM 1  to COM 3  are sent to the CPU LIU    320 . Therefore, one can check whether there is a variation in the amount of current flowing through the line by checking whether there is a variation in the outputs LCVO to LCV 2 . The accuracy of the current sensing circuit of FIG. 4 can be improved by increasing the number of comparators and resistors. 
     The amount of current flowing through the line is reduced when a terminal is coupled to the line or parallel telephones share the same line. The line interface unit  370  senses the amount of current flowing through the line and transmits information to the CPU LIU    320 . Based on the transmitted information, the CPU LIU    320  recognizes a hook-on/off status of the parallel telephones or a line cut-off state which may continue for an extended period of time during the execution of a dialing operation. In the latter case, the CPU LIU    320  generates a desired message and transmits the message to the CPU FAX    101  via the communication port  340  to inform the user that a dialing operation is taking place. 
     FIGS. 5 and 6 are flow charts depicting illustrative line interfacing operations in accordance with the present invention. FIG. 5 depicts line interfacing in the power-on mode and FIG. 6 depicts line interfacing in the emergency mode. 
     The line interfacing operation according to the present invention will now be described in conjunction with the above-mentioned configuration. When the user lifts the handset  180 , the LIU  108  is powered by line current supplied from the exchange system EXH causing the handset  180  to be in the hook-off state. The CPU LIU    320  then checks whether or not the CPU FAX    101  is the power-on state, i.e., whether the main power of the facsimile machine is supplied (ON). This checking may be accomplished using various methods. In accordance with one method, the CPU LIU    320  transfers a command to the CPU FAX    101  via the communication port  340  and for a predetermined time waits for a response from the CPU FAX    101 . When a response is received made from the CPU FAX    101  within the predetermined time, the CPU LIU    320  recognizes the CPU FAX    101  to be in the power-on state. If a response is not received, the CPU LIU    320  assumes the CPU FAX    101  to be in the power-off state or the emergency mode. 
     In accordance with another method, a hardware port between the CPU FAX 101  and LIU  108  is used to determine the power state of the CPU FAX    101 . The CPU LIU    320  checks the state of this port to determine the power-on/off state of the CPU FAX    101 . In accordance with another method, the CPU FAX    101  periodically transfers a command to the CPU LIU    320  of the LIU  108  via the communication port  340 . When the CPU FAX    101  receives a response from the CPU LIU    320 , the CPU FAX    101  sends to the CPU LIU    320  a command to initiate execution of a normal operation. 
     With reference to FIG. 5, at step  501  the CPU LIU    320  transmits a hookoff status signal to the CPU FAX    101  via the communication port  340 . Simultaneously, the CPU LIU    320  initiates a timer equipped therein. The timer is set to check whether CPU FAX    101  will generate a response to the hook-off status signal within a predetermined time. If there is no response from the CPU FAX    101  within the predetermined time, the procedure advances to step  506 . Step  506  checks whether the timer has clocked more than the predetermined time. If it is determined at step  506  that the timer has clocked more than the predetermined time, an emergency mode control routine is executed. 
     However, if at step  506  it is determined that the timer has not clocked more than the predetermined time, than the procedure returns to step  502 . At step  502 , the CPU LIU    320  waits for a response from the CPU FAX    101 . If there is a response from the CPU FAX    101 , the CPU LIU    320  determines that the CPU FAX    101  is in the power-on mode and executes step  503 . At step  503 , the CPU LIU    320  generates a key input disable signal to prevent a key code value from being received from the key matrix  250 . Then the CPU LIU    320  records a code indicative of this disable state in a key register. Therefore, the CPU LIU    320  of the LIU  108  can execute a control to carry out a dialing operation only for dial information transferred from the CPU FAX    101  via the communication port  340  while ignoring the key code value generated from the key matrix  250  by reading the disable code recorded in the key register. 
     After disabling the input from the key matrix  250 , the procedure advances to step  504 . At step  504 , the handset  180  is connected to the telephone line path in order for a dial tone or signal from the exchange system EXH to be heard through the handset  180 . Referring to FIG. 3, this connection is accomplished by sending a switch control signal S 3  from the CPU LIU    320  to the switch unit  350  instructing a connection with the voice path (VP). Upon receiving the switch control signal S 3 , the switch unit  350  connects the handset  180  to the telephone via the speech network  150 . As a result, the user can hear the confirm tones generated during the dialing operation. 
     The above procedure is an example for explaining the procedure of checking whether or not the CPU FAX    101  is in its power-on state under the condition that the LIU  108  is powered on, and then executing an initial operation based on the results of the checking. 
     Referring once again to FIG. 5, at step  505  the procedure checks whether the CPU LIU    320  received a desired command or data from the CPU FAX    101  via the transmission port  340  after setting the LIU  108  to the power-on mode. If there is a command or data received from the CPU LIU    320 , the procedure advances to a routine for executing a procedure associated with the received command or data. If a command or data is not received, the CPU LIU    320  will wait until the desired command or data is received. 
     When the CPU LIU    320  receives data indicative of dial information, the CPU LIU    320  sends signals for controlling the dialer  360  in order to execute a dialing operation. Where the dialing mode is set to the DTMF mode, the dialer  360  generates a DTMF signal by composing high and low group signals of dial digits and then transmits the DTMF signal to the CPU LIU    320 . 
     FIG. 6 shows a dialing operation in the emergency mode, i.e., when the CPU FAX    101  is in the power-off state. When the LIU  108  determines that the CPU FAX    101  is in its power-off state, i.e., in the emergency mode, the CPU LIU    320  receives key code values from the key matrix  250  for initiating the dialing operation at step  601 . A key input enable signal is generated by the CPU LIU    320  which is recorded in the internal key register. At step  602 , the handset  180  is connected to the telephone line via the voice path (VP) in order for the user to hear the dialing tones generated during the dialing operation. At step  603 , the dial numbers pressed on the key matrix  250  to execute the dialing operation are determined by known methods of scanning and decoding. The decoded key input is then sent to the CPU LIU    320  as dial information. At step  604 , the CPU LIU    320  sends the dial information to the tip and ring terminals T and R of the telephone line by passing the dial information through the switch unit  350  via the dialer  360  for the terminals to transmit DTMF dial tones or dial pulses. 
     As shown in FIG. 5, the LIU  108  waits for a command or dial information from the CPU FAX    101  at step  505  after completing the initialization in the power-on mode. Since in the power-on mode, the input transferred to the LIU  108  has been disabled, the key code values from the key matrix  250  are recognized only by the CPU FAX    101 . The recognized key code values are then transferred from the CPU FAX    101  to the CPU LIU    320  via the communication port  340 . This transferring process will now be described in detail with reference to FIG.  7 . 
     FIG. 7 shows commands associated with data transmitting/receiving protocols for exchanging data between the CPU LIU    320  and CPU FAX    101  in the power-on mode. When a hook event signal indicative of the hook-off state of the handset  180  is generated by a manipulation of the user at step  701 , the CPU LIU    320  informs the CPU FAX    101  of the generation of the hook event signal at step  702 . In response to the hook event signal, the CPU FAX    101  transmits a handset path setting command, namely, a voice path command, to the CPU LIU    320  at step  703 . The CPU LIU    320  then transmits a desired response to the voice path command to the CPU FAX    101  at step  704 . When the user presses the keys of the key matrix  250  to generate key inputs for a dialing operation at step  705 , the key inputs are sensed by the CPU FAX    101 . Hence, at step  706 , dial information is sent to the CPU LIU    320 . 
     In response to the dial information, the CPU LIU    320  controls dialing of dialer  360  at step  707  for the dialer  360  to start the dialing operation. The procedure then advances to step  708  where a response to the received dial information is sent to the CPU FAX    101 . Upon receiving the response, the CPU FAX    101  continuously transmits the corresponding digit information to the CPU LIU    320 . 
     At step  709 , a dial key release signal is generated upon the user not pressing any keys for a predetermined time. When the CPU FAX    101  senses the dial key release input, the procedure advances to step  710  where the CPU FAX    101  sends a key release command to the CPU LIU    320 . In accordance with the key release command, the CPU LIU    320  stops the dialer  360  from further dialing at step  711 . The CPU LIU    320  then checks to determine whether the tone transmission time i.e., the time elapsed from the time the key release command is received equals the predetermined tone transmission time. If the tone transmission time is shorter than the predetermined tone transmission time, then generation of the dial tone is continued until the tone transmission time equals the predetermined tone transmission time. Once the tone transmission time equals the predetermined tone transmission time, the generation of the dial tone is stopped. If it is determined that the tone transmission time is longer than the predetermined tone transmission time, the generation of the dial tone is stopped instantaneously. The CPU LIU    320  then sends a response to the CPU FAX    101  at step  712 . 
     When the dialing operation is switched from the DTMF mode to the DP mode or when the dialing operation is executed in the DP mode, the CPU FAX    101  sends a “DP mode setting” command to the CPU LIU    320  at step  713 . In response to the DP mode setting command, the CPU LIU    320  controls its internal register to set the dialing mode to the DP mode at step  714 . At step  715 , the CPU LIU    320  sends a response to the CPU FAX    101 . When the CPU FAX    101  subsequently receives a key code value from the key matrix  250  at step  716 , the procedure advances to step  717 . At step  717 , the CPU FAX    101  transfers dial information to the CPU LIU    320 . In response to the dial information, the CPU LIU    320  checks its internal register to identify the current dialing mode. Based on the identification, the CPU LIU    320  controls the dialer  360  at step  718  in order for the dialer  360  to execute a dialing operation for the dial digit in the DP mode. After the completion of the dialing operation, the procedure advances to step  719  where the CPU LIU    320  sends a response to the CPU FAX    101 . After receiving this response, the CPU FAX    101  determines whether there is another dial number. When there is another dial number, the CPU FAX    101  re-executes the above operation to transfer the dial digit until there are no more dial digits. 
     When data or a command is not exchanged between the CPU LIU    320  and CPU FAX    101  during the execution of the dialing operation or even after a response is made from a counterpart telephone, assuming that the dialing operation has been completed, the CPU FAX    101  periodically transmits a “hook status advice” command to the CPU LIU    320  at step  720 . In response to the hook status advice command, the CPU LIU    320  generates a responding signal, namely, a hook status signal at step  721 . However, if the CPU FAX    101  does not transmit the hook status advice command for a predetermined time at step  720 , the CPU LIU    320  determines that the CPU FAX    101  is in the power-off state. In this case, the CPU LIU    320  sets the current operation mode to the emergency mode and a control for the emergency mode is initiated. 
     When a hook event signal indicative of a hook-on status of the handset is generated at step  722  by the user placing the handset  180  on a handset cradle, the signal is sensed by the CPU LIU    320  which, in turn, informs the CPU FAX    101  at step  723 . 
     FIG. 8 shows the procedure of switching the internal paths of the LIU  108  in accordance with different commands generated by the CPU FAX    101 . A dialing operation is first initiated at steps  801  and  802  similarly to the dialing operation depicted by FIG.  7 . When the current operational mode is switched to the facsimile mode for a facsimile transmission at step  803  during the execution of the dialing operation, the mode switching is sensed by the CPU FAX    101 . The CPU FAX    101  thus transfers a “modem mode setting” message to the CPU LIU    320  at step  804 . The CPU LIU    320  then switches the current internal path to a desired internal path at step  805  in accordance with the message received from the CPU LIU    320  at step  805 . The switching is achieved by the generation of a switch control signal S 1  in a first state, for example, a logic high level. Upon receiving the switch control signal S 1 , the switch unit  350  connects the telephone line terminals T and R, to the modem  107  via the non-voice path (NVP). 
     Referring to FIG. 8 the CPU LIU    320  transmits a response to the CPU FAX    101  at step  806  after switching its internal path. Subsequently, it may be required to switch the current operational mode to the telephone mode due to the user&#39;s desire for a voice conversation or for other reasons. If this is the case, then the CPU FAX    101  at step  807  sends a “handset mode setting” command to the CPU LIU    320  at step  808 . FIG. 9 illustrates a format message of the type transferred from the CPU FAX    101  to the CPU LIU    320 . The message format may be associated with data or a command. 
     Upon receiving the mode setting command, the CPU LIU    320  sends a switch control signal S 1  which is generated in a second state, for example, a logic low level, to the switch unit  350  at step  808 . In accordance with this switch control signal S 1 , the switch unit  350  switches the internal path of the LIU  108  in order for the telephone line terminals T and R to be connected to the handset  180  via the voice path and speech network  150 . After switching the internal path, the procedure advances to step  809  where the CPU LIU    320  sends a response to the CPU FAX    101  informing the receipt of a desired message and the execution of an operation associated with the message. 
     FIG. 10 illustrates message formats as transferred from the CPU LIU    320  to the CPU FAX    101 . FIG. 10A is associated with a response message whereas FIG. 10B is associated with a hook event message. In the case of the response message, the lower five bits b 0  to b 4  are echo bits of the last received data bits d 1  to d 5  (see FIG.  9 ). These echo bits are those which have been shifted one bit to the right from the last received data bits d 1  to d 5 . In both message formats as shown by FIGS. 10A and 10B, the upper three bits b 7  to b 5  indicate the hook-on/off status of a manual or electronic hook switch in the dial mode. For example, when the bit b 5  is  0  in the dial mode, pulse dialing is executed, when the bit b 5  is  1 , tone dialing is executed. 
     FIG. 11 is a block diagram of a circuit configuration of a LIU in accordance to another embodiment of the present invention. An explanation of the operation of the CPU FAX    101  in the power-off state, i.e., in the emergency mode as depicted in FIG. 6 will be given hereinafter. When it is confirmed that the CPU FAX    101  is in the power-off state, the CPU LIU    320  receives key code values from the key matrix  250  to control the dialing operation at step  601 . The CPU LIU    320  then generates a key input enable signal and records the generated key input enable signal in the internal key register. At step  602 , the handset  180  is connected to the telephone line via the voice path in order for the user to hear the dialing tones generated during the dialing operation. At step  603 , the dial numbers pressed are determined by scanning the key input received from the key matrix  250 . The scanned key input is then decoded and sent to the CPU LIU    320  as dial information. At step  604 , the CPU LIU    320  sends the dial information to the tip and ring terminals T and R of the telephone line by passing the dial information through the switch unit  350  via the dialer  360  in order for DTMF dial tones or dial pulses to be transmitted. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the present invention. Therefore, it is as disclosed in the accompanying claims.