Abstract:
A screen communication method and system for communicating screen data between a plurality of interconnected terminal units each having a display part and an input part, wherein each time screen modifying data is input from the display part, each of the terminal units transmits a modifying right request to another terminal unit before transmitting the screen modifying data, and obtains a modifying right under such a condition that each terminal unit receives no screen modifying request from another terminal unit within a predetermined period of time after the transmission of the first modifying right request.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a screen communications system that enables quick across to content-of-screen-display modifying information between a plurality of mutually communicable terminal units which display the same contents on the respective display screens. 
     2. Description of the Related Art 
     Heretofore the screen communications system of the above-described type is exemplified by a personal computer conference system. 
     In this known computer conference system, in order to guarantee that the screen displays of all terminal units are identical, a screen modifying right is moved by transferring a modifying right request command and a modifying right moving command. Since these commands are executed according to a store-and-forward protocol similar to the transfer of screen modifying data, a user needs to perform at least two steps of requesting a modifying right and modifying the content of the screen display. 
     With this prior art, the operation for requesting a modifying right must be performed before actually modifying the content of screen display, which is laborious and time-consuming. 
     To this end, a graphics conference system has been proposed, such as disclosed in Japanese Patent Laid-Open Publication No. SHO 62-269585, in which the individual terminal unit has a built-in switch so that a modifying right can be moved by receiving a signal requesting the opening of the switch and by transmitting a signal indicating the completion of the opening of the switch. 
     In this graphics conference system, the terminal unit whose switch is closed has a modifying right; when a screen modifying operation is performed at the terminal unit whose switch is open, the terminal unit transmits a signal requesting the opening of the switch and does not close the switch until a signal indicating the completion of the opening of the switch is returned. 
     Since only one of the switches will be open at any point in time, it is possible to move a modifying right in a single screen modifying operation, while guaranteeing the uniformity of screen display. 
     However, until a signal indicating the completion of the opening of the switch is returned after a signal requesting the opening of the switch is transmitted, the terminal unit cannot discriminate whether or not the built-in switch may be closed. As a result, the terminal unit cannot discriminate whether or not it has a modifying right by itself, and the delay time from the screen modifying operation to the reaction of the display of the terminal unit will become large, thus diminishing operation of the terminal unit. Therefore quick movement of a modifying right cannot be achieved. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a screen communications system which enables quick movement of a modifying right, while guaranteeing that the contents of screen display on different terminal units are identical. 
     In order to accomplish the above object of the invention, a modifying right request is transmitted and that each terminal unit detects by itself whether or not it has a modifying right. 
     According to a first aspect of the invention, there is provided a screen communications method for communicating screen data between a plurality of interconnected terminal units, each having a display part and an input part, wherein each time screen modifying data is input from the input part, each of the terminal units transmits a modifying right request to another terminal unit before transmitting the screen modifying data, and obtains a modifying right, although no terminal unit receives a screen modifying request from another terminal unit within a predetermined period of time after the transmission of the first modifying right request. Each terminal unit releases the modifying right within a predetermined period of obtaining it, and no screen modifying data is input from the input part. 
     Preferably the modifying right request transmitting means may transmit a modifying right request without processing it into a packet. According to a second aspect of the invention, there is provided a screen communications system for communicating screen data between a plurality of interconnected terminal units, each having a display part and an input part, the system comprising a first transmitter for transmitting a modifying right request and a second transmitter for transmitting screen modifying data. The second transmitter may perform transmission by using transmission procedures according to a storage-and-forward protocol. The first transmitter may be an analog immediate forward type, the modifying right request being an audio signal having a predetermined frequency. Alternatively, the first transmitter may be a digital immediate forward type, the modifying right request being a binary signal. 
     Each terminal unit may further include: means for transmitting a modifying right request to another terminal unit each time screen modifying data is input from the input part, before transmitting the screen modifying data; means for receiving a modifying right request transmitted from another terminal unit; means for transmitting the screen modifying data input from the input part; means for receiving the screen modifying data transmitted from another terminal unit; and means for modifying the display contents of the display part based on the screen modifying data input from the input part or that transmitted from another terminal unit. 
     Further, in order that each terminal unit can detect by itself whether or not it has a modifying right with it, timing to confirm a modifying right and timing to release the modifying right are determined. 
     Generally, to confirm a modifying right requires confirmation that the plural terminal units are not in simultaneous operation. This requirement can be satisfied by transmitting a modifying right request from a certain terminal unit, then receiving a modifying right from another terminal unit before the modifying right request from the first-named terminal unit arrives at the second-named terminal unit, and monitoring the modifying right request from the second-named terminal unit until this modifying right request arrives at the first-named terminal unit, i.e., during the reciprocating transmission of the modifying right request. 
     Further, generally, to release a modifying right requires confirmation that the input of screen modifying data is terminated. This requirement can be satisfied by monitoring the input of screen modifying data as long as such input is regarded as being continuous. 
     For this purpose, each terminal unit may further include: a first measuring means for measuring at least a time period of reciprocating transmission to a modifying right request each time the modifying right request is transmitted; a second measuring means for measuring a time period of input of the screen modifying data to be regarded as a continuous input at the input part; means for confirming a modifying right within a time period of measurement by the first measuring means if the modifying right receiving means receives no modifying right request from another terminal unit; and means for releasing a modifying right within a time period determined by the second measuring means if no screen modifying data is input from the input part. 
     Further, a timing measurement to confirm that another terminal unit has released a modifying right may be carried out. 
     Generally, to confirm that another terminal unit has released a modifying right requires a determination as to whether or not the receiving of screen modifying data has been terminated. This requirement can be satisfied by monitoring a modifying right request from another terminal unit only during the time period from the receipt of the modifying right until all of the screen modifying data has been received, i.e., during the total time period of the measuring time of the second measuring means and the transmitting time of the screen modifying data. 
     For this purpose, each terminal unit further includes: a third measuring means for measuring a total time of the measuring time period of the second measuring means and the transmitting time period of the screen modifying data each time the modifying right request receiving means receives a modifying right request from another terminal unit; and means for confirming, within a time period of measurement by the third measuring means and if the modifying right request receiving means receives no modifying right request from another terminal unit, that another terminal unit has released a modifying right. 
     Each of the first, second and third measuring means may be a timer. 
     Since it is possible to discriminate whether or not there is a modifying right with the terminal unit itself from only a modifying right request, the delay time from the screen modifying operation until the display part of the terminal unit reacts becomes smaller, thus causing quick movement of a modifying right. It is therefore possible to display the presence/absence of a modifying right. 
     In order to determine a timing to confirm that another terminal unit has released a modifying right, each terminal unit further includes: means for transmitting data notifying that the modifying right release confirming means has confirmed the release of a modifying right; means for receiving the release notifying data from another terminal unit; and means for confirming, if the release notifying data receiving means receives the release notifying data from another terminal unit, that another terminal unit has released a modifying right. 
     In operation, a modifying right request can be transmitted in a reduced time, without being processed into a packet or subjected to error correction which are needed for screen modifying data. Therefore, even if a modifying right request is transmitted after performing the screen modifying operation and even if modification of the content of display on the screen is started after a modifying right is confirmed, a user at the terminal unit will not discern a time difference. 
     If it is transmitted in an audio signal having a predetermined frequency, a modifying right request can be detected by a filter. 
     If it is transmitted in a binary signal having a predetermined number of bits, a modifying right request can be detected by a flip-flop. 
     Further, since the individual terminal unit can determine by itself a timing to confirm a modifying right and a timing to release the modifying right, it is possible to reduce the delay time from the screen modifying operation until the display part of the terminal unit reacts, thus realizing an improved degree of operativity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing a screen communications system according to a first embodiment of this invention; 
     FIG. 2 is a block diagram showing hardware of the system of the first embodiment; 
     FIG. 3 is a state transition diagram showing the process in which a modifying right is discriminated by a control circuit; 
     FIG. 4 is a sequence diagram corresponding to the state transition diagram of FIG. 3, representing the operation in the first embodiment; 
     FIG. 5 is a state transition diagram showing the process in which a modifying right is discriminated by a control circuit according to a second embodiment of the invention; 
     FIG. 6 is a sequence diagram corresponding to the state transition diagram of FIG. 5, representing the operation in the second embodiment; 
     FIG. 7 is a block diagram showing a screen communications system according to a third embodiment; 
     FIG. 8 is a schematic view showing an example of display screen according to a fourth embodiment; and 
     FIG. 9 is a block diagram showing a screen communications system according to the fourth embodiment, 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Several embodiments of this invention will now be described with reference to the accompanying drawings. 
     FIG. 1 shows a screen communications system according to the first embodiment of this invention. 
     In FIG. 1, three terminal units 1a, 1b, 1c are connected to an exchanger 2 via three transmission lines 10a, 10b, 10c, respectively. The terminal unit 1a has a display 120a, a keyboard 121a and a telephone 110a. The other terminal units 1b, 1c have the same construction as that of the terminal unit 1a, and the display 120b, 120c of both of the terminal units 1b, 1c displays the content of screen display identical with that on the display 120a of the terminal unit 1a. 
     When the keyboard 121a is operated, the terminal unit 1a transmits an operating signal 500, for inhibiting any input from the other terminal units 1b, 1c, to the terminal units 1b, 1c via the exchanger 2. In each of the other terminal units 1b, 1c when the operating signal 500 is detected, an input inhibition display 510b, 510c will be displayed on the display screen, and any input from the associated keyboard 121b, 121c will be inhibited. 
     The terminal unit 1a also transmits screen modifying data 520 to the other terminal units 1b, 1c via the exchanger 2. In each of the other terminal units 1b, 1c, when the screen modifying data 520 is received, the content of screen display of each display 120b, 120c will be changed into the same content as that on the display 120a of the terminal unit 1a. 
     In the terminal units 1b, 1c, with the lapse of a predetermined time after the operating signal 500 is detected, the input inhibition will be cancelled, and the input inhibition display 510b, 510c on the display screen will be erased. 
     As shown in FIG. 2, the terminal unit la includes an audio communication circuit 11a, a data communication circuit 12a, a transmitter circuit 13a for multiplexing the outputs of these two communication circuits 11a, 12a and then transmitting the multiplexed output to the transmission line 10a, and a control circuit 14a. The terminal unit 1a also includes, as external units, a telephone 110a to be connected to the audio communication circuit 11a, and a display 120a and a keyboard 121a which are connected to the control circuit 14a. 
     The control circuit 14a is connected to the data communication circuit 12a directly and is also connected to the audio communication circuit 11a via an operating signal transmitting circuit 141a, which outputs a tone having a predetermined frequency, and an operating signal detecting circuit 142a, which detects a tone of a predetermined frequency. The transmitting circuit 141a and the detecting circuit 142a are connected in parallel. 
     Each of the other terminal units 1b, 1c has the same construction as that of the terminal unit 1a. 
     The exchanger 2 includes a line switch 21 for immediate forward channel and a packet switch 22 for store-and-forward channel. The exchanger 2 also includes three transmitter circuits 23a, 23b, 23c each of which multiplexes the outputs from the two switches 21, 22 and transmits the multiplexed output to the associated transmission line 10a. 
     In the screen communications system of this embodiment, a call in voice is set between the audio communication circuits 11a, 11b, 11c via the exchanger 2, and a call in packet is set between the data communication circuits 12a, 12b, 12c. 
     The displays 120a, 120b, 120c indicate the same contents by the call in packet. 
     When the keyboard 121a is operated while the other keyboards 121b, 121c are not operated, the control circuit 14a causes the operating signal transmitting circuit 141a to output an operating signal. The audio communication circuit I la totals the operating signal output from the operating signal transmitting circuit 141a and an audio signal output from the telephone 110a and outputs the result to a transmitter circuit 13a. 
     Further, the control circuit 14a changes the content of the display 120a. In order to change the contents of displays 120b, 120c similarly, screen modifying data is processed into a packet, and the packetted data is output to the transmitter circuit 13a. 
     Meanwhile, in each of the other terminal units 1b, 1c, when the operating signal detecting circuit 142b, 142c detects the operating signal output from the transmitter circuit 13a via the exchanger 2, the operating signal detecting circuit 142b, 142c outputs a detection signal to the associated control circuit 14b, 14c. The control circuit 14b, 14c ignores the operation of the keyboard 121b, 121c and modifies the content of the display 120b, 120c based on the screen modifying data detected by the data communication circuit 12b, 12c. At that time, even if the keyboard 121b, 121c is operated, the control 14b, 14c does not activate the operating signal transmitting circuit 141b, 141c. 
     FIG. 3 is a state transition diagram showing the process in which a modifying right is discriminated by the control circuit 14a, 14b, 14c. 
     As the control circuits 14b, 14c are identical with the control circuit 14a, the manner of discrimination of a modifying right will now be described only in connection with the control circuit 14a. 
     The control circuit 14a, as shown in FIG. 2, includes three timers 81a, 82a, 83a and a buffer 143a which stores data input from the keyboard 121a. 
     In the beginning, the control circuit 14a is in an initial state 900. In the initial state 900, when the keyboard 121a is operated, the control circuit 14a causes the operating signal transmitting circuit 141a to output an operating signal, stores in the buffer 143a the data input from the keyboard 121a, causes the timers 81a, 82a to be reset and started, and then transits into a wait-for-modifying-right state 910. 
     This timer 81a is a timer for detecting whether or not the keyboard 121a has been operated simultaneously with the keyboards 121b and 121c. In the terminal units 1b or 1c, assuming that an operating signal is output as the keyboard 121b or 121c is operated immediately before detecting an operating signal from the keyboard 121a, the value of the timer 81a may be set to be longer than the longer of either the total time of the transmission delay time in transmission of an operating signal from the terminal unit 1a to the terminal unit 1b and the time required to detect an operating signal by the operating signal detecting circuit 142b, or the total time of the transmission delay time in transmission of an operating signal from the terminal unit 1a to the terminal unit 1c and time required to detect an operating signal by the operating signal detecting circuit 142c. 
     In the wait-for-modifying-right state 910, when the keyboard 121a is operated, the control circuit 14a causes the operating signal transmitting circuit 141a to output an operating signal, stores in the buffer 143a the data input from the keyboard 121a, and causes the timer 82a to be reset and started. 
     Namely, if no operating signal from the terminal unit 1b, 1c is detected before the timer 81a expires, it may be regarded that the terminal unit 1a has a modifying right, and then the control circuit 14a will transit into a presence-of-modifying-right state 920. 
     In the presence-of-modifying-right state 920, the control circuit 14a changes the content of the display 120a depending on the content of the buffer 143a and the input from the keyboard 121a, and causes the timer 82a to be reset and started each time the keyboard 121a is operated. 
     This timer 82a is a timer for guaranteeing a modifying right as long as a series of input from the keyboard 121a continues. The value of the timer 82a may be set to be larger than the value of the timer 81a and longer than the time interval between successive series of input from the keyboards 121a. 
     When the timer 82a expires, the control circuit 14a will transit into the initial state 900. 
     In the wait-for-modifying-right state 910, when the operating signal detecting circuit 142a detects an operating signal from the terminal unit 1b or 1c before the timer 81a expires, the control circuit 14a will be cleared to reset the timer 82a to start and will transit into a collision state 930. 
     In the collision state 930, each time the keyboard 121a is operated and each time the operating signal detecting circuit 142a detects an operating signal, the timer 82a will be reset and started. Also in the collision state 930, any input from the keyboard 121a is invalid, and each time any input from the keyboard 121a is made, the operating signal transmitting circuit 141a will transmit an operating signal. When the timer 82a expires, the control circuit 14a will transit into the initial state 900. 
     In the initial state 900, when the operating signal detecting circuit 142a detects an operating signal from the terminal unit 1b or 1c, the control circuit 14a will cause the timer 83a to be reset and started and will transit into an absence-of-modifying-right state 940. 
     In the absence-of-modifying-right state 940, since any input from the keyboard 121a is invalid, the operating signal transmitting circuit 141 a does not operate. 
     In the absence-of-modifying-right state 940, each time the operating signal detecting circuit 142a detects an operating signal, the control circuit 14a will cause the timer 83a to be reset and start and will change the content of the display 120a based on the screen modifying data received by the data communication circuit 12a. 
     The tinier 83a is a timer for guaranteeing the transmission delay time in store and forward channel compared to immediate forward channel. The value of the timer 83a may be set to be larger than the value of the tinier 82a at least by the transmission delay time when transmitting the screen modifying data. As a result, it is possible to exclude any input from the keyboard 121a while the content of the display 120a is being changed by a series of input from the keyboard 121b or 121c of the terminal unit 1b or 1c. 
     In the absence-of-modifying-right state 940, when the timer 83a expires, the control circuit 14a will transit into the initial state 900. 
     In the state other than the absence-of-modifying-right state 940, when it receives the screen modifying data, the terminal unit 1a will notify a user that there possibly is a difference in the content of display from the terminal units 1b, 1c. 
     FIG. 4 is a sequence diagram corresponding to the state transition diagram of FIG. 3, representing the operation in the first embodiment. 
     This operation between the terminal units 1a, 1b, for example, will now be described. In FIG. 4, reference numerals 500-504 designate operating signals; and 520 and 521, screen modifying data. 
     FIG. 4A shows a sequence when both the keyboards 121a, 121b are operated. 
     When the keyboard 121a is operated, the control circuit 14a will cause the operating signal transmitting circuit 141a to output an operating signal 500, will store in the buffer 143a the data input from the keyboard 121a, will cause the timers 81a, 82a to be reset and started, and will transit from the initial state 900 into the wait-for-modifying-right state 910. 
     When the keyboard 121a is operated before the control circuit 14a detects the operating signal 500, the control circuit 14b will cause the operating signal transmitting circuit 141b to output an operating signal 500, will store in the buffer 143b the data input from the keyboard 121b, will cause the timers 81b, 82b to be reset and started, and will transit from the initial state 900 into the wait-for-modifying-right state 910. 
     In the wait-for-modifying-right state 910, since they receive respective operating signals 501, 500 before the timers 81a, 81b expire, both the control circuits 14a, 14b will clear the contents of the buffers 143a, 143b, will cause the timers 82a, 82b to be reset and started and will transit to the collision state 930. 
     In the collision state 930, any input from the keyboards 121a, 121b is invalid, and this fact is notified to the operator by beeping and is displayed on the display screen. 
     Since the control circuits 14a, 14b transit from the collision state 930 into the initial state 900 when the timers 82a, 82b time out, the contents of the displays 120a, 120b will not be changed. 
     In the first embodiment, the timers 81a, 81b, 81c are used for detecting any collision and are set to more than several tens of ms, for example. This is because the transmission delay time when the maximum-step relay is performed on the telephone network of NTT (Nippon Telephone and Telegram) is about 23 ms. In this case, since the reaction of the display or the occurrence of the beeping sound in response to any input from the keyboards are delayed only by several tens of ms, the operator will not discern a time difference of reaction of the terminal unit. 
     The timers 82a, 82b, 82c are set to more than several hundreds of ms, and the timers 83a, 83b, 83c are set from about several seconds to severals tens of seconds. 
     If the transmission delay time of immediate forward channel is more than several hundreds of ms, the operator might feel a time difference of reaction of the terminal unit. In this case, it is preferable to display the content of the buffer on a part of the own display by using, for example, a multi-window. 
     FIG. 4B shows the sequence until the disappearance of a modifying right when only the keyboard 121a is operated. 
     The control circuit 14a transits into the presence-of-modifying-right state 920 when the timer 81a times out after an operating signal 503 is transmitted, by which time the control circuit 14b transits into the absence-of-modifying-right state 940. 
     In the presence-of-modifying-right state 920, the control circuit 14a changes the content of the display 120a depending on the content of the buffer 143a and the input from the display 120a and causes the timer 82a to be reset and started each time the keyboard 121a is operated. Each time the keyboard 121a is operated, the control circuit 14a outputs operating signals 503, 504 and also outputs screen modifying data 520, 521 input from the keyboard 121a. 
     Further, in the absence-of-modifying-right state 940, each time the operating signals 503, 504 are detected, the control circuit 14b will cause the timer 83b to be reset and started and will modify the content of the display 120b based on the screen modifying data 520, 521. At that time, since any input from the keyboard 121b is invalid, this fact is notified by beeping and is displayed on the display screen. 
     The control circuit 14b transits from the absence-of-modifying-right state 940 into the initial state 900 when the timer 83b times out. Meanwhile, the control circuit 14a transits from the presence-of-modifying-right state 920 into the initial state 900, when the timer 82a expires. Therefore both the two terminal units 1a, 1b will not assume the absence-of-modifying-right state 920. 
     In this embodiment, the display of the collision state and the display of the absence-of-modifying-right state are different but they may be identical with each other. 
     FIG. 5 shows another state transition of the control circuit 14a according to the second embodiment of this invention. 
     In FIG. 5, in the presence-of-modifying-right state 921, when the timer 82a expires, the control circuit 14a will transit into a wait-for-completion-of-modifying state 951. After an operating signal is transmitted, the control circuit 14a can be adopted in the case where the transmission delay time of store-and-forward channel is not guaranteed. Thus, since it is unclear when the screen modifying data is to be received, the modifying right releasing data representing the completion of modifying is notified to the operator. 
     In FIG. 5, in the presence-of-modifying-right state 921, each time the keyboard 121a is operated, the control circuit 14a will activate the operating signal transmitting circuit 141a and will cause the timer 82a to be reset and started. 
     Upon completion of modification, i.e., when the timer 82a expires, the control circuit 14a will transit into the wait-for-completion-of-modifying state 951 and will transmit the modifying right releasing data by using the data communication circuit 12a, and then will transit into the initial state 901. 
     In the absence-of-modifying-right state 941, when the modifying right releasing data is received irrespective of the timer 83a, the control circuit 14a will transit into the initial state 901. 
     Alternatively, in the wait-for-completion-of-modifying state 951, when the keyboard 121a is operated before transmission of the screen modifying data from the data communication circuit 12a has completed, the control circuit 14a may activate the operating signal transmitting circuit 141a, may cause the tinier 82a to be reset and started and may transit into the presence-of-modifying-right state 921. 
     FIG. 6 shows a sequence corresponding to the state transition of FIG. 5, representing the operation of the second embodiment. 
     In FIG. 6, when the keyboard 121a is operated, the control circuit 14a will cause the operating signal transmitting circuit 141a to output an operating signal 500, will store in the buffer 143a the data input from the keyboard 121a, will cause the timers 81a, 82a to be reset and started and will transit from the initial state 901 into the wait-for-modifying-right state 911. In the wait-for-modifying-right state 911, when the timer 81a expires, the control circuit 14a will transit into the presence-of-modifying-right state 921. 
     In the presence-of-modifying-right state 921, each time the content of the display 120a is changed based on the content of the buffer 142a and the input from the keyboard 121a, and each time the keyboard 121a is operated, the control circuit 14a will cause the timer 82a to be reset and started. 
     When the timer 82a expires, the control circuit 14a will transit into the wait-for-completion-of-modifying state 951 and will soon thereafter transmit the modifying right releasing data 530 and then transit into the initial state 901. 
     Upon receipt of an operating signal 500, the control circuit 14b transits from the initial state 901 into the absence-of-modifying-right state 941. 
     In the absence-of-modifying-right state 941, the control circuit 14b modifies the content of the display 120b based on the screen modifying data 520. At that time, since any input from the keyboard 121b is invalid, this fact is notified to the operator by beeping and it is indicated on the display that there is no modifying right. 
     In the absence-of-modifying-right state 941, upon receipt of the modifying right releasing data 530, the control circuit 14b transits into the initial state 901. 
     As long as it is in the presence-of-modifying-right state 921 or the wait-for-completion-of-modifying state 951, the screen modifying data 520 may be transmitted at any time. If the keyboard 121a is operated before the data communication circuit 12a has completed transmission of the screen modifying data 530, the control circuit 14a may transit into the presence-of-modifying-right state 921 again. 
     In the foregoing embodiments, the modifying signal transmitting circuits 141a, 141b, 141c of FIG. 2 are set so as to respectively output operating signals of different frequencies. A plurality of resonance circuits may be provided in the operating signal detecting circuit 142a; by detecting which resonance circuit has detected the received operating signal, it is possible to detect which one of the terminal units 1b, 1c has been operated, and so it is possible to display on the display 120a the name of the operator of the terminal unit. 
     FIG. 7 shows a screen communications system according to the third embodiment of this invention. 
     This system generally comprises a plurality of (three in the illustrated embodiment) terminal units 3a, 3b, 3c and an exchanger 4. Each of the terminal unit 3a, 3b, 3c includes, instead of the transmitting circuit 13a, 13b, 13c of the previous embodiments, a multiplexing circuit 33a, 33b, 33c for multiplexing an audio signal, screen modifying data and an operating signal into an immediate forward exchange channel and then transmitting them. Each terminal unit 3a, 3b, 3c also includes, instead of the operating signal transmitting circuit 141a, 141b, 141c of the previous embodiments, a flip-flop 341a, 341b, 341c and, instead of the detecting circuit 142a, 142b, 142c of the previous embodiments, a flip-flop 342a, 342b, 342c. 
     The manner of state transition of control circuits 34a, 34b, 34c in the terminal units 3a, 3b, 3c is identical with that described in connection with FIG. 3. 
     When the control circuit 34a is in the wait-for-modifying-right state 910, the presence-of-modifying-right state 920 or the collision state 930, the flip-flop 341a is set to &#34;1&#34;. 
     In this embodiment, a bit multiplexing method can be considered for use as a method of multiplexing an operating signal into an immediate forward channel. To perform this method, the flip-flops 341a, 341b, 341c and the flip-flops 342a, 342b, 342c are used. This bit multiplexing method is exemplified by a method of transmitting immediate channel data of, for example, 64 kb/s 8 bits for every 8 kHz and allocating 7 bits to an audio signal and 1 bit to an operating signal. 
     FIG. 8 shows an example of screen display according to the fourth embodiment of this invention, and FIG. 9 shows the hardware of the screen communications system of the fourth embodiment. 
     As shown in FIG. 8, the display 120 is divided into a plurality of (three in the illustrated embodiment) areas 70, 71, 72; the area 70 is shared with a terminal unit group 60, the area 71 is shared with a terminal unit group 61, and the area 72 is used only by the terminal unit 6 itself. 
     The terminal unit 6 has operating signal transmitting and receiving circuits 641,642 connected to the terminal unit groups 60, 61 via an exchanger 2. 
     When an operating signal is transmitted, as a terminal unit 601 of the terminal unit group 60 is operated, an operating signal receiving circuit 641 will detect this operating signal, and a control circuit 64 will cause an input inhibition display 701 of the area 70. The control circuit 64 will cause the cursor to move out of the area 70 and will not allow the cursor to move into the area 70 while any input is inhibited. 
     At that time, if a screen modification is made within the area 71, the control circuit 64 will cause the operating signal transmitting and receiving circuit 642 to transmit an operating signal to inhibit any input to the terminal unit group 61. 
     Thus in this embodiment, the multi-window method is adopted so that a screen communications system can be constructed for each window. Since a single window is only for the terminal unit itself, it is convenient to use. 
     According to this invention, since by only operating the keyboard each terminal unit can discriminate by itself whether or not there is a modifying right, with guaranteeing that the contents of screen displays of the individual terminal units are identical, the delay time from operating the terminal unit until the operator&#39;s own display reacts can be reduced. Therefore it is possible to quickly move a modifying right so that the operativity of the terminal unit can be improved. 
     Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the claims.