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BACKGROUND OF THE INVENTION
The present invention relates to a modem pooling system for enabling
selective connection of a plurality of data terminal units to a modem
through exchange service.
A modem is mainly used to connect a data terminal unit to an analog line
such as a public telephone line. The modem pooling system is a system for
enabling a number of data terminal units to get access to an analog line
by use of a few modem, and practically realized in a digital key telephone
system, for instance.
In the digital key telephone system of this type, conventionally, at least
one modem is connected to a key service unit including an exchange circuit
via an interface (e.g. RS-232C), and further an analog office line is
connected to the modem. Further, a plurality of extension key telephone
sets connected to the key service unit are provided with each interface
unit (e.g. RS-232C) respectively, through which each personal computer is
connected, as a data terminal unit, to each extension key telephone set.
In the conventional system, when an incoming call arrives at the modem
through the office line, the incoming call signal is first transmitted
from the modem to a control unit in the key service unit. In response to
this signal, the control unit controls a switching circuit for exchange
service to connect the modem to a data terminal unit, and further
indicates the incoming call arrival to the data terminal unit. In response
to the incoming call information, the data terminal unit starts data
communication with the office line.
In the conventional system, however, where an automatic responsive modem is
used, there exists a problem in that when the control unit is getting
access to the data terminal unit to be called, the modem responds to an
incoming call signal, so that before the modem is connected to the data
terminal unit, data are transmitted from the office line to the modem and
therefore the head portion of the transmitted data cannot be received by
the data terminal unit.
In more detail, the automatic responsive modem usually responds
automatically to an incoming call signal arrival by an independent control
unit incorporated in the modem, after a predetermined time has elapsed,
irrespective of the operation of the key service unit. However, when the
control unit in the key service unit is under overload conditions (the
state where a plurality of outgoing or incoming call signals are generated
simultaneously and therefore each control processing is delayed
momentarily), since the exchange service is delayed from the automatic
response of the modem, the head portion of the data inputted from the
modem to the key service unit will not be transmitted to the data terminal
unit.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a modem pooling system
which can prevent the head portion of data to be received by the data
terminal unit via the modem from being omitted, even when an automatic
responsive modem is incorporated in the system.
To achieve the above-mentioned object, the present invention provides a
modem pooling system for enabling selective connection of a plurality of
data terminal units to a modem, comprising: data terminal state detecting
means for detecting whether each of the data terminal units is active or
inactive; responsive terminal selecting means for selecting one data
terminal unit as a responsive terminal unit from among the data terminal
units determined to be active by said data terminal state detecting means,
in accordance with a predetermined priority order; and connection control
means for previously connecting the modem to the selected responsive
terminal unit, prior to an incoming call response of the modem.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system block diagram showing an embodiment of the modem pooling
system according to the present invention;
FIGS. 2A-2C are block diagrams showing a key service unit of the system
shown in FIG. 1;
FIG. 2 is a diagram showing the arrangement of FIGS. 2A-2C.
FIG. 3 is a block diagram showing the configuration of a key service unit
interface unit and a data terminal interface unit in a digital key
telephone set of the system shown in FIG. 1;
FIG. 4 is a block diagram showing a ping-pong transmission circuit provided
in the key service unit interface shown in FIG. 3;
FIG. 5 is a block diagram showing a microprocessor for controlling a
telephone set, which is provided in the key service unit interface shown
in FIG. 3;
FIG. 6 is a block diagram showing a microprocessor for controlling data
communications, which is provided in the data terminal unit interface
shown in FIG. 3;
FIG. 7 is a flowchart for assistance in explaining the responsive PC
selecting processing executed by the control unit shown in FIG. 2; and
FIGS. 8A-8B are sequence diagrams for assistance in explaining the
operation sequence of the embodiment shown in FIG. 1.
FIG. 8 is a diagram showing the arrangement in FIGS. 8A-8B.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will be described hereinbelow with
reference to the attached drawings.
FIG. 1 is a block diagram showing an embodiment of the modem pooling system
according to the present invention, in which a digital key telephone
system is shown by way of example.
In this system, a plurality (n) of digital key telephone sets (DKT) 3.sub.1
to 3.sub.n are connected to a key service unit (KSU) provided with
exchange and control service functions, and further a plurality (n) of
personal computers (PC) 5.sub.1 to 5.sub.n are connected to the digital
key telephone sets (DKT), respectively. An analog office line 7.sub.1 and
a modem 9 are connected to the key service unit 1. Another office line
7.sub.2 is connected to the modem 9. Each personal computer 5.sub.1 to
5.sub.n have access to the office line 7.sub.2 via the modem 9. Further,
the interface between the modem 9 and the key service unit 1 is RS-232C.
The key service unit 1 includes a plurality of digital key telephone set
(DKT) interface units 11.sub.1 to 11.sub.n connected to the digital key
telephone sets 3.sub.1 to 3.sub.n, an office line interface unit 13
connected to the office line 7.sub.1, and a modem interface unit 15
connected to the modem 9. All of these interface units are connected to a
time switch 17 for controlling the exchange service between these
interface units. Hereinafter, data transmission from the time switch 17 to
these interface units is referred to as "down-stream data transmission"
and data transmission in the opposite direction is referred to as
"up-stream data transmission". Further, these interface units and the time
switch 17 are connected to the control unit 19 for controlling the system
operation, via a control data bus 21.
The digital key telephone set 3 includes a key service unit (KSU) interface
unit 23 connected to the DKT interface unit 11 in the key service unit 1,
a telephone set circuit 25 having a speech network, a dial circuit, a
handset, etc. to enable various key telephone set functions, and a data
terminal unit interface unit 27 connected to the data terminal unit 5.
Data are ping-pong transmitted between the KSU interface unit 23 and the
DKT interface unit 11 via (2B+D) time-division channel, for instance. In
more detail, data packets obtained through two time-division signal
channels B1 and B2 and a time-division data channel D in combination are
transmitted between the KSU interface unit 23 and the DKT interface unit
11. The B1 channel is used to transmit speech data when the key telephone
set 3 is in speech operation, and the B2 channel is used to transmit data
when the personal computer is in data communication operation. On the
other hand, the D channel is used to transmit various control commands (to
control various operations such as incoming call, outgoing call, response,
interrupt and restart of data communications, speech end, etc.) and result
codes. Data transmitted through B channel is referred to as "communication
data", and data transmitted through D channel is referred to as "control
data", hereinafter.
FIGS. 2A-2C show each section of the key service unit 1. The DKT interface
unit 11 includes a (2B+D) forming circuit 29 and a (2B+D) separating
circuit 31. The (2B+D) forming circuit 29 receives communication data from
two designated down-stream time slots outputted from the time switch 17
and control data from the control unit 19 via the bus 21, and forms (2B+D)
data packets by multiplexing these data to transmit data to the key
telephone set 3. Further, the (2B+D) separating circuit 31 receives the
(2B+D) data packets from the key telephone set 3, separates the data for
each channel, transmits the separated communication data to two designated
up-stream time slots inputted to the time switch 17 and the separated
control data to the control unit 19 via the bus 21. The transmission of
the (2B+D) forming circuit 29 and the reception of the (2B+D) separating
circuit 31 are executed independently in time division mode.
The modem interface unit 15 is an interface equivalent to so-called
three-wire RS-232C interface. That is, this modem interface unit 15
receives data from the modem 9 via the RD line and transmits the received
data to one designated down-stream time slot of the time switch 17, and
further receives data from one designated up-stream time slot of the time
switch 17 and transmits the received data to the modem 9 via the SD line.
This modem interface unit 15 includes an RD analyzing circuit 33 and a SD
analyzing circuit 35. These analyzing circuits 33 and 35 monitor and
analyze data (RD and SD) transmitted between the modem 9 and the time
switch 17 via the interface unit 15, and transmit control data
corresponding to the results to the control unit 19 via the bus 21.
Further, a modem control circuit 36 receives control data from the control
unit 19 and transmits control commands corresponding thereto to the modem
9, while having access to the control unit 19 to check the presence or
absence of carriers required for control.
The office line interface unit 13 is provided with various functions such
as detection function of outgoing and incoming call signals to and from
the office line 7.sub.1 ; transmission function of communication data
between the office line 7.sub.1 and the time switch 17, etc. The office
line interface unit 13 is so selected as to be applicable to the sort of
the office lines 7.sub.1 (e.g. analog office line, ISDN office line,
etc.). In the case of ISDN (integrated service digital network) office
line, (2B+D) forming circuit and (2B+D) separating circuit both similar to
those provided in the DKT interface unit 29 are incorporated in the office
line interface unit 13.
The control unit 19 includes a PC state control unit 41, a modem state
control unit 43, a DKT/office line state control unit 45, a time switch
controller 47, a RAM 49, and an idle PC search unit 51. The PC state
control unit 41 monitors the states (e.g. standby, incoming call, data
communication, etc.) of the personal computers 5.sub.1 to 5.sub.n on the
basis of control data given by the DKT interface units 11.sub.1 to
11.sub.n, and transmits control data to the DKT interface units 11.sub.1
to 11.sub.n to control the personal computers 5.sub.1 to 5.sub.n. The
modem state control unit 43 monitors the state of the modem 9 on the basis
of the control data given by the modem interface unit 15, and transmits
necessary control data to the modem interface unit 15. The DKT/office line
state control unit 45 monitors and controls the states of the key
telephone sets 3.sub.1 to 3.sub.n and the office line 7.sub.1 by
transmitting control data between the DKT interface units 11.sub.1 to
11.sub.n and the office line interface unit 13.
The time switch controller 47 controls the time switch 17 according to the
state of each terminal unit and office line to connect/disconnect the
communication path.
Further, the control unit 19 includes a PC search unit 51. This PC search
unit 51 refers to a state table in a RAM 49 to select a single personal
computer now being in standby state. The selected computer is designated
as a personal computer in charge of response to an incoming call given
through the office line 7.sub.2 (referred to as a responsive PC,
hereinafter).
When a responsive PC has been decided, the information is given to the time
switch controller 47, so that the time switch controller 47 keeps the
connection between the modem and the responsive PC. Therefore, when an
incoming call signal arrives at the modem 9 via the office line 7.sub.2,
since the modem 9 has already been connected to the responsive PC, no
exchange service is required; that is, the data inputted through the
office line 7.sub.2 can be immediately received by the responsive PC. The
configuration of the control unit 19 will be described in detail later.
FIG. 3 shows a detailed configuration of the KSU interface unit 23 and the
data terminal interface unit 27 both incorporated in the digital key
telephone set 3.
The KSU interface unit 23 includes a ping-pong transmission circuit 53 and
a telephone set control microprocessor 53. The ping-pong transmission
circuit 55 is connected to the DKT interface unit 11 of the key service
unit 1 via a two-wire signal line 57, and provided with a (2B+D)
separating circuit 59 and a (2B+D) forming circuit 61 as shown in FIG. 4.
The (2B+D) separating circuit 59 receives down-stream packets from the key
service unit 1, separates the received data packet into down-stream data
PH01, PH02 and DOUT for each channel, and outputs these data. The
outputted B1-channel down-stream communication (speech) data PH01 are
transmitted to the telephone set circuit 25; the B2-channel down-stream
communication data are transmitted to the data terminal interface unit 27;
and the down-stream control data are transmitted to the telephone set
circuit control microprocessor 55 and the data terminal interface unit 27.
Further, B1-channel up-stream communication (speech) data PHI1 are
inputted from the telephone set circuit 25 to the ping-pong transmission
circuit 53; and B2-channel up-stream communication data PHI2 and up-stream
control data DIN are inputted from the data terminal interface unit 27 to
the transmission circuit 53. These up-stream data PHI1, PHI2 and DIN are
multiplexed into the up-stream (2B+D) packet by the (2B+D) forming circuit
61, and then transmitted to the key service unit 1 via the signal line 57.
The down-stream packet reception and the up-stream packet transmission are
executed independently in time division manner.
The telephone set control microprocessor 55 controls the operation of the
telephone set circuit 25. This microprocessor 55 includes a telephone set
control data analyzing section 63, a key/hook switch signal receiving
section 65, an up-stream control data transmitting section 67, and a
D-channel control signal forming section 69, as shown in FIG. 5. The
telephone set control data analyzing section 63 receives down-stream
control data DOUT from the ping-pong transmission circuit 53, analyzes the
control data (if DOUT are data for controlling the corresponding key
telephone set), and transmits control signals to the telephone set circuit
25 in order to turn on a display LED, generate a call tone, etc. according
to the analyzed result. The key/hook switch signal receiving section 65
receives key signals (generated when dial keys or function keys of the key
telephone set are depressed) and hook signals indicative of handset on/off
hook transmitted from the telephone set circuit 25, forms control data
corresponding to the received signals, and transmits the formed control
data to the up-stream control data transmitting section 67. The up-stream
control data transmitting section 67 transmits the received control data
to the data terminal interface unit 27 as the first up-stream control data
DIN 1.
Further, the key/hook switch signal receiving section 65 gives an
information signal indicative of the presence or absence of signals
inputted from the telephone set circuit 25, to the D-channel control
signal forming section 69. On the basis of the information signal, the
D-channel control signal forming section 69 forms a D-channel control
signal DCNT ("H" if the signal is inputted but "L" if not inputted), and
gives it to the data terminal interface unit 27.
As described later, the D-channel control signal DCNT controls the control
data transmission from the data terminal interface unit 27 to the
D-channel.
The data terminal interface unit 27 will be explained hereinbelow. As shown
in FIG. 3, the data terminal interface unit 27 constitutes an RS-232C
interface to the personal computer 5 (only the connection lines ER, SD and
RD between the two are shown).
An equipment-ready signal inputted from the personal computer 5 via the ER
line 70 indicates whether the personal computer 5 is active or inactive.
This equipment-ready signal ER is inputted to a data communication control
microprocessor 77. This communication control microprocessor 77 outputs
data indicative of "active" or "inactive" as the second D-channel
up-stream data DIN 2, whenever the personal computer 5 changes from
"inactive" to "active" or from "active" to "inactive". Data SD transmitted
from the personal computer 5 are inputted to the data terminal interface
unit 27 via the SD line 71, irrespective of communication data or control
data. The transmitted data SD are inputted to a data communication control
microprocessor 77 and also to an AND gate 73. The output data of this AND
gate 73 are inputted to the ping-pong transmission circuit 53 as the
B2-channel up-stream communication data PHI2. Further, the B2-channel
down-stream data PH02 from the ping-pong transmission circuit 53 are
applied to an input terminal B of a selector 75. The output data of the
selector 75 are received by the personal computer 5 via the SD line 79.
The AND gate 73 and the selector 75 are controlled in response to a mode
signal MODE of high/low ("H"/"L") level transmitted by a data
communication control microprocessor 77. In more detail, the mode signal
MODE is kept at "H" level during data communication to open the AND gate
73 and to allow the selector 75 to select an input terminal B. Therefore,
the data SD transmitted from the personal computer 5 become the B2-channel
up-stream communication data PHI2, and the B2-channel down-stream
communication data PHO2 become the data RD received by the personal
computer 5. As a result, communication data can be transmitted between the
personal computer 5 and the key service unit 1.
On the other hand, when outgoing and incoming call signals are controlled,
the mode signal MODE is kept at "L" level to close the AND gate 73 and to
allow the selector 75 to select an input terminal A.
To this terminal A, an output terminal 78 of the data communication control
microprocessor 77 is connected to output control data to the personal
computer 5. Therefore, the data SD transmitted by the personal computer 5
are transmitted to only the data communication control microprocessor 77
(without being transmitted to the B-channel), and only the control data
outputted by the data communication control microprocessor 77 are received
by the personal computer 5 as data RD. As a result, control data can be
transmitted between the personal computer 5 and the data communication
control microprocessor 77. When the control data are being transmitted,
the communication data are blocked (not transmitted).
FIG. 6 shows the internal configuration of the data communication control
microprocessor 77. The microprocessor 77 includes an ER signal decoder 82,
a down-stream data receiving section 83, a down-stream data decoder 85, a
command receiving section 87, a command decoder 89, various result code
generating sections 91, 93, 95 and 97, various up-stream data generating
sections 98, 99 and 101, flip-flops 105, 107 and 108, and an up-stream
data transmitting section 109.
The ER signal decoder 82 determines whether the personal computer 5 is
active or inactive, in response to the equipment-ready signal ER
transmitted from the personal computer 5. Whenever the personal computer 5
changes from "inactive" to "active" or from "active" to "inactive", the ER
signal decoder 82 outputs data indicative of "active" or "inactive". These
data are inputted to a buffer 127, and then outputted from the up-stream
data transmit section 109 as the second D-channel up-stream data DIN 2.
The down-stream data receiving section 83 receives the D-channel
down-stream data DOUT from the ping-pong transmission circuit 53, extracts
control data for the data communication control microprocessor 77
therefrom, and transmits the extracted control data to the down-stream
data decoder 85. The down-stream data decoder 85 decodes the received
down-stream control data. These down-stream data are an incoming call
signal data indicative of an incoming call, a connection data indicative
of connection to another extension telephone set via the time switch 17, a
disconnection data indicative of disconnection from the extension
telephone set, a modem connection data indicative of connection to the
modem via the time switch 17, and a modem disconnection data indicative of
disconnection from the modem. If the decoded data is the incoming call
data, a ring generating section 93 is activated; if the connection data, a
connect generating section 95 is activated; if the disconnection data, a
no-carrier generating section 97 is activated, respectively, so that
result codes of RING, CONNECT, NO CARRIER are outputted by the generating
sections, respectively. Further, if the modem connection data is decoded,
the flip-flop 105 is set; and if the modem disconnection data is decoded,
the flip-flop 105 is reset, when this flip-flop 105 is kept set, since the
switch 121 is kept open, the data SD transmitted by the personal computer
5 are not inputted to the command receiving section 87.
The command receiving section 87 receives the data SD transmitted by the
personal computer 5, extracts control commands (referred to as "AT
commands", in general) from the data stream, and transmits the extracted
data to the command decoder 89. The command decoder 89 decodes the
transmitted control commands. These control commands are an outgoing call
command to generate an outgoing call, an incoming call command to generate
an incoming call, an interrupt command to interrupt data communication, a
restart command to restart interrupted data communication, and an end
command to end data communication. If the decoded command is an outgoing
call command, an outgoing call data generating section 98 is activated to
output an outgoing call data; if an incoming call response command, an
incoming call response data generating section 99 is activated to output
an incoming call response data; and if an end command, an end data
generating section 101 is activated to output an end data. These data are
inputted to the buffer 127 and then outputted by the up-stream data
transmitting section 109 as the second D-channel up-stream data DIN 2.
Further, the outgoing call command, the interrupt command or the end
command is decoded, an OK generating section 91 is activated to output a
result code OK. This result code OK and the aforementioned other result
codes RING, CONNECT and NO CARRIER are all transmitted to the input
terminal A of the selector 75 via an OR gate 115 and then received by the
personal computer 5.
The flip-flop 107 generates a mode signal MODE. When the connection data,
the outgoing call command or the restart command is decoded or when the
flip-flop 105 is set, this flip-flop 107 is set to generate an H-level
mode signal MODE. On the other hand, when the disconnection data, the
modem disconnection data or the interrupt command is decoded, this
flip-flop 107 is reset to generate an L-level mode signal MODE.
The flip-flop 108 is set in response to an outgoing call command and reset
in response to the connection data. When this flip-flop 108 is being set,
the switch 121 is turned off, so that the data SD transmitted by the
personal computer 5 are not inputted to the command receiving section 87.
The microprocessor 77 has three operation modes of control mode, connection
standby mode and communication mode. In the control and connection standby
modes, the mode signal MODE changes to L-level, so that the personal
computer 5 is disconnected from the B2-channel. Further, in the connection
standby mode, since the flip-flop 108 is set and the switch 121 is turned
off, control commands transmitted by the personal computer 5 are not
received. On the other hand, in the communication mode, since the mode
signal MODE changes to H-level, the personal computer 5 is connected to
the B2-channel.
Further, there exists a modem direct connection mode as a special state of
speed modes. In this mode, the flip-flop 105 is set to open the switch 12.
To the up-stream data transmitting section 109, D-channel control signals
DCNT from the telephone set control microprocessor 55 are applied. Only
when this control signal DCNT is at L-level (i.e. the first D-channel
up-stream data DIN 1 are not outputted from the telephone set control
microprocessor 55), the up-stream data transmitting section 109 receives
data from the buffer 127 and outputs the second D-channel up-stream data
DIN 2. Both the first and second D-channel up-stream data DIN 1 and DIN 2
are inputted to the ping-pong transmission circuit 53 via the OR gate 81
as the D-channel up-stream data DIN 1. The control operation of the
D-channel control signal DCNT prevents the first and second D-channel
up-stream data DIN 1 and 2 from interfering with each other.
With reference to FIG. 3, voltage supply circuits 123 and 125 provided in
the KSU interface unit 23 and the data terminal interface unit 27 convert
a dc voltage (e.g. 24 V) supplied via a signal line 57 into another dc
voltage (e.g. 5 V) to supply it to both the interface units 23 and 27.
With reference to FIG. 2 again, the configuration of the control unit 19 of
the key service unit 1 will be described in detail hereinbelow.
The PC state control unit 41 receives D-channel control data transmitted
from each DKT interface unit 11. In practice, the control data are data
indicative of active/inactive state of each personal computer 5
transmitted from each data terminal interface unit 27 and various control
commands transmitted from each personal computer 5. These received control
data are inputted to a command/PC No. separating section 133 via a data
buffer 131 to separate the control data from an identification number
(referred to as PC No.) of the corresponding personal computer (which is
attached to the control data as a header). The control data (command) are
inputted to a command decoder 135, and the PC No. is inputted to the
command decoder 135 and a PC No./data composition section 137.
The command decoder 135 decodes the inputted command to detect a change in
state of each personal computer (i.e. change from active to inactive or
vice versa, idle (standby) state in active state, incoming call state,
change in mutual communication state, etc.). The changed state of each
personal computer is written in a PC state register 139. Therefore, the PC
state register 139 can store the updated state of each personal computer
(i.e. active or inactive, idle in active state, incoming call state,
communication state, etc.) for each PC number.
The command decoder 135 is provided with plural output lines one to one
corresponding to the above-mentioned state change, so that a trigger pulse
is outputted via each line corresponding to each decoded result. In this
case, where the decoded state change is an idle state in active state, an
incoming call state or a mutual communication state, each corresponding
response code data is read in response to each corresponding trigger
pulse, and then inputted to the PC No./data composition section 137 via a
response code data buffer 143. The PC No./data composition section 137
attaches a PC No. transmitted by the command/PC No. separating section 133
to the response code data as a header and then transmits the data to the
bus 21.
Further, all the trigger pulses outputted from the command decoder 135 are
inputted to an OR gate 145 irrespective of the detected state change, and
then applied to a PC state checking section 149 in an idle PC searching
unit 51, as PC state change detection signals. In response to these PC
state change detection signal, the PC state checking section 149 checks
the PC state, as described later in further detail.
A modem state control unit 43 receives control data from the modem
interface unit 15. The received control data are inputted to a
command/code decoder 153 via a command/code buffer 151. The command/code
decoder 153 decodes the inputted control data to detect the state of modem
(i.e. standby state, incoming call state, communication state, etc.), and
transmits a data indicative of the detected state to a comparator 155 and
a modem state buffer 157. The comparator 155 compares the modem state data
already stored in a modem state register 159 with that transmitted from
the command/code decoder 153, and outputs a trigger pulse when both the
data mismatch with each other. In response to this trigger pulse, the
modem state buffer 157 writes the held modem state data in the modem state
register 159, so that it is possible to store an updated modem state in
the modem state register 159.
In the case of mismatch of the compared result (i.e. the modem state has
been changed), the trigger pulse generated by the comparator 155 is also
applied to a modem state checking section 161 of the idle PC searching
unit 51. In response to the trigger pulse, the modem state checking
section 161 reads the data from the modem state register 159 in the modem
state control unit 43 to check the current modem state. As a checked
result, if the modem is in standby state, the modem state checking section
161 gives a standby state return pulse to the PC state checking section
149 and further applies an H-level logical signal to one input terminal of
an AND gate 163. In response to the standby state return pulse, the PC
state checking section 149 checks the PC state as described later, in the
same way as when the afore-mentioned PC state change detection signal is
received.
An output signal of the PC state checking section 149 is applied to the
other input terminal of the AND gate 163. As described above, the PC state
checking section 149 checks the state of each personal computer, whenever
the modem state is returned to a standby state from other states or
whenever the state of either one of the personal computers changes. The
above-mentioned check is executed as follows:
First, the PC state checking section 149 outputs a search pulse to the AND
gate 163. This search pulse is applied to a priority encoder 165 via the
AND gate 163 only when the modem 9 is in standby state and therefore the
output signal of the modem state checking section 161 is at H-level. In
response to the search pulse, the priority encoder 165 reads data from a
priority table 167 in which the priority order of predetermined responsive
personal computers is stored together with the PC Nos. In response to a
first search pulse, the priority encoder 165 reads a PC No. with the
highest priority. The read PC No. is written in a response PC No. register
169 as a temporary response PC No. and further given to the PC state
checking section 149. Then, the PC state checking section 149 transmits
the temporary responsive PC No. to a PC state inquiry section 147 in the
PC state control unit 41. The PC state inquiry section 147 gives the
received temporary responsive PC No. to the PC state register 139 to read
the corresponding PC state data, and returns the read PC state data to the
PC state checking section 149.
The PC state checking section 149 checks the returned PC state data. If the
PC state is not in active and idle (standby) state, the PC state checking
section 149 outputs a search pulse again. In response to the search pulse,
the priority encoder 165 reads a PC No. with the second higher priority,
and checks the state of PC of the read PC No. in the same way as described
above.
As described above, the PC states are repeatedly checked in the order of
priority. During the PC No. checking process, if a personal computer of
active and further idle state is found, the PC state checking section 149
selects this personal computer as a responsive PC (which can respond), and
stops outputting further searching pulses. Instead, the PC state checking
section 149 transmits a check-end signal to the priority encoder 165. In
response to the check end signal, the priority encoder 165 outputs a time
switch control data and a modem connection data each including a PC No. of
the finally read or selected responsive PC. The time switch control data
is transmitted to the time switch controller 47. In response to the
control data, the time switch controller 47 controls the time switch 17 so
that a communication path between the modem 9 and the selected responsive
PC can be connected. Further, the modem connection data is received by the
DKT interface unit 11 corresponding to the selected responsive PC, and
further transmitted to the data terminal interface unit 27.
With reference to FIGS. 7, 8A and 8B, the operation of the key telephone
system configured as described above will be described hereinbelow.
FIG. 7 is a flowchart for assistance in explaining the process for
selecting a responsive PC, which is executed by the idle PC search unit 51
of the control unit 19.
The idle PC search unit 51 checks the state of modem 9 with reference to
the state register 159 in the modem state control unit 43, whenever the
state of the modem 9 changes (in step S1). When the state of the modem 9
returns from other states to the standby state or when the state of either
one of personal computers changes in the modem standby state, the search
unit 51 reads the state data of each personal computer 5.sub.1 to 5.sub.n
from the state register 139 in the PC state control unit 41 (in step S2).
The state data are read in the order of priority previously determined in
the priority table 167.
The searching unit 51 reads the state data of a personal computer with the
highest priority, checks whether the read personal computer is in standby
state or not (in step S3). When the personal computer is in speech or
incoming-call state (irrespective of the active or inactive state), the
recorded state of another personal computer with the second higher
prior | | |
