Abstract:
A ring trip circuit is incorporated in a telephone exchanger together with a call signal source and a relay unit connected between the call signal source and user&#39;s terminal, and determines current status of the user&#39;s terminal at a certain timing close to the zero level of a call signal changed across the zero level without influence of fluctuation of the call signal source, a loop resistance and a leak resistance, thereby preventing the user&#39;s terminal from undesirable interruption of the call signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to a ring trip circuit and, more particularly, to a ring trip circuit incorporated in a telephone exchanger for producing a reset signal at a different timing from the zero-crossing point of a call signal. 
     DESCRIPTION OF THE RELATED ART 
     The ring trip circuit is incorporated in a telephone exchanger for controlling a call signal. When the telephone exchanger selects a telephone subscriber, the telephone exchanger supplies the call signal to the telephone subscriber. If the telephone subscriber responds to the call signal, the ring trip circuit stops the call signal. A typical example of the ring trip circuit is disclosed in Japanese Patent Publication of Unexamined Application No. 6-188972, and FIG. 1 illustrates the circuit configuration of the prior art ring trip circuit disclosed in the Japanese Patent Publication of Unexamined Application. 
     The prior art ring trip circuit comprises a call signal source  1  which superimposes an ac call signal VAC on a dc potential VDC so as to produce a call signal. The call signal source  1  has one end connected to a lease resistor  2 , and the lease resistor  2  is used for current detection and current control. The lease resistor  2  is connected to a pair of photo-emitting diodes  3   a / 3   b , and the photo-emitting diodes  3   a  and  3   b  are connected in parallel between the lease resistor  2  and a call signal transfer relay unit  4 . The photo-emitting diodes  3   a  and  3   b  are inverted to each other, and form in combination an optical signal transmitting unit  3 . The anode of the photo-emitting diode  3   a  and the cathode of the photo-emitting diode  3   b  are connected to the lease resistor  2 , and the anode of the photo-emitting diode  3   b  and the cathode of the photo-emitting diode  3   b  are connected to a first relay contact r 1  of the call signal transfer relay unit  4 . 
     The call signal source  1  has the other end connected to the ground. The other end of the call signal source  1  is further connected to a current limiting resistor  5 , which in turn is connected to a second relay contact r 2  of the call signal transfer relay unit  4 . The call signal transfer relay unit  4  selectively connects a call signal source  1  to telephone subscriber lines  6 . The telephone subscriber lines  6  are connected to a terminal  7 . Each of the telephone subscriber lines  6  provides a loop resistance  6   a  and a leak resistance  6   b  depending upon individual conditions such as the distance between the telephone exchanger and the terminal  7 . The terminal  7  includes a bell circuit equivalent to a series combination of a condenser CB, a resistor RB and a coil LB and a series combination of a hook switch S and a dc end resistor RS. 
     The photo-emitting diodes  3   a / 3   b  of the optical signal transmitting circuit  3  supply an optical signal Sop 1  to a photo-detecting transistor  8   a  of an optical receiver  8 . The photo-detecting transistor  8   a  has a collector connected to a positive potential line PT and an emitter connected through a resistor  8   b  to the ground, and flows emitter current into the resistor  8   b . The emitter current is equivalent to the current passing through the lease resistor  2 . Thus, the optical receiver detects the current passing through the lease resistor  2 . The resistor  8   b  converts the emitter current to an output potential signal Scrt, and the output potential signal Scrt is supplied to the output node of the optical receiver  8  between the photo-detecting transistor  8   a  and the resistor  8   b.    
     The output potential signal Scrt is supplied to a logic circuit  9 . The logic circuit  9  has a threshold equivalent to a zero-crossing value of the call signal, and compares the output potential signal Scrt with the threshold so as to give a binary value to the output potential signal. Thus, the logic circuit  9  converts the output potential signal to a binary signal Db, and the binary signal Db is supplied to a counter  10 . 
     While the binary signal Db is in the high level, the counter  10  is enabled, and increments the value in response to a clock pulse CLK. The output signal CNT of the counter  10  is indicative of the number of clock pulses CLK, and is compared with a digital threshold value DTH. If the value of the output signal CNT is less than the digital threshold, the comparator  11  determines the terminal  7  to be in on-hook status or off-state of the hook switch S. On the other hand, if the value of ,the output signal CNT is equal to or greater than the digital threshold, the comparator  11  determines the terminal  7  to be in off-hook status or on-state of the hook switch S. Thus, the comparator  11  produces a status signal STUS representative of the status of the terminal  7 , and the status signal STUS is supplied to a reset signal source  12 . 
     The reset signal source  12  is gated with the status signal STUS, and supplies a reset signal RST to a flip flop circuit  13  for controlling the relay unit  4  at transition timings of the binary signal Db from the high level to the low level and vice versa. As described hereinbefore, the threshold of the logic circuit  9  is equivalent to the value of the call signal at the zero-crossing point. Therefore, the reset signal RST is produced at the zero-crossing timings of the call signal. 
     When a relay-on signal RLY is supplied to the flip flop circuit  13  as a command from an upper level, the flip flop circuit  13  is changed to set-status, and causes the relay unit  4  to change the relay contacts r 1 /r 2  to the call signal source  1 . On the other hand, the flip flop circuit  13  is changed to the reset status in response to the reset signal RST, and causes the relay unit  4  to isolate the relay contacts r 1 /r 2  from the call signal source  1 . 
     The resistance of the lease resistor  2 , the resistance of the current limiting resistor  5 , the dc potential level VDC, the ac call signal VAC, the loop resistance  6   a , the leak resistance  6   b , the capacitance of the capacitor CB, the resistance of the resistor RB, the inductance of the coil LB and the resistance of the resistor RS are assumed to be 1 kilo-ohm, 0.15 kilo-ohm, 48 volts, 75 Vrms at 25 Hz, zero ohm, infinity, 0.45 micro-F, 54 H, 3.65 kilo-ohm and 0.4 kilo-ohm, respectively. When the relay-on signal RLY is supplied to the flip flop circuit  13 , the flip flop circuit  13  is changed to the set-status, and the call signal transfer relay unit  4  connects the call signal source  1  to a telephone subscriber line  6 . Then, the call signal is supplied from the call signal source  1  through the telephone subscriber line  6  to the terminal  7 . 
     If the terminal  7  is in the on-hook status or the hook switch S is in the off-state, the capacitor CB allows only the ac call signal to pass through the bell circuit (see FIG.  2 A), and the dc potential does not pass through the capacitor CB. The current IR 2  passing through the lease resistor  2  is expressed by equation 1. 
     
       
         IR2=13.4×Sin (50πt)[mA]  equation 1 
       
     
     When the potential exceeds over 0.7 volt as a clamp voltage of the photo-emitting diode  7   a , the photo-emitting diode  3   a  emits the light, and supplies the optical signal Soc 1  to the photo-detecting transistor  8   a . For this reason, the output potential signal Scrt is in the high level for 19.34 millisecond, and remains in the low level for 20.66 millisecond. 
     If a frame signal at 8 kliz is used as the clock pulse CLK, the count value CNT 1  during the high level is given as 
     
       
         CNT1=19.34[ms]/0.125[msec]=154  equation 2 
       
     
     The duty ratio is 48.4 percent. 
     On the other hand, if the terminal is in the off-hook status, the hook switch S provides a dc current loop, and the ac call signal VAC is superimposed on the dc potential as shown in FIG.  2 B. The dc potential component offsets the call signal from that under the on-hook status, and makes the current passing through the photo-emitting diodes unbalance. The current IR 2  is calculated as 
      IR2=68.4×Sin (50πt)+31[mA]  equation 3 
     As a result, the output potential signal S is in the high level for 25.84 millisecond, and is maintained in the low level for 14.16 millisecond. The count value CNT 1  during the high level is given as 
     
       
         CNT1=25.84[ms]/0.125[msec]=206  equation 4 
       
     
     The duty ratio is 64.6 percent. 
     The count value is different between the on-hook status and the off-hook status, and the digital threshold DTH is determined to be an intermediate value between  154  and  206 . The comparator  11  compares the output signal CNT with the digital threshold DTH to see whether the terminal  7  is in the on-hook status or the off-hook status. The count value does not reach the digital threshold DTH in the on-hook status, and the status signal STUS remains low as shown in FIG.  2 A. On the other hand, when the terminal  7  is in the off-hook status, the comparator  11  changes the status signal STUS to the high level (see FIG.  2 B). 
     Even if the comparator  11  changes the status signal STUS to the high level, the reset signal source  12  produces the reset signal in response to a zero-crossing signal ZC. The binary signal Db is directly supplied to the reset signal source  12 , and the reset signal source  12  produces the zero-crossing signal ZC at the leading edge and the trailing edge of the binary signal Db. For this reason, time delay Δt is introduced between the rising edge of the status signal STUS and the zero-crossing signal ZC as shown in FIG.  2 B. The reset signal source  12  supplies the reset signal RST to the flip flop circuit in response to the zero-crossing signal ZC after the change of the status signal STUS to the high level (see FIG.  2 B), and, accordingly, the flip flop circuit output signal  13  falls  13  falls the output signal. 
     The reason why the reset signal source  12  supplies the reset signal RST to the flip flop circuit  13  in response to the reset signal RST is that of prevention of excess voltage. In detail, if the reset signal RST is produced at a certain timing different from the zero-crossing point, the flip flop circuit  13  causes the call signal transfer relay unit  4  to change the relay contacts r 1 /r 2  from the call signal source  1  while the call signal has either positive or negative potential level, and an excess voltage takes place. The excess voltage is causative of damage on the relay contacts r 1 /r 2  and the power supply circuit, and malfunction takes place in associated circuits. For this reason, the reset signal source  12  supplies the reset signal RST to the flip flop circuit  13  in response to the zero-crossing signal ZC so as to prevent the associated circuits from the above described troubles. 
     The telephone subscriber line  6  provides the loop resistance  6   a  and the leak resistance  6   b  as described hereinbefore. The loop resistance  6   a  and the leak resistance  6   b  are not constant. In fact, the loop resistance  6   a  ranges from zero to 800 ohms, and the leak resistance falls the range between 15 kiloohms and infinitive. The loop resistance  6   a  decays the call signal, and the leak resistance  6   b  superimposes the ac signal on a certain dc potential under the on-hook status. Moreover, the signal source  1  may produce the call signal under different conditions. This means that there is a possibility that the count value in the on-hook status becomes larger than the count value in the off-hook status. If the count value in the on-hook status becomes larger than the digital threshold value DTH, the comparator  11  changes the status signal STUS to the high level, and the flip flop circuit  13  changes the output signal to the low level. The call signal transfer relay unit  4  isolates the telephone subscriber line  6  from the call signal source  1 . This results in that the terminal  7  stops the ringing. 
     For example, assuming now that the prior art call signal source  1  is operative under the dc potential level VDC of 48±10 volts and the ac signal VAC of 60-120 Vrms at 14-33 Hz. If the dc potential and the ac amplitude are maximized and minimized, respectively, the count value is maximized in the on-hook status. The most undesirable conditions are resulted from the loop resistance  6   a  of 800 ohms, the leak resistance  6   b  of 15 kilo-ohms, the dc component of 58 volts and the ac signal of 60 Vrms. The frequency of the ac signal VAC does not have any influence on the count value, and is assumed to be 25 Hz. In this situation, the current IR 2  is given as 
     
       
         IR2=12.2×Sin (50 90  t)+3.46[mA]  equation 5 
       
     
     The output potential signal Scrt is in the high level for 22.98 millisecond, and is in the low level for 17.02 millisecond. For this reason, the count value under the on-hook status is calculated as 
     
       
         CNT1=22.98/0.125=183  equation 6 
       
     
     The duty ratio is 57.5 percent. 
     On the other hand, the count value is maximized in the off-hook status under the maximum dc potential component and the maximum ac amplitude. The loop resistance  6   a  is 800 ohms, the leak resistance  6   b  is infinity, the dc potential component VDC is 38 volts, and the ac signal VAC is 120 Vrms at 25 Hz. The current IR 2  is given by equation 7. 
     
       
         IR2=60.0×Sin (50πt)+11.45[mA]  equation 7 
       
     
     The output potential signal Scrt is in the high level for 22.70 millisecond, and is in the low level for 17.30 millisecond. The count value CNT 1  is calculated as follows. 
     
       
         CNT1=22.70/0.125=181  equation 8 
       
     
     The duty ratio is 56.8 percent. Thus, the count value CNT 1  in the on-hook status becomes larger than the count value CNT 1  in the off-hook status, and the terminal  7  undesirably stops the ringing. 
     Another problem is unavoidable time delay inherent in the call signal transfer relay unit  4 . When the output signal of the flip flop circuit  13  is changed to the low level, the call signal transfer relay unit  4  completes the change of relay contacts r 1 /r 2  after 1 to 3 millisecond. Even if the reset signal source  12  produces the reset signal RST in synchronism with the zero-crossing signal ZC, the call signal transfer relay unit  4  changes the relay contacts r 1 /r 2  after the zero-crossing point, and the time delay is causative of the excess voltage. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide a ring trip circuit, which is free from the undesirable interruption of a call signal and the excess voltage. 
     In accordance with one aspect of the present invention, there is provided a ring trip circuit associated with a call signal source electrically connected through a relay unit and a selected subscriber line to user&#39;s terminal for supplying a call signal changed across zero level, and the ring trip circuit comprises a photo-emitting unit connected to a signal line assigned to the call signal, and producing an optical signal representative of the magnitude of the call signal, a photo-detecting unit optically connected to the photo-emitting unit, and producing a first electric signal changed from a first potential level to a second potential level and vice versa when the call signal is decreased to a certain magnitude close to the zero level, a status discriminator connected to the photo-detecting unit discriminating status of the user&#39;s terminal at the change of the first electric signal changed from the first potential level to the second potential level, and producing a second electric signal representative of the status of the user&#39;s terminal and a relay controller connected to the status discriminator and a command source for controlling the relay unit, and supplying a third electric signal to the relay unit depending upon the status for changing the relay unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the ring trip circuit will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a circuit diagram showing the prior art ring trip circuit incorporated in the telephone exchanger; 
     FIG. 2A is a view showing the waveforms of the essential signals produced in the prior art ring trip circuit in the on-hook status; 
     FIG. 2B is a view showing the waveforms of the essential signals produced in the prior art ring trip circuit in the off-hook status; 
     FIG. 3 is a circuit diagram showing a ring trip circuit incorporated in a telephone exchanger; 
     FIG. 4A is a view showing the waveforms of the essential signals produced in the ring trip circuit in the on-hook status; and 
     FIG. 4B is a view showing the waveforms of the essential signals produced in the ring trip circuit in the off-hook status. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 3 of the drawings, a ring trip circuit embodying the present invention  11  is incorporated in a telephone exchanger  12 . The telephone exchanger  12  is connected through telephone subscriber lines  13  to terminals  14  as similar to the prior art. Only one terminal  14  is illustrated in FIG. 3, and is connected through an associated telephone subscriber line  13  to the telephone exchanger  12 . The terminal and the telephone subscriber line  13  are similar to those of the prior art, and component elements are labeled with references designating corresponding component elements of the prior art terminal/telephone subscriber line  14 / 13  without detailed description. 
     A relay unit  15 , a call signal source  16  and a command source  17  are further incorporated in the telephone exchanger  12 . The relay unit  15  has relay contacts r 1 /r 2  which are connected through resistors  18 / 19  to the call signal source  16  and a ground line GND, respectively. The call signal source  16  is implemented by a series combination of an ac signal source VAC and a dc potential source VDC. The ac signal source VAC is connected to the resistor  18 , and the dc potential source VDC is grounded. When the telephone exchanger  12  determines the terminal  14  to be called, the command source  17  supplies a relay-on signal Son to the ring trip circuit  11 , and the ring trip circuit  11  changes a control signal CTL 1  to high level. The control signal CTL 1  of the high level causes the relay unit  15  to change the relay contacts r 1 /r 2  so as to connect the call signal source  16  to the telephone subscriber line  13 . 
     The ring trip circuit  11  includes a photo-coupling unit  11   a . The photo-coupling unit  11   a  converts current IR 18  passing through the resistor  18  to a potential signal Sv, and a photo-emitter  11   b  and a photo-detector  11   c  form in combination the photo-coupling unit  11   a.    
     The photo-emitter  11   b  includes a resistor  11   d , a photo-emitting diode  11   f , a diode  11   e  and a Zener diode  11   g . The resistor  11   d  is connected to one end of the resistor  18  so as to restrict the current flowing into the photo-emitter  11   b , and the photo-emitting diode  11   f  has the cathode connected to the other end of the resistor N 2 . The Zener diode  11   g  has the anode connected to the anode of the photo-emitting diode  11   f  and the cathode connected to the cathode of the diode  11   e . The anode of the diode  11   e  is connected to the other end N 2 , and the diode  11   e  prevents the photo-emitting diode  11   f  from damage. The resistor  18  converts the current IR 18  to a potential difference, and the photo-emitting diode  11   f  controls the intensity of light depending upon the potential difference. For this reason, an optical signal Sop 2  represents the amount of current IR 18 . 
     The photo-detector  11   c  includes a photo-detecting transistor  11   h  connected at the collector node thereof to a positive power supply line PT and a resistor  11   j  connected between the emitter node N 3  of the photo-detecting transistor  11   h  and the ground line GND. The optical signal Sop is incident onto the photo-detecting transistor  11   h , and the photo-detecting transistor  11   h  converts the optical signal Sop to the potential signal Sv. The potential signal Sv swings the potential level between the negative potential range and the positive potential range, and periodically crosses zero volt. 
     The ring trip circuit  11  further includes a first counter  11   k , a second counter  11   m , two&#39;s complement generator  11   n , an adder  11   o , a reset signal source  11   p  and a flip flop circuit  11   q . The potential signal Sc and a clock signal CLK are supplied to the first counter  11   k  and the second counter  11   m . The first counter is enabled with the potential signal Sv of a low potential range, and counts the pulses supplied thereto during the potential range. On the other hand, the second counter  11   m  is enabled with the potential signal Sv of a high potential range, and counts the pulses supplied thereto during the high potential range. 
     The first counter  11   k  supplies a first digital signal DS 1  representative of a first binary number stored therein to the two&#39;s complement generator  11   n . The two&#39;s complement generator  11   n  produces two&#39;s complement of the first binary number, and supplies a second digital signal DS 2  representative of the two&#39;s complement to the adder  11   o . The second counter  11   m  supplies a third digital signal DS 3  representative of a second binary number stored therein to the adder  11   o , and the adder  11   o  adds the two&#39;s complement of the first binary number to the second binary number. The potential signal Sv is supplied to the adder  11   o , and the adder  11   o  calculates the sum at the decay from the high level to the low level. 
     The adder  11   o  produces a control signal CTL 2  representative of the status of the terminal  14  on the basis of a fourth digital code DS 4  representative of the sum, and supplies the control signal CTL 2  to the reset signal source  11   p . As will be described hereinlater in detail, when the terminal  14  is in the on-hook status, the carry bit of the fourth digital code DS 4  is changed to zero. On the other hand, when the terminal  14  is in the off-hook status, the carry bit of the fourth digital code DS 4  is changed to 1. The control signal CTL 2  is representative of the value of the carry bit, and the reset signal source  11   p  determines whether to produce a reset signal RST or not. The reset signal RST is supplied to the flip flop circuit  11   q , and the flip flop circuit  11   q  is responsive to the reset signal of active high level so as to change the control signal CTL 1  to low level. The control signal of low level is supplied to the relay unit  15 , and the relay unit  15  changes the relay contacts r 1 /r 2  from the call signal source  16 . 
     In this instance, the first and second counters  11   k / 11   m , the two&#39;s complement generator  11   n  and the adder  11   o  as a whole constitute a status discriminator  11   r , and the reset signal source  11   p  and the flip flop circuit  11   q  form in combination a relay controller  11   s.    
     The ring trip circuit  11  behaves as follows. The resistors  18 ,  19  and  11   d  are assumed to have 1 kilo-ohm, 0.15 kilo-ohm and 10 kilo-ohms, respectively, and the dc potential source VDC and the ac signal source VAC respectively produce 48±10 volts and 60 to 120 Vrms at 14 to 33 Hz. The loop resistance  6   a  and the leak resistance  6   b  are assumed to be zero to 800 ohms and 15 kilo-ohms to infinity. In the terminal  14 , the capacitor CB, the resistor RB and the coil LB are assumed to have 0.45 μF, 3.65 kilo-ohms and 54 H, respectively. The clamp voltage of the photo-emitting diode  11   f  is assumed to be 0.7 volt, and the Zener voltage of the Zener diode  11   g  is assumed to be 4.3 volts. 
     When the command source  17  supplies the relay-on signal Son to the flip flop circuit  11   q , the flip flop circuit  11   q  changes the control signal CTL 1  to the high level, and the relay unit  15  connects the call signal source  16  to the telephone subscriber line  13 . The current IR 18  flows through the resistor  18 , and the potential difference DV 1  takes place between the nodes N 1  and N 2 . 
     When the potential difference DV 1  exceeds the total of the Zener voltage and the clamp voltage, current flows through the resistor  11   d , the Zener diode  11   g  and the photo-emitting diode  11   f , and the photo-emitting diode  11   f  radiates the optical signal Sop 2 . In this instance, when the current IR 18  exceeds 5 milli-ampere, the potential difference DV 1  exceeds the total of the Zener voltage and the clamp voltage, i.e., 5 volts, and the photo-emitting diode  11   f  starts to radiate the optical signal Sop 2 . On the other hand, when the current IR 18  is decreased to 5 milli-ampere, the photo-emitting diode  11   f  stops the optical signal Sop 2 . The photo-detecting transistor  11   h  is responsive to the optical signal Sop 2  so as to produce the potential signal Sv. When the current IR 18  is 5 milli-ampere, the potential signal Sv is changed from the high potential range to the low potential range and vice versa. When the dc current component of the call signal is 5 milli-ampere, the duty ratio of the potential signal becomes 50 percent. 
     When the terminal  14  is in the on-hook status, the capacitor CB interrupts a dc loop between the telephone exchanger  12  and the terminal  14 . However, the leak resistance  6   b  of 15 kilo-ohms provides a current path for the dc current component. For this reason, the ac signal is superimposed on the dc current component of 3.6 milli-ampere, and the potential difference DV 1  between the nodes N 1  and N 2  is 3.6 volts at the maximum. As a result, the duty ratio of the potential signal Sv is less than 50 percent as shown in FIG.  4 A. 
     On the other hand, when the terminal  14  is in the off-hook status, the switch unit S is closed, and the dc loop is established between the telephone exchanger  12  and the terminal  14 . For this reason, the dc potential component is increased to 11.45 milli-ampere at the minimum, and the potential difference DV 1  of 11.45 volts takes place between the nodes N 1  and N 2  at the minimum. As a result, the duty ratio of the potential signal Sv is greater than 50 percent as shown in FIG.  4 B. 
     The first counter  11   k  maximizes the count value under the conditions where the dc potential component is minimum and the amplitude of the ac signal is maximum. The conditions are equivalent to the loop resistance S 1  of 800 ohms, the leak resistance  6   b  of infinity, the dc potential VDC of 38 volts and the ac signal VAC of 120 Vrms at 25 Hz. The current IR 18  is calculated by equation 9. 
     
       
         IR 18 =60.0×Sin (50πt)+11.45[mA]  equation 9 
       
     
     The potential signal Sv is in the low level for 18.38 millisecond, and is maintained in the high level for 21.62 millisecond. The duty ratio is 54.05 percent. 
     The first counter  11   k  is enabled with the potential signal of the low level so as to count the clock pulses CLK. The clock signal CLK is assumed to be 8 KHz. 
     As described hereinbefore, when the terminal  14  is in the on-hook status, the potential signal Sv remains in the low level for 20.75 millisecond, and the first counter  11   k  counts 166 pulses, i.e., 20.75/0.125. The first binary number is [9:1]=[010100110]. The number of pulses  166  is represented by nine bits, and the tenth bit is the carry bit. The two&#39;s complement generator  11   n  produces two&#39;s complement of the first binary number [101011010] as shown in FIG.  4 A. 
     On the other hand, when the terminal  14  is in the off-hook status, the potential signal Sv remains in the low level for 18.38 millisecond, and the first counter  11   k  counts 147 pulses CLK, i.e., 18.38/0.125. The first binary number is [9:1]=[010010011], and the two&#39;s complement of the first binary number is [101101101] as shown in FIG.  4 B. 
     The second counter  11   m  is enabled with the potential signal Sv of the high level so as to count the clock pulses CLK. When the terminal  14  is in the on-hook status, the second counter  11   m  counts 19.25/0.125=154 pulses, and the second binary number is [9:1]=[010011010] as shown in FIG.  4 A. On the other hand, when the terminal  14  is in the off-hook status, the second counter  11   m  counts 21.62/0.125=172 pulses, and the second binary number is [9:1]=[010101100]. 
     The adder  11   o  calculates the sum of the two&#39;s complement of the first binary number and the second binary number so as to produce the fourth digital signal DS 4 . When the terminal  14  is in the on-hook status, the sum is represented as 
     
       
         SUM=[101011010]+[010011010]=[0111110100]  equation 10 
       
     
     On the other hand, when the terminal  14  is in the off-hook status, the sum is represented as 
     
       
         SUM=[101101101]+[010101100]=[1000011001]  equation 11 
       
     
     Thus, the carry bit “0” is representative of the on-hook status, and the carry bit “1” is representative of the off-hook status. 
     As described hereinbefore, when the potential signal Sv is changed from the high level to the low level, the adder  11   o  calculates the sum, and supplies the control signal CTL 2  to the reset signal source  11   p . If the control signal CTL 2  is representative of the carry bit of “0”, the reset signal source  11   p  determines the terminal  14  to be in the on-hook status, and maintains the reset signal RST in the low level (see FIG.  4 A). For this reason, the flip flop circuit  11   q  keeps the control signal CTL 1  high, and the call signal is continuously supplied through the relay unit  15  and the telephone subscriber line  13  to the terminal  14 . 
     On the other hand, if the control signal CTL 2  is representative of the carry bit of “1”, the reset signal source  11   p  determines the terminal  14  to be changed to the off-hook status, and the reset signal source  11   p  changes the reset signal RST to the high level. As a result, the flip flop circuit  11   q  changes the control signal CTL 1  to the low level. Then, the relay unit  15  changes the relay contacts r 1 /r 2  to the other side, and isolates the telephone subscriber line  13  from the call signal source  16 . 
     As described hereinbefore, the adder  11   o  calculates the sum at the decay of the potential signal Sv, and the reset signal source  11   p  determines whether to produce the reset signal RST or not on the basis of the value of the carry bit. For this reason, the relay unit  15  changes the relay contacts r 1 /r 2  around the decay of the potential signal Sv. When the current IR 18  is decreased to a critical value represented as (Zener voltage of Zener diode  11   g +clamp voltage of photo-emitting diode  11   f )/resistance of the resistor  18 . In the above described example, the critical value is 5 milli-ampere. As will be seen from FIGS. 4A and 4B, the critical value is close to the zero-crossing point, but is not equal to the zero-crossing point. Time delay is introduced between the decay of the potential signal Sv and the switching action of the relay unit  15 . For this reason, the relay contacts r 1 /r 2  are changed at a certain timing much closer to the zero-crossing point than the critical timing. 
     As will be appreciated from the foregoing description, the trip timing is not affected by fluctuation of the call signal source, the loop resistance and the leak resistance, and the ring trip circuit never produces the reset signal in the on-hook status. Moreover, the reset signal RST is produced before reaching the zero-crossing point, and the relay unit  15  changes the relay contacts r 1 /r 2  as close to the zero-crossing point as possible. As a result, the associated circuits are prevented from the excess voltage. 
     Although a particular embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention.