Abstract:
A telephone terminal device, which is activated by a predetermined number of ringing signals even if a telephone call occurred immediately after a previous caller had abandoned a call before engagement of the telephone line. When the telephone terminal device is set to standby mode, the polarity of telephone line (L1, L2) is stored in microprocessor CPU-1 via photocoupler PC-3 (or PC-4) and contact y2-1. If there is a call thereafter, ringing portion of the ringing signal is detected by photocoupler PC-3 (or PC-4) via capacitor C2. During a silent period, the polarity of the telephone line (L1, L2) is checked by the aforementioned PC-3 or PC-4 via contact y2-1 which is closed. When the polarity of the telephone line at the time of abandonment of call by a caller prior to activation of the telephone terminal device coincides with the aforementioned polarity stored, the telephone terminal device is restored to standby mode (with the counter for ranging signals being cleared).

Description:
BACKGROUND OF THE INVENTION 
     The present invention is an improvement of Japanese Patent No. 1107913 (Number of Examined Application Published: S56-4951) invented by the same applicant, and relates to processing in a telephone terminal device which takes place when a caller hangs up before the device is activated. 
     For example, the telephone terminal device which was disclosed in the above-mentioned Japanese patent occasionally would not be activated after a predetermined number of ringing signals. The latter arose due to an unstable state of the telephone line resulting from abandonment of a call before engagement of the telephone line, which caused a malfunction of the so-called &#34;toll saver&#34; function. It may be recalled that with the toll saver function, if no messages have been recorded on the device, it is activated after four rings. If at least one message has been recorded, it is activated after two rings. 
     Even though the probability of such a malfunction is low, it can not be ignored. Therefore, the present invention is designed to prevent such a malfunction from ever occurring. Specifically, the present invention is directed to provide a means for activating a telephone terminal device in response to a predetermined number of ringing signals even if a new ringing signal arrives immediately after a previous call that was abandoned during ringing. 
     SUMMARY OF THE INVENTION 
     Unlike conventional devices which determine whether a call was abandoned before engagement of the telephone line by detecting discontinuation of ringing signals, the present invention achieves its objective not only by detecting such a discontinuation but also by detecting a change in voltage on the telephone line. 
     In the present invention, polarity of telephone line (DC voltage) during standby mode of a telephone terminal device is stored in a memory means. If the polarity of the telephone line after the voltage change resulting from caller&#39;s abandonment of call matches the polarity which was memorized, the call will be judged to have been abandoned. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating one embodiment of the present invention. 
     FIG. 2 is a flowchart illustrating sequence of operations of the present invention. 
     FIG. 3 is a timing chart for the telephone terminal device illustrating its activation in response to ringing signals and restoration. 
     FIG. 4 is a timing chart Illustrating the situation wherein a ringing signal is broken off. 
     Legend 
     1=CPU 
     2, 3, 4, 5=Inverters 
     6=Line transformer 
     Y-1, Y-2=Relays 
     PC-1, PC-2, PC-3, PC-4=Photocouplers 
     L-1, L-2=Telephone line 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following is an explanation of the structure and operation of the present invention. FIG. 1 illustrates the main components of the present invention. In this figure, numeral 1 represents a microprocessor (CPU). The program which controls the present device is stored in a ROM. 
     Numerals 2 through 5 represent Schmitt-type inverters. Numeral 6 is a line transformer. Symbols PC-1 through PC-4 are photocouplers. Symbols Y-1 and Y-2 are relays having contacts y1-1 and y2-1, respectively. Symbol L1 is the positive terminal of a telephone line and L2 is the negative terminal. There can be two loops between L1 and L2. A first loop can be formed by the &#34;make&#34; contact y1-1, the primary winding of line transformer 6, and the parallel circuits of LED-1 and LED-2 in the LED section of the respective photocouplers PC-1 and PC-2. A second loop can be formed by resistor R1, the parallel circuits of LED-3 and LED-4 in the LED section of the respective photocouplers PC-3 and PC-4, the &#34;make&#34; contact y2-1 and the parallel circuit of C-2. 
     Next is a description of the operation of the present invention. The subroutine shown in FIG. 2 is invoked when the &#34;Absent&#34; button (not shown in the drawings) is pressed. At Step 10, output port 0-2 drops to a low (&#34;L&#34;) level by means of a command from the microprocessor, thereby turning loaded relay Y-2 on. 
     Accordingly, contact y2-1 closes, and the telephone line (L1, L2) is connected to photocoupler PC-3 and PC-4 via resistor R1, which has a high resistance value, so that direct current can flow to either PC-3 or PC-4. At this time, if the polarity of L1 is positive and L2 is negative, LED-3 of photocoupler PC-3 will turn on and the resulting light will be received by phototransistor PH3. (Henceforth, the latter will be referred to as &#34;PC-3 on.&#34;) Then, input port I-3 drops from a high (&#34;H&#34;) level to a low (&#34;L&#34;) level by means of inverter 4. 
     When PC-3 turns on and input port I-3 drops to a low (&#34;L&#34;) level, Step 11 will become affirmative. At the next Step 12, FLAG is set to &#34;1&#34; and polarity of the telephone line is stored in memory. 
     On the other hand, if the telephone line is of the opposite polarity, the program branches to Steps 13 and 14 wherein FLAG is set to &#34;0.&#34; Regardless of the polarity of the telephone line, relay Y-2 turns off in response to the command at Step 15. At Step 16, a counter for counting the number of ringing signals is cleared. At Step 17, standby mode, wherein the device awaits arrival of a ringing signal, is activated. It may be Observed that the above memorization of the polarity of the telephone line should take place before the arrival of ringing signals, perhaps when the device is turned on. 
     When ringing signals arrive during standby mode, photocouplers PC-3 and, PC-4 turn on and off repeatedly. By checking the period of the ringing signal, it is possible to confirm that it is indeed a ringing signal. If the signal is a ringing signal, Step 17 will be affirmative. After the ringing portion of the ringing signal has ended and the silent portion of the ringing signal (see t2 of FIG. 3) begins, Step 18 will be negative. Then the counter is incremented by 1 at Step 19. 
     The present device counts the number of ringing signals. The user has the option of selecting the number of rings after which the device will be activated. However, it is also possible to have a structure in which the means for counting ringing signals utilizes a timer which will activate the device at a predetermined time period after the first ringing signal has been received. Such a structure would be practically equivalent to a counter. 
     Assuming the device is set to be activated after two rings, when the first ringing portion of the ringing signal arrives, Step 20 will be negative. Next, at Step 21, relay Y-2 is turned on. As stated above, a connection is thus established in which direct current flows from L1 and L2 to photocouplers PC-3 and PC-4. The purpose of establishing the connection is to detect a change in polarity on L1 and L2 of the telephone line which results from a caller&#39;s abandonment of call. 
     At Step 22, a two-second timer (4 second timer in the U.S.) which corresponds to the silent portion of the ringing signal (see t2-t3 of FIG. 3) is started and during that time Y-2 is maintained in the on position. 
     At Step 23, a test is performed to determine whether FLAG is set to &#34;1.&#34; As mentioned earlier, L1 is positive and L2 is negative during standby mode of the present device. Therefore, FLAG is equal to &#34;1&#34; and Step 23 will be positive. 
     Next, at Step 24 a test is performed to determine whether photocoupler PC-3 is on. 
     As mentioned above, photocoupler PC-3 is on provided that L1 is positive and L2 is negative. However, as long as the caller does not abandon a call (operations for the case in which the caller abandons a call will be described later), the voltage at L1 will be at a negative level, as indicated by t2-t3 in FIG. 3. Therefore, Step 24 will be negative. The program will continue in a loop between Step 24 and Step 26 for two seconds. 
     After the two seconds have elapsed, the aforementioned relay Y-2 is turned off at Step 28 and the program returns to Step 17. Then, the second ring arrives (see t3-t4 of FIG. 3). Steps 17 through 19 are as described above. When the counter for ringing signals has reached the predetermined value, Step 20 will be affirmative. Relay Y-1 is turned on at Step 29, whereby the telephone line is engaged (t5 in FIG. 3) and the device is activated. 
     When the caller places the telephone on hook (t6 in FIG. 3) after the completion of operations of the present device, the on-hook condition will be detected by means of photocouplers PC-1 or PC-2. The program then returns to Step 17, wherein standby mode is restored, via Step 16. 
     Next, referring to FIG. 4, operations of the present device when a caller abandons a call during ringing will be described. 
     in the case that a caller abandons a call after one ring has been received on the present device, when the switchboard of the telephone exchange (not shown in the diagram) releases the call at any time during the silent portion from t11 to t12 as shown FIG. 4, voltage at L1 immediately returns to +48 V. 
     In this situation, photocoupler PC-3 will be turned on. In terms of the flowchart that PC-3 is turned on during the aforementioned loop between Step 24 and 26. Therefore, Step 24 will become affirmative. Next, the program returns to Step 17 via Steps 15 and 16. The device is then restored to standby mode to await the next ringing signal. 
     As shown above, there is no time delay in restoring the present device to standby mode after a caller abandons a call. Even if a new ringing signal arrives immediately after a previous call was abandoned, the present device will still be activated in response to a predetermined number of rings. 
     Even if the exchange office terminates the transmission of the ringing signal in the middle of the ringing portion after a caller hangs up during ringing, the present device will be restored to standby mode because L will return to +48 V as shown by t11 of FIG. 4. 
     The present invention should not be considered to be limited to the aforementioned embodiment. It is possible to obtain various embodiments by employing a number of variations on the same technical concept. For example, a semiconductor switching element can be used instead of relay Y-2. Also, the present invention is applicable not only to telephone answering devices, but also to autodialing equipment or other telephone terminal devices. 
     As shown above, the present invention is structured so that when a caller hangs up during ringing of the telephone, the resultant change in DC voltage on the telephone line is detected. Unlike conventional devices, the present device is restored to standby mode immediately after a caller hangs up. Even if a ringing signal arrives immediately after a previous caller hung up during ringing of the telephone, the present device will still be activated after a predetermined number of rings. Thus, for example, the toll saver feature which is found in telephone answering devices will operate correctly, which is of great practical importance.