Telephonic communications – Supervisory or control line signaling – Using line or loop condition detection
Reexamination Certificate
1997-11-26
2001-02-06
Chan, Wing F. (Department: 2743)
Telephonic communications
Supervisory or control line signaling
Using line or loop condition detection
C379S377000
Reexamination Certificate
active
06185296
ABSTRACT:
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 (50&pgr;t)[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&
Chan Wing F.
NEC Corporation
Scully Scott Murphy & Presser
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