Electronic prepay adapter circuit for central office telephone equipment

An electronic prepay adapter circuit is provided for use in converting semi-postpay central office telephone equipment to operation in a full prepay mode. In one arrangement, a prepay adapter circuit is connected on a line by line basis between each pay station and the associated line relay of the central office telephone equipment. In another arrangement, the prepay adapter circuit is connected between the linefinder and the first selector of the central office telephone equipment and utilized on a common or shared basis for a plurality of pay station telephone lines. The prepay adapter circuit is easily connected as a self-contained unit or module into the central office telephone equipment by the connection of the central office battery supply, ringing generator, collect and return voltage supplies, sleeve control lead, and the tip and ring line connections.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The present invention relates generally to the field of telephone apparatus 
and more particularly to a prepay adapter circuit for converting central 
office telephone equipment from semi-postpay operation to full prepay 
operation. 
B. Description of the Prior Art 
Pay stations are generally available for full prepay operation with central 
office telephone equipment that includes control circuitry for operation 
of the pay station in a full prepay mode. One pay station of this general 
type is the Model 201 pay station manufactured by the Communication 
Equipment & Engineering Company of Melrose Park, Ill. 60160. 
In many locations existing central office telephone equipment provides 
semi-postpay operation and is not compatible or capable of operation with 
full prepay, pay stations. 
Thus, the need arises for circuitry to adapt or convert existing 
semi-postpay central offices for operation as full prepay central offices 
without extensive modifications of the existing central office telephone 
equipment and without the entire replacement of the central office 
equipment. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide an electronic 
prepay adapter circuit for converting semi-postpay central office 
telephone equipment to operation in a full prepay mode operation in an 
efficient, economical manner and without extensive modification of the 
existing central office telephone equipment. 
It is a further object of the present invention to provide an electronic 
prepay adapter circuit for converting existing semi-postpay central office 
telephone equipment to full prepay operation with a minimum of 
interconnections and changes to the existing central office telephone 
equipment. 
Briefly, in accordance with one important aspect of the present invention 
there is provided an electronic prepay adapter circuit for converting 
existing semi-postpay central office telephone equipment to full prepay 
operation. In one arrangement, a prepay adapter circuit is connected on a 
line by line basis between each pay station and the associated line relay 
of the central office telephone equipment. In another arrangement the 
prepay adapter circuit is connected between the linefinder and first 
selector of the central office telephone equipment and utilized on a 
common or shared basis for a plurality of pay station telephone lines. The 
prepay adapter circuit is easily connected as a self-contained unit or 
module into the central office telephone equipment by the connection of 
the standard central office battery supply, ringing generator, collect and 
return voltage supplies, sleeve control lead, and the tip and ring 
telephone line connections. 
The invention both as to its organization and method of operation together 
with further objects and advantages thereof will best be understood by 
reference to the following specification taken in connection with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1, 2 and 3, the prepay adapter circuit of the 
present invention referred to generally at 10 and a tone control circuit 
referred to generally at 12 are shown in connection with central office 
telephone equipment in FIGS. 2 and 3. The tone control circuit 12 is of 
the general type that receives tone signalling from a toll center, decodes 
the received tone signalling, and controls the collect and return 
functions of a pay station. In a specific embodiment of the tone control 
circuit 12 is a Model TD-4 tone control system manufactured by 
Communication Equipment & Engineering Company. However, it should also be 
understood that the prepay adapter circuit 10 of the present invention in 
other embodiments is connected to control various other coin control 
arrangements and devices. 
The prepay adapter circuit 10 in the arrangement illustrated in FIG. 2 is 
connected along with the tone control circuit 12 to central office 
telephone equipment on a shared or multi-line basis to service a plurality 
of telephone pay stations 14. Each of the pay stations 14 associated with 
the central office equipment of FIG. 2 is connected to a respective line 
relay 16 over a telephone line pair 18 commonly referred to as tip and 
ring. A predetermined number of the line relays 16 are associated with a 
particular connector bank referred to generally at 20. The tip and ring 
leads along with the control lead or sleeve from the connector bank 20 are 
connected from the line relay 16 to a respective point on a linefinder 
bank referred to generally at 24. Thus, the linefinder 24 controls a 
plurality of pay stations 14 through respective line relays 16. The tip, 
ring and control leads connecting the line relay 16 to the linefinder bank 
are referred to at 22 and identified as T, R, C, (F). 
In accordance with important aspects of the present invention, the prepay 
adapter circuit 10 and the tone control circuit 12 are connected between 
the linefinder equipment 24 and the pay station first selector equipment 
28 of the existing semi-postpay central office equipment thus converting 
the existing semi-postpay central office equipment to a full prepay 
central office. The prepay adapter 10 and the tone control circuit 12 are 
connected to the linefinder equipment 24 by the tip, ring and control 
lines referred to generally at 26 and identified as T,R, C, (OUT). 
The prepay adapter circuit 10 and the tone control circuit 12 are connected 
to the pay station first selector 28 of the central office equipment over 
the lines 30 identified as T, R, C, (IN) and corresponding to the tip, 
ring and control lead inputs from the first selector. The pay station 
first selector 28 and the local first selector 31 of non-coin operated 
equipment are connected to the trunk circuitry of the central office 
equipment referred to generally at 32. For example, the trunk may be a 
TSPS trunk connected to a toll center and thus the central office 
equipment illustrated in FIG. 2 is of the remote type. 
Referring now to FIG. 3, and in accordance with other important aspects of 
the present invention, each of the prepay adapter circuits 10 and the tone 
control circuits 12 are connected to the existing semi-postpay central 
office equipment on a line by line or dedicated pay station basis to 
convert the existing central office equipment to full prepay operation. 
Eacy pay station 14 is connected to a respective prepay adapter circuit 10 
and a tone control circuit 12 over a telephone line pair 36 corresponding 
to the tip and ring leads and identified as T, R, (OUT) in FIG. 3. Each of 
the prepay adapter circuits 10 and the tone control circuits 12 associated 
with each respective pay station 14 are connected to a line relay 16 over 
the lines 38 corresponding to the tip, ring and control input lines 
referred to at 38 and identified as T, R, C (IN). Each of the line relays 
16 associated with a particular connector bank 20 is connected over the 
lines 22 identified as T, R, C (F) to a linefinder bank of the linefinder 
equipment 24. The linefinder 24 is connected to the pay station first 
selector 28 over the lines 30. The pay station first selector 28 and the 
local first selector 31 of non-coin operated equipment are connected to 
the trunk arrangement 32. 
Thus, the converted, full prepay central office equipment arrangements of 
FIGS. 2 and 3 with the interconnected prepay adapter circuits 10 and the 
tone control circuits 12 differ only in the manner in which the prepay 
adapter circuits 10 and the tone control circuits 12 are interconnected in 
the central office equipment; i.e. on a dedicated or per line basis in the 
arrangement of FIG. 3 and on a shared line basis in the arrangement of 
FIG. 2. 
In the arrangement of FIG. 2, a prepay adapter circuit 10 and a tone 
control circuit 12 is provided for each linefinder bank that services a 
predetermined plurality of pay stations 14. In the arrangement of FIG. 3, 
a prepay adapter circuit 10 and a tone control circuit 12 is provided for 
each line pair to a pay station 14. 
In accordance with important aspects of the present invention and referring 
now again to FIG. 1, the prepay adapter circuit 10 includes a line 
reversal detector 40 for the answer supervision detection of the reversal 
of the polarity of the tip and ring leads when the called party answers on 
a pay station originated call. Upon the detection of a line reversal 
condition, the line reversal detector 40 generates an output at 42 that is 
connected as an input to the coil control stage 44. The coin control stage 
44 upon the line reversal at 42 is conditioned to a "set to collect" state 
or mode. The coin control stage 44 includes a second input 46 generated by 
a C-lead status detector stage 48. The C-lead status detector stage 48 is 
connected to the control lead identified as C-IN of the lines 38 of FIG. 3 
or the lines 30 of FIG. 2 to detect the condition of the pay station 14 
going on-hook at the end of a call. When the hang-up condition is 
detected, the C-lead status detector stage 48 generates an output at 46 to 
the coin control stage 44. After these conditions, the coin control stage 
48 generates an output at 50 to control the coin control relays referred 
to generally at 52 of the coin control circuit 12 to produce a collect 
condition on the tip and ring lines, T, R, (OUT), corresponding to either 
the lines 26 of FIG. 2 or the lines 36 of FIG. 3. The coin control stage 
44 generates a collect control signal 50 to the coin control relay 
arrangement 52 a predetermined time interval, one second in a specific 
embodiment, after the C(IN) status changes, denoting an on-hook condition. 
The collect control signal 50 remains active for a second predetermined 
time interval, one second in a specific embodiment. 
The coin control stage 44 is then reset to a "set to return" condition 
until again actuated by the above or other conditions. If a free or 
incomplete call is made, the coin control stage 44 remains in the "set to 
return" state and a return control signal is generated at 50 conditioning 
the coin control relays at 52 to generate a return signal to the pay 
station on the tip and ring lines, T, R (OUT). 
The prepay adapter circuit 10 also includes a ring voltage detector stage 
56 that is connected to the ring lead to generate an output at 58 upon the 
detection of ringing voltage to the pay station 14. The output 58 is 
connected to the coin control stage 44 to set the coin control stage 44 to 
the "set to collect" condition on the detection of ringing voltage. When 
the hang-up signal is detected from the pay station 14 by the C-lead 
status detector stage 48, the coin control stage 44 will generate the 
collect control signal at 50 to the coin control relays 52. In accordance 
with an important aspects of the present invention this sequence is 
effective during an incoming call to the pay station 14 to provide 
automatic collection of coins during an incoming collect call. On 
non-collect incoming calls, no coins will be deposited and thus the 
collect signal will not collect any coins. However on incoming collect 
calls, the collect signal insures that all deposited coins will be 
automatically collected after termination of the call in the event that 
the operator forgets to manually actuate the collect control signals or if 
the telephone equipment fails to perform the collection. If an incoming 
call is not answered at the pay station, the coin control stage 44 will be 
automatically reset one second after the C-lead status detector stage 48 
detects a change in condition of the C-lead from ground potential to the 
level of the central office battery, normally -48 VDC. 
In accordance with other important aspects of the present invention, the 
prepay adapter circuit 10 includes an audio muting stage 62 to selectively 
place a capacitive shunt across the tip and ring lines to the pay station 
14 at output 64 upon the occurrence of predetermined conditions. These 
predetermined conditions are the detection of a line reversal, the 
detection of loop current, and no deposit acknowledgement signal from the 
pay station 14. The capacitive shunt across the tip and ring conductors at 
64 by the audio muting stage 62 drastically attenuates and essentially 
mutes the transmit and receive audio signals without effecting the DC loop 
current to the pay station 14. 
The pay station 14 acknowledges receipt of an appropriate deposit by the 
placing of an inductive ground on the telephone line pair. This causes an 
imbalance between the current flowing within tip and ring lines and thus a 
change in the voltage drops across the tip and ring lines as measured 
across the respective relay winding impedances in the central office 
equipment. This change or differential voltage condition is utilized by a 
line imbalance detector stage 66 to detect the presence of the inductive 
ground and thus the acknowledgement of a deposit. The line imbalance 
detector stage 66 generates an output at 68 to the audio muting stage 62 
to indicate the deposit acknowledgement condition. 
The audio muting stage 62 also includes an input at 70 from a loop current 
status detector stage 72 connected to detect the loop current over the 
telephone line pair to the pay station 14. The audio muting stage 62 is 
operative to mute the audio path to the pay station when the line reversal 
condition is detected by stage 66, loop current is detected by stage 72 
and no line imbalance condition is detected by the stage 66 indicating no 
deposit acknowledgement. When a proper deposit is acknowledged and the 
line imbalance detector 66 detects this condition, the audio muting 
condition is removed by the audio muting stage 62. When the normal 
polarity condition is present on the tip and ring lines, the line 
imbalance detector 66 inhibits generation of any audio muting function by 
the audio muting stage 62. Thus, audio muting is inhibited for free calls 
to the operator, a 911 emergency number of other calls that do not result 
in reverse line polarity (answer supervision). The audio muting stage 62 
is enabled by the input 70 only during the detection of loop current to 
prevent false operation due to ringing voltages for example. This reduces 
the possibility of damage to the components in the audio muting stage 62. 
In accordance with yet other important aspects of the present invention, 
the prepay adapter circuit 10 includes a tone generator stage 76 to 
generate an audible identification tone on the telephone line pair in 
situations where it is desirable to alert the operator that a public phone 
or pay station is being called. The tone generator stage 76 is selectively 
operable as an optional feature by means of a selection arrangement. The 
tone generator 76 is operative to generate a preselected audible tone to 
alert the operator when ringing voltage has been detected by the stage 66 
at output 78 connected as an input to the tone generator stage 76 and 
after a loop current has been detected by the stage 72. Thus, the tone 
generator 76 generates the audible tone for a predetermined interval of 
time after a pay station has been answered. The loop current detector 
stage 72 generates an output 80 indicating that the pay station 14 has 
been answered or placed off-hook. The output 78 of the ring voltage 
detector stage 56 indicates that the pay station 14 has been rung. 
In accordance with other important aspects of the present invention, the 
prepay adapter circuit 10 includes a hold stage 84 to present an 
appropriate termination impedance to the central office equipment during 
coin control operations to prevent release of the line relay 16 of the 
central office equipment. Further, during coin control operations for 
local calls when the pay station 14 is in an on-hook condition, the 
termination must not appear on the telephone line to the central office. 
The hold stage 84 is effective to satisfy the above conditions and present 
an appropriate termination of the central office equipment during toll 
calls and while coin control operations are operative. The hold stage is 
controlled by an output 86 of the C-lead status detector stage 48. Thus, 
if the pay station 14 is in an off-hook condition when coin control 
signalling occurs, the hold stage 84 at an output 88 through the coin 
control relay arrangement 52 connects the fixed termination to the tip 
line T (IN) of the central office equipment. If the pay station 14 is 
on-hook when the coin control functions occur, the C-lead status detector 
48 will inhibit operation of the hold stage 84. 
When the prepay adapter circuit 10 is connected on a shared line basis to 
service a plurality of pay stations 14 as illustrated in FIG. 2, a C-lead 
control stage 90 is utilized by the prepay adapter circuit 10 to 
appropriately condition the state of the C-OUT lead, the control lead to 
the linefinder equipment 24. This conditioning of the C-OUT lead is 
required to prevent premature release by the linefinder 24 of a particular 
line to a pay station 14 that is placed in an on-hook condition. The 
premature release could occur before the appropriate collect or return 
control functions are performed. With the pay station 14 off hook, the 
C-lead from the linefinder 24 identified as C-OUT will switch to a ground 
potential. In this state, the C-OUT condition is passed to the C-IN line 
to the first selector 28. The C-lead status detector stage 48 generates a 
signal at an output 94 to the lead C-lead control stage 90 to condition 
the C-lead control stage 90 to output a ground potential on the C-OUT line 
to the linefinder equipment 24 for a predetermined interval of time after 
the pay station 14 has been placed in an on-hook condition and independent 
of the condition of the C-IN line returning to the central office battery 
voltage of approximately -48 volts. Thus a predetermined time interval 
after hand-up of the pay station 14 is provided during which the coin 
control functions are performed before the linefinder equipment 24 
releases the line to the pay station 14. 
The tone control circuit 12 in a specific embodiment includes an amplifier 
stage 96 connected to the T, R (IN) lines of the lines 30 of FIG. 2 or the 
lines 38 of FIG. 3 to amplify the tone signalling. The output of the 
amplifier stage 96 is connected to a tone decoder stage 98 that generates 
decoded signals to control the coin control relay arrangement 52. The T, R 
(IN) lines are also connected to the coin control relay arrangement 52. 
The coin control relay arrangement 52 also includes an uninterrupted 
ringing voltages input and two coin control input voltages. The coin 
control input voltages conventionally are +120 VDC for the collect 
function and a -120 VDC line for the return function. The coin control 
relay arrangement 52 is operative to output +120 VDC on the T, R (OUT) 
lines for a collect function and the -120 VDC on the T, R (OUT) lines for 
a return function. During the coin control periods, the coin control relay 
arrangement 52 isolates the tip and ring input lines T, R (IN) at 30 or 38 
from the T, R (OUT) lines at 26 or 36 and places the collect voltage (+120 
VDC) or return voltage (-120 VDC) on the output tip and ring lines T, R 
(OUT) at 26 or 36. When no coin control function or ring function is being 
performed, the coin control relay arrangement 52 connects the T (IN) line 
to the T (OUT) line and the R (IN) line to the R (OUT) line. For the 
details of the interconnections of the collect, return and ring relays, 
reference may be made to the Model TD-4 tone control circuit discussed 
hereinbefore. 
Referring now to FIG. 4 and considering a specific embodiment of the prepay 
adapter circuit 10 of FIG. 1, the line reversal detector stage 40 is 
connected to the tip side T (IN) of the telephone line connections at 30 
or 38. The anode of a zener diode 110 is connected to the T (IN) line and 
the cathode of the zener diode 110 is connected through a resistor 112 to 
one side of a capacitor 114 and also to the base lead of a PNP transistor 
116. The other end of the capacitor 114 is connected to the ground 
reference potential 118. A resistor 120 is connected across the capacitor 
114. The emitter lead of the transistor 116 is also connected to the 
ground references 118. The collector lead of the transistor 116 is 
connected through a resistor 122 to the output 42 of the line reversal 
detector stage 40 that is connected to the coin control stage 44. A 
resistor 124 is connected between the output 42 and the negative supply 
reference potential 126. In a specific embodiment where a -48VDC central 
office battery is utilized, the zener diode 110 is a 24 volt diode and the 
negative supply reference potential is -12 VDC. 
When answer supervision occurs with a reversal of the normal polarity of 
the tip and ring lines, the voltage on the T (IN) line will change from a 
range of approximately -4 to -18 volts to a range of approximately -30 to 
-44 volts. This reverse polarity level on the T (IN) line will render the 
transistor 116 conductive and produce a change in the output state at the 
output line 42. Upon reverse polarity detection, the output at 42 changes 
to a high level defined as a logicl level near the ground reference 
potential 118. A low level is defined as a reference level near the minus 
supply reference potential 126, i.e. near -120 VDC. 
The coin control stage 44 is responsive to the input at 42 to be 
conditioned to a "set to collect" state as discussed hereinbefore. The 
coin control stage 44 includes a latch arrangement provided by two, 
two-input NOR gates 130 and 132. One input of the NOR gate 130 is 
connected to the input 42 from the line reversal detector stage 40. The 
second input of the NOR gate 130 is connected to the output of the NOR 
gate 132. The output of the NOR gate 130 is connected to one input of the 
NOR gate 132. The second input of the NOR gate 132 is connected to the 
negative supply reference 126 through a resistor 134. A capacitor 136 is 
connected across the resistor 134. A capacitor 138 is connected between 
the output 42 and the negative supply 126. A diode 140 is connected anode 
to cathode between the output 42 and one end of a capacitor 142. The other 
end of the capacitor 142 is connected to the input of the NOR gate 132 
that is connected to the resistor 134. The output of the NOR gate 130 is 
connected through the series combination of a resistor 144 and a diode 146 
arranged cathode to anode to the base of a PNP transistor 148. 
The collector of the transistor 148 is connected through a diode 150, anode 
to cathode, to the collect control line output 50a that is connected to 
the coin control relay arrangement 52. The output of the NOR gate 132 is 
connected through the series combination of a resistor 152 and a diode 
154, arranged cathode to anode, to the base lead of a PNP transistor 165. 
The collector lead of the transistor 156 is connected anode to cathode 
through a diode 158 to the return control line 50b that is connected to 
the coin control relay arrangement 52. The emitter leads of the two 
transistors 148 and 156 are connected to each other and to the collector 
of a PNP transistor 160. 
The emitter lead of the transistor 160 is connected to the ground reference 
118 and the base lead of the transistor 160 is connected through a 
resistor 162 to the cathode of the diode 140. The cathode of the diode 140 
is also connected to the output of a first inverter gate 164. The input of 
the inverter gate 164 is connected to the output of another inverter gate 
166. The input of the inverter gate 166 is connected through the parallel 
combination of a resistor 168 and a capacitor 170 to the ground reference 
118. The input of the inverter gate 166 is also connected through a diode 
172 arranged anode to cathode to the output 46 of the C-lead status 
detector stage 48. 
The latch of the coin control stage 44 formed by the gates 130 and 132 is 
set to the "set to collect" condition by the high logic level at output 42 
of the line reversal detector stage 40 upon detection of the reverse 
polarity answer supervision. The latch forward biases the base-emitter 
junction of the transistor 148. However, transistor 148 does not conduct 
and generate the collect control signal at 50a until the transistor 160 is 
activated through the output 46 by the C-lead status detector stage 48. 
The output 46 of the C-lead status detector stage 48 activates transistor 
160 when the pay station 14 is placed in an on-hook condition with the 
C-IN lead to stage 48 switching to a battery voltage of approximately -48 
VDC. 
The C-lead status detector stage 48 utilizes the C (IN) control lead as an 
input connected through a resistor 176 to the input of an inverter gate 
178. A capacitor 180 is connected between the input of the inverter gate 
178 and the negative supply 126. The output of the inverter gate 178 is 
connected to the output 94 of the C-lead status detector stage 48 that is 
connected as an input to the C-lead control stage 90. The output of the 
inverter gate 178 is also connected through a resistor 182 to the input of 
an inverter gate 184. A diode 186 is connected cathode to anode between 
the output of the inverter gate 178 and the input of the inverter gate 
184. A capacitor 188 is connected between the input of the inverter gate 
184 and the negative supply 126. The output of the inverter gate 184 is 
connected through an inverter gate 190 to one end of a capacitor 192. The 
other end of the capacitor 192 is connected to the input of an inverter 
gate 194. The output of the inverter gate 194 is connected to the output 
46 of the C-lead status detector stage 48. A resistor 196 is connected 
between the inverter gate 194 and the negative supply 126. 
When the pay station 14 is placed in an on-hook condition at the 
termination of a conversation, the C (IN) lead switches to a voltage of 
-48 volts. This generates a negative going pulse at the output of inverter 
gate 194; the output 46 to the coin control stage 44. The negative pulse 
at output 46 charges the capacitor 170 of the coin control stage 44. Once 
charged, the capacitor 170 discharges through the resistor 168. 
When the C (IN) lead switches to a -48 VDC level, the generation of the 
negative pulse at 46 is delayed by the RC time constant determined by the 
resistor 182 and the capacitor 188 for a first predetermined time 
interval, approximately one second in a specific embodiment. Thus, the 
transistor 160 is rendered conductive after the first predetermined time 
interval and remains conductive for a second predetermined time interval, 
approximately one second in a specific embodiment, as determined by the 
time constant of the resistor 168 and the capacitor 170. 
If the coin control stage 44 has been previously placed in a "set to 
collect" condition and with transistor 160 rendered conductive, a current 
path will be established from ground reference 118 through the 
emitter-collector junction of the transistor 160, the emitter-collector 
junction of the transistor 148, diode 150 and via the collect control line 
50a through the collect relay of the coin control relay arrangement 52. 
The coin control relay arrangement 52 then outputs the +120 VDC collect 
signal to the pay station 14. After the discharge of the capacitor 170, a 
high logic level is generated at the output of the inverter gate 164 
denoting the end of the second predetermined time interval. The latch 
formed by gates 130 and 132 is then conditioned or reset to a "set to 
return" condition through the capacitor 142, and transistor 160 is 
rendered non-conductive. 
If a free or incomplete call is made, line reversal does not take place and 
the reversal detector stage 40 does not generate an output at 42 and the 
coin control stage 44 is in a "set to return" state. When the pay station 
14 goes on hook, the C (IN) lead switches to a -48 VDC level and the 
transistor 160 is activated after the first predetermined time interval 
for the second predetermined time interval and transistor 156 is rendered 
conductive as the coin control stage 44 is in a "set to return" state. 
Thus, a current path is established via the return line control line 50b 
to activate the return relay in the coin control relay arrangement 52. The 
coin control relay arrangement 52 generates a -120 VDC return signal to 
the pay station 14. 
The ring voltage detector stage 56 of the prepay adapter circuit 10 
utilizes the R (IN) line as an input. The R (IN) line is connected through 
the series combination of a capacitor 200 and a resistor 202 to a diode 
rectifying arrangement including two diodes 204 and 206 arranged to 
rectify the ringing voltage. One diode 204 is connected cathode to anode 
between the resistor 202 and the negative supply 126. The second diode 206 
is connected anode to cathode between the cathode of the diode 204 and one 
end of a capacitor 208. The other end of capacitor 208 is connected to the 
negative supply 126. A resistor 210 is connected across the capacitor 108. 
The cathode of the diode 206 is connected through a resistor 212 to an 
input of an inverter gate 214. A diode 216 is connected anode to cathode 
between the input of the inverter gate 214 and the ground reference 118. 
Thus, the ringing voltage is rectified by the diodes 204 and 206 and 
filtered by the capacitor 208. The output of the inverter gate 214 is 
connected through a diode 218 arranged cathode to anode to the input of an 
inverter gate 220. The output of the inverter gate 220 is connected to the 
output 78 of the ring voltage detector stage 56 that is connected as an 
input to the tone generator stage 76. The output of the inverter gate 220 
is also connected through a resistor 222 and a selective connection 
arrangement 224 to the output 58 of the ring voltage detector stage 56. 
The selective connection arrangement 224 in a specific embodiment is a 
jumper wire which is connected to render the ring voltage detector stage 
operable to output a signal at the output 58. The jumper wire 224 is 
omitted or disconnected if the feature to output a signal at 58 is not 
required. The parallel combination of a capacitor 226 and a resistor 228 
are connected between the input of the inverter gate 220 and the ground 
reference 118. 
Upon the detection of ringing voltage, the output 58 is effective to set 
the latch formed by gates 130 and 132 of the coin control stage 44 to the 
"set to collect" condition. As explained hereinabove, this sequence is 
effective to provide an automatic collection feature for incoming collect 
calls to the pay station to thereby assist the operator. 
The line imbalance detector stage 66 of the prepay adapter circuit 10 
includes a comparator stage 232. The output of the comparator stage 232 is 
connected to the output 68 of the line imbalance detector stage 66. One 
input 234 of the comparator stage 232 is connected to a voltage divider 
arrangement connected between the ground reference 118 and the -48 volt 
central office battery connection 236. The voltage divider arrangement is 
provided by the series combination of a resistor 238, a potentiometer 240 
and a resistor 242 connected between the ground reference 118 and the -48 
volt central office battery supply line 236. 
A second input 244 to the comparator 232 is connected to the junction of 
two resistors 246 and 248. The series combination of resistors 246 and 248 
is connected between the T (IN) line and the R (IN) line. The resistors 
246 and 248 are equal resistances and thus the voltage supplied to the 
input 244 is the midpoint between the T (IN) and R (IN) potentials when 
the line polarity is in a reverse condition. The input 244 of the 
comparator 232 is also connected through the series connection of a 
resistor 250 and a diode 252 arranged cathode to anode to the T (IN) lead. 
The voltage divider provided by resistor 238, resistor 242 and 
potentiometer 240 is selected and adjusted to provide a voltage at the 
input 234 that is approximately 48% of the full central office battery 
voltage at 236 with respect to the ground reference 118. With the line 
polarity reversed and balanced, the input 244 is exactly 50% of the 
battery voltage. In this condition the input 244 is lower or more negative 
than the input at 234 and thus the output at 68 is a low state. 
This is the condition for a pay station originated call, low output state 
at 68, when the called party has answered and the proper coin deposit 
acknowledgement has not yet occurred. In this state the muting stage 62 in 
response to the output at 68 provides an audio mute condition between the 
T (IN) and R (IN) lines to the pay station 14. 
The audio mute stage 62 includes a capacitor 256 connected between the R 
(IN) line and one contact 257 of a pair of normally open relay contacts 
258. The other contact 259 of the relay contacts 258 is connected to the T 
(IN) line. The relay contacts 258 are controlled by an associated relay 
coil 260. The relay coil 260 is connected between the output 68 of the 
line imbalance detector stage 66 and the cathode of an LED indicator 262. 
The anode of the LED indicator 262 is connected to the output 70 of the 
loop current status detector stage 72. A capacitor 264 is connected across 
the relay coil 260. 
The loop current status detector stage 72 includes a PNP transistor 266. 
The collector lead of the transistor 266 is connected to the output 70. 
The emitter lead of the transistor 266 is connected through a diode 268 
arranged cathode to anode to the ground reference 118. The emitter lead of 
the transistor 266 is also connected through a resistor 270 to the T (IN) 
line. The base lead of the transistor 266 is connected through a resistor 
272 to the T (IN) line. A capacitor 274 is connected between the base and 
emitter leads of the transistor 266. 
In a reverse polarity condition, the transistor 266 is rendered conductive 
by the negative level on the T (IN) line. Thus, the loop current status 
detector stage 72 provides a current path from the ground reference 118 
through the diode 268, the emitter to collector junction of the transistor 
266, the LED indicator 262, the relay coil 260, and via the low state at 
output 68 through the line imbalance comparator 232. Thus, with current 
through the relay coil 260, the normally open contacts 258 are closed to 
place the capacitor 256 across the ring and tip lines to the pay station 
14 to mute the audio path. 
Upon the proper deposit of coins into the pay station 14, the imbalanced 
line condition from the pay station 14 acknowledges the receipt of the 
proper deposit. This line imbalance condition is detected by the line 
imbalance detector stage 66. Specifically, the voltage present at the 
input 244 of the comparator 232, now changes to about 35 to 45% of the 
battery voltage dependent upon the pay station line loop length and the 
ground resistance conditions. In this condition, the input 244 is higher 
or less negative than the input at 234. Thus, the output 68 of the 
comparator stage 232 will switch to a high level state. This high output 
state will inhibit any current flow through the relay coil 260 and thus 
the relay contacts 258 will return to the normally open state. The audio 
mute condition to the pay station 14 is then removed and normal 
conversation and progress of the call ensues. 
Under normal line polarity conditions, the diode 252 and the resistor 250 
present a voltage level at the input 244 equal to approximately 25% of the 
central office battery voltage. This level is always higher or less 
negative then the input at 234 and the relay contacts 258 will remain in 
the open condition regardless of the presence or absence of the imbalance 
condition from the pay station 14. This prevents any audio muting during 
normal line polarity to allow free calls to the operator, 911 emergency 
calls or any other number designations which do not result in reverse 
answer supervision. It should be understood that all the voltage 
comparisons are determined as a percentage of the central office battery 
voltage and thus fluctuations in the battery voltage have no effect on the 
operation of the line imbalance detector stage 66. Indicator 262 serves to 
indicate the status of the comparator stage 232 during adjustments of the 
potentiometer 240. Transistor 266 further prevents the relay contacts 258 
from operating during idle line conditions such as might be caused by 
voltage transients or ringing voltage on the line to prevent a false of 
the comparator 232 from operating the relay contacts 258 by supplying 
current to the relay coil 260. Zener diodes 278, 280 are connected to the 
supply inputs of the comparator 232 and to the ground reference 118 and to 
the -48 volt battery line 246 to provide a 12 volt supply to the 
comparator stage 242. 
The tone generator stage 76 of the prepay adapter circuit 10 includes a 
diode 284 having an anode lead connected to the output 80 of the loop 
current status detector stage 72. The cathode of the diode 284 is 
connected through a resistor 286 to the input of an inverter gate 288. The 
output of the inverter gate 288 is connected through a capacitor 290 to 
the input of an inverter gate 292. The output of the inverter gate 292 is 
connected through a diode 294 arranged anode to cathode to the input of an 
inverter gate 296. The output of the inverter gate 296 is connected to one 
input of a two input NOR gate 298. The two input NOR gate 298 along with 
another two input NOR gate 300 provides an audio oscillator. 
The output of the NOR gate 298 is connected to one input of the NOR gate 
300. The output of the NOR gate 300 is connected through the series 
combination of a capacitor 302 and a resistor 304 to the second input of 
the NOR gate 298. The output of the NOR gate 298 is also connected through 
the series combination of a resistor 306 and a variable resistor 308 to 
the junction of the resistor 304 and the capacitor 302. The output of the 
NOR gate 300 is connected through a resistor 310 to the base lead of an 
NPN transistor 312. The emitter lead of the transistor 312 is connected to 
the negative supply 126 through a resistor 314. The collector lead of the 
transistor 312 is connected to the T (IN) line through a capacitor 316. 
The collector of the transistor 312 is also connected to the ground 
reference 118 through the series combination of a resistor 318 and a diode 
320 arranged cathode to anode. 
The second input of the NOR gate 300 is connected to the output of an 
inverter gate 322. The input of the inverter gate 322 is connected through 
a diode 324 arranged anode to cathode to the base lead of the transistor 
266 of the loop current status detector 72. The input of the inverter gate 
322 is also connected to the output 78 of the ring voltage detector stage 
76. The input of the inverter gate 288 is connected to the negative supply 
126 through the parallel combination of a resistor 326 and a selective 
connection arrangement 328. The selective connection arrangement 328 in a 
specific embodiment is a jumper wire which is connected to inhibit the 
generation of the identification tone by the tone generator stage 76. The 
jumper 328 is omitted or removed to allow operation of the tone generator 
stage 76. The input of the inverter gate 292 is connected to the ground 
reference 118 through a resistor 330. The parallel combination of a 
resistor 332 and a capacitor 334 is connected between the input of the 
inverter gate 296 and the negative supply 126. 
The high level at the output 78 of the ring voltage detection stage 76 
during ring voltage detection produces a low input to the inverter gate 
300. The low input to the inverter gate 300 continues throughout ringing, 
the silent intervals between ring intervals and approximately six seconds 
after the last ring cycle. After six seconds past the last ring cycle, the 
input to the NOR gate 300 will return to a high level and the tone 
generator stage 76 will be inhibited. 
Upon the pay station 14 being rung and going off-hook, the transistor 266 
is rendered conductive and current flows through the diode 284 and the 
resistor 286 to produce a high level at the input of the inverter gate 
288. The high to low transistion at the output of the inverter gate 288 is 
coupled through a capacitor 290 to the input of the inverter gate 292. A 
relatively short positive pulse occurs at the output of the inverter gate 
292 that immediately charges the capacitor 334. The capacitor 334 then 
discharges through the resistor 332. Thus, a negative pulse of 
approximately one second duration as determined by the RC time constant of 
the capacitor 334 and the resistor 332 is generated at the inverter gate 
296. Assuming that ringing voltage has previously been detected and a low 
input is present to the NOR gate 300 from the inverter 322, the tone 
generator stage 76 will be enabled for approximately a one second period 
to generate an audible tone on the T (IN) line. The oscillator formed by 
gates 298 and 300 along with the capacitor 302, the resistor 306 and the 
variable resistor 308 determine the frequency of the audible tone. This 
audible tone serves as an identification tone to the operator to indicate 
that a pay station has answered. 
The hold stage 84 of the prepay adapter circuit 10 includes a PNP 
transistor 340. The collector lead of the transistor 340 is connected to 
the output 88 of the hold stage 84. The emitter lead of the transistor 340 
is connected to the T (IN) line. The base lead of the transistor 340 is 
connected through a resistor 342 to the output 86 of the C-lead status 
detector stage 48. The fixed line termination discussed hereinbefore is 
included in the coin control relay arrangement 52 and is connected during 
coin control (return or collect) operations and when the hold output 88 is 
active. The hold output 88 is active when the output 86 of the C-lead 
status detector stage 48 indicates that the pay station 14 is off-hook. 
Referring to a specific embodiment of the tone control circuit 12, the 
Model TD-4 system referenced hereinbefore, the hold stage 84 is connected 
between the T (IN) line of the TD-4 system and the contact identified as 
number 11 of the collect relay (drawing number 434-227B at page 7 of the 
CEECO TD-4 technical description form 1015-6). The existing connection 
between the T (IN) line and the contact 11 is then disconnected and the 
hold stage 84 is connected in lieu of the connection. The hold output 88 
is connected to contact 11 of the collect relay. 
The C-lead control stage 90 includes the parallel combination of a resistor 
346 and a diode 348 arranged cathode to anode, between the output 94 of 
the C-lead status detector stage 48 and one end of a resistor 350. The 
other end of the resistor 350 is connected to the base lead of a PNP 
transistor 352. A capacitor 353 is connected between the junction of the 
resistors 350 and 346 and the negative supply 126. The emitter lead of the 
transistor 352 is connected to the ground reference 118. The collector 
lead of the transistor 352 is connected through a resistor 354 to the base 
lead of an NPN transistor 356. The emitter lead of the transistor 356 is 
connected to the negative supply 126. The collector lead of the transistor 
356 is connected through a relay coil 358 to the ground reference 118. A 
diode 360 is connected anode to cathode between the collector of the 
transistor 356 and the ground reference 118. 
The relay coil 358 controls a movable center contact 362 between a pair of 
contacts 364 and 366. The contact 366 is normally open and connected 
through a diode 368 cathode to anode to the ground reference 118. The 
movable center contact 362 is connected to the C (OUT) line. The normally 
closed contact 364 is connected to the C (IN) line and through a diode 370 
anode to cathode to the C (OUT) line. The diode 370 provides a path for 
ANI signals or the like from the C (IN) line to the C (OUT) line 
regardless of the status of the center contact 362. 
The relay coil 358 receives current through the transistor 356 upon the C 
(OUT) lead from the line finder 24 switching from -48 VDC (C.O. battery) 
to approximately ground potential. This occurs when the pay station is 
placed off-hook. This signal C (OUT) is supplied through the normally 
closed contact 364 to the C (IN) lead to the pay station first selector 
28. Further, the potential on C (IN) also provides a high level to the 
gate 178 of the C-lead status detector stage 48. Thus, the output 94 is at 
a low level. With relay coil 358 energized, the normally open contact 366 
is connected to the center contact 362, and the C (OUT) lead is connected 
to the ground reference 118 thru diode 368. 
This condition continues for a predetermined time interval after the pay 
station is placed on hook, thus allowing time for the coin control 
function. In a specific embodiment, the time interval is approximately two 
seconds. Upon the pay station being placed on hook, the C (IN) lead 
switches to -48 v. The output 94 switches to a high level and the 
capacitor 353 begins to charge. After the two second time interval, the 
current through the relay coil 358 will cease and the relay contact 366 
will be placed in the normally open state. At this point, the pay station 
line will be released. The two second time interval is provided by the RC 
time constant of the resistor 346 and the capacitor 353. 
The various NOR and INVERTER logic gates identified hereinbefore in 
connection with the prepay adapter circuit 10 of FIG. 4 include power 
supply connections between the ground reference 118 and the negative 
supply 126 to provide a 12 VDC power supply. 
While there has been illustrated and described several embodiments of the 
present invention, it will be apparent that various changes and 
modifications thereof will occur to those skilled in the art. It is 
intended in the appended claims to cover all such changes and 
modifications as fall within the true spirit and scope of the present 
invention.