Low resistance origination scan circuit

An origination scan circuit for detecting circuit closure state in a loop circuit has a much lower impedance than does the typical loop circuit. Its source impedance is split into separate resistive parts for connecting opposite terminals of an office battery to the tip and ring leads of the loop. Currents in the separate resistive parts are sensed and summed for comparison against a threshold current to indicate whether or not a subscriber station on the loop is off hook. Threshold current magnitude is variable to accommodate loop start and ground start applications. When the off-hook state is detected, the origination scan circuit is disabled by switching open the resistive part connections, and loop current is thereafter provided by other suitable means. Automatic current limiting arrangements are included for converting the switching functions to high impedance circuit elements.

FIELD OF THE INVENTION 
This invention relates to circuit closure detecting arrangements and 
particularly to those useful for off-hook detection in telephone 
subscriber loop circuits. 
BACKGROUND OF THE INVENTION 
Numerous on-hook/off-hook circuit state detectors are known in the art for 
indicating, in response to attainment of at least a predetermined minimum 
current level, in a telephone subscriber loop circuit that the subscriber 
has taken the station from the on-hook state to the off-hook state. In 
that state the station set hook switch is closed and telephone central 
office battery current can flow at a sufficient level to sustain operation 
of the subscriber station set for communication purposes. Some detectors 
of that type sense current in each line circuit conductor, i.e., the tip 
and ring conductors, and determine from the sensed current levels at least 
whether or not the subscriber station is in the off-hook state. Some 
examples are noted below. 
A U.S. Pat. No. 4,115,664 to C. Laface et al., is directed to a loop 
current detecting circuit in which current is sensed and information 
coupled via optical couplers in each conductor to decision logic 
circuitry. The sensed signals are logically combined to distinguish 
between current flow in one or both conductors. 
A telephone system supervisory circuit is shown in a U.S. Pat. No. 
3,855,420 to R. E. Steidl. That circuit uses current sensing transistors 
to detect currents, and the combined states of the transistors define 
predetermined supervisory states. 
In U.S. Pat. No. 4,360,709 to K. Hamazato et al., telephone loop closure 
detection is performed by circuits which generate signals proportional to 
tip and ring currents, respectively. Sensing resistors are effective in 
the circuit between office battery and the loop at all times. Sensing 
signals are summed and compared to a reference voltage to produce a signal 
indicative of the results of the comparison. 
A hook status detector in a subscriber loop interface circuit of U.S. Pat. 
No. 4,406,929 to W. D. Pace et al. responds to the occurrence of loop 
current to develop a voltage which is compared to a reference to indicate 
an off-hook state. Sensing currents are coupled through series resistors 
which are large compared to the closed-circuit resistances usually found 
in loop circuits. In the off-hook state of loop operation, the sensing 
circuits are always energized to maintain the operative state of 
associated battery feed circuits. 
SUMMARY OF THE INVENTION 
A hook state detecting circuit, herein usually called an origination scan 
circuit, is provided separately from the battery feed circuit of a 
telephone line circuit and has such a low source impedance, compared to 
the subscriber loop circuit impedance, that even in the worst case 
operative currents for on-hook and off-hook states are so widely different 
that the origination scan circuit can readily detect the difference 
without expensive, sophisticated, high-sensitivity circuits. That source 
impedance is resistive and is split into separate parts for connecting 
opposite terminals of an office battery to tip and ring connection 
terminals of the circuit. Currents in the separate resistors are scaled, 
summed, and thresholded to determine whether a subscriber station set on 
the loop is off hook or on hook. 
In one embodiment, the detection of the off-hook state causes the 
origination scan circuit to be powered down as the line feed circuit 
powers up. Because the low-valued resistors in the origination scan 
circuit conduct a large current for only a short time, they are of very 
low wattage and are thus easily implemented in integrated circuit 
technology. A current limiting function is also provided.

DETAILED DESCRIPTION 
In FIG. 1, a line interface unit 10, or channel unit, interfaces between 
tip and ring leads 11 and 12 of a subscriber line, or loop, circuit and a 
pair of two-wire circuits 16 and 17 extending to office equipment such as 
switching or multiplexing equipment of a carrier system. A subscriber 
station set is schematically represented by a block 13 including a 
resistor and switch hook contacts to connect the resistor between the tip 
and ring leads. Office battery supply rails (not separately shown) are 
included in the circuit blocks of this figure. An electronic line, or 
battery, feed circuit 18 advantageously provides current to the loop 
circuit during off-hook operating times when the switch hook contacts are 
closed. Circuit 18 is, for example, of the type shown in a copending U.S. 
patent application Ser. No. 635,209, filed July 27, 1984, of R. Toumani 
(Case 6), entitled "Electronic Battery Feed Circuit For Telephone 
Systems," and which is assigned to the same assignee as the present 
application. Such an electronic battery feed circuit is conveniently 
integratable and performs other circuit functions, including hybrid 
transformation between two-wire and four-wire circuits. A protection 
circuit 19 is connected in parallel with the line feed circuit 18, and 
that parallel combination is connected through protection resistors 20 and 
21 to the tip and ring leads, respectively, of the subscriber loop. 
Resistors 20 and 21 are typically low valued resistors compared to loop 
resistance, and in one embodiment they had values of 68 ohms each. 
In accordance with the present invention, one interface function not 
performed in the line feed circuit 18 is the initial off-hook state 
detection. This function is advantageously performed by an origination 
scan circuit 22 which is also connected, through terminals 23 and 26, in 
parallel with circuits 18 and 19. As will be subsequently described, 
origination scan circuit 22 is in an active, or powered up, condition 
during the on-hook state on the subscriber loop; and in that condition it 
supplies and monitors the tip and ring current that may be required. At 
such times the line feed circuit 18 is powered down to reduce power 
dissipation. When the subscriber goes off hook, the resulting increase in 
loop current flow is detected in the origination scan circuit; and a 
corresponding signal is sent on a lead 27 to a control logic circuit 28, 
which is also part of the channel unit. Logic circuit 28 correspondingly 
turns on the line feed circuit 18 power conditioning function by applying 
a signal on a lead 29. Likewise, a signal is applied from logic circuit 28 
via a lead 30 to turn off the origination scan circuit 22 in a manner to 
be described. It will be appreciated that the control logic function just 
described is that of a set/reset bistable circuit having its complementary 
outputs connected to leads 29 and 30. In its off state, circuit 22 
presents such a high impedance between terminals 23 and 26 that it has no 
significant effect on channel unit operation; and line feed circuit 18 
supplies line current to the subscriber loop in the usual manner. 
When the subscriber goes on hook again, the resulting high loop resistance 
and low loop current cause the line feed circuit to produce a 
corresponding signal on a lead 31 to the control logic circuit 28. For 
example, in a line feed circuit, such as in the aforementioned line feed 
circuit, a direct current profiling circuit is provided, and its output in 
a direct current feedback path changes abruptly when the station set goes 
on hook; and that change is detected, by circuits not separately shown, to 
produce the signal on lead 31. Logic circuit 28 responds by changing the 
signals on leads 29 and 30 to power down the line feed circuit 18 and 
power up the origination scan circuit 22. 
FIG. 2 is a functional block diagram of the origination scan circuit 22 of 
FIG. 1 and shows the use of tip and ring switches 32 and 33 to implement 
the on/off function of the circuit 22. When circuit 22 is actively 
monitoring loop circuit state, those switches are closed; and the loop is 
in its highest impedance state; and any necessary leakage currents are 
coupled from the office supply rails, schematically represented by a 
negative supply 36 and ground, through the switches 32 and 33 and two 
current scalers 37 and 38. Scaler 38 is illustratively implemented as a 
current mirror. Supply 36 is illustratively minus 48 volts. Switch 32 is 
in series with the input to current scaler 37, and switch 33 includes the 
master transistor in the input side of current mirror scaler 38. When the 
subscriber station set 13 goes off hook, current from the supply rails 
increases in a path from ground through current scaler 37, tip switch 32, 
tip terminal 23, the loop, ring terminal 26, and ring switch 33 and 
current mirror scaler 38 to supply 36. The amount of current flowing 
depends upon the length of the subscriber loop. 
Current scaler 37 and current mirror scaler 38 provide reduced proportional 
versions of tip and ring currents to different inputs of an analog summing 
circuit 39, the output of which is applied to an input of a comparator 
circuit 40. In the comparator, the sum signal is compared to a 
predetermined threshold current advantageously selected to represent the 
worst case (longest loop) minimum required operating loop current. If the 
threshold is exceeded, the comparator output changes to so indicate to the 
control logic via lead 27. A lead 34 is provided for applying a 
loop-start/ground-start (LS/GS) signal which is used in a manner to be 
described for modifying the magnitude of the threshold to accommodate the 
type, loop-start or ground-start, of subscriber loop to be served by the 
line interface unit. It will be appreciated that this method of summing 
scaled versions of the currents sensed in the tip and ring conductors of 
the subscriber loop lends the origination scan circuit insensitivity to 
longitudinal signals on the loop. Thus, a 60 Hertz longitudinal signal on 
the subscriber loop increases and decreases at terminals 23 and 26 
together. That longitudinal effect combines with metallic current at any 
given instant to enhance the total current on one of the tip or ring 
switches 32 or 33 and decrease the current in the other switch. 
Consequently, the scaled versions of the longitudinal component cancel in 
summer 39. 
FIG. 3 depicts a schematic diagram of an illustrative embodiment of the 
functions of the circuit of FIG. 2. Like reference characters are employed 
to indicate circuit elements which are the same as those in FIG. 1. The 
embodiment is illustratively implemented using plus and minus, 5-volt, 
ground referenced supplies 41 and 42. 
A binary signal on lead 30 from control logic 28 determines whether the 
circuit 22 of FIG. 3 is on (powered up) or off (powered down). In the high 
state, e.g., +5 volts, of that signal applied through a coupling resistor 
43 to a base terminal of a pnp transistor 46, that transistor is biased 
off. Turning off transistor 46 allows pnp transistors 47 and 48 to conduct 
current through their respective equal emitter resistors 50 and 51 from 
supply 41 through a common resistor 49. The level of that conduction is 
set by the emitter series resistances 50 and 51, the common resistor 49, 
and the base bias established by a potential divider including resistors 
52 and 53 connected in series across supply 41. That base bias, 
illustratively about 2 volts, is selected to establish a reasonable 
switching threshold for the binary signal on lead 30. The resistors 49, 50 
and 51 are selected to cause transistors 47 and 48 to supply sufficient 
current to enable conduction in switches 32 and 33. The same base bias is 
also supplied to comparator 40 for a purpose to be described. In the low 
state, e.g., ground, of the lead 30 signal, transistor 46 conducts thereby 
lowering the emitter voltages of transistors 47 and 48 so that they are 
rendered nonconducting. Therefore, switches 32 and 33 are necessarily 
disabled in their open circuit condition. 
Switches 32 and 33 are Darlington type switches of essentially the same 
configuration. In switch 33, the Darlington connection includes npn 
transistors 56 and 57; and transistor 56 receives at its base electrode 
the transistor 47 collector current. Collector terminals of transistors 56 
and 57 are connected to ring terminal 26, and the emitter of transistor 57 
is connected to supply 36 by the series combination of resistors 58 and 
59. The combined resistance of those two resistors and their counterparts 
on the tip side of the circuit is less than the anticipated loop circuit 
resistance to which the origination scan circuit will be connected. In one 
embodiment, resistor 58 was about 3.75 ohms and resistor 59 was about 80 
ohms. Such low resistances necessarily results in relatively high current 
flow when a connected subscriber station set goes into its off-hook state; 
but, as previously mentioned, the origination scan circuit will be powered 
down in response to that event. That large current is advantageously 
accommodated by employing multiple, e.g., sixteen in one embodiment, 
parallel-connected transistors to implement the transistor 57; and in that 
implementation the resistor 58 schematically represents the combined 
resistance of the emitter ballasting resistors normally employed in such a 
combination. 
A resistor 54, between base and emitter of transistor 56, and a resistor 
55, between the base of transistor 57 and the common circuit point between 
resistors 58 and 59, conduct the collector-base capacitor currents caused 
by fast voltage variations on terminal 26 to prevent false turn-on of 
switch 33. 
Darlington transistor 57 also performs the master transistor function of 
the current mirror circuit 38. Accordingly, less semiconductor area is 
required than would be the case if the switch and master transistor 
functions were accomplished by separate multiple transistor sets able to 
handle the ring current. The current of transistor 57 is mirrored to a 
slave current path including an npn transistor 60 and its emitter resistor 
61. The mirrored current is scaled down by a factor of about 100 by 
properly ratioing resistors 61 and 59. 
Control signal current from transistor 48 for switch 32 is inverted in a 
current mirror 62 so it can be utilized in the switch, which employs pnp 
transistors. Aside from conductivity type, switch 32 is identical to 
switch 33; and it is connected in series with a resistor 63, corresponding 
to resistor 59, between ground and the tip terminal 23. The current scaler 
37 includes a voltage-to-current conversion arrangement connected across a 
resistor 63. In that arrangement are a diode-connected npn transistor 66 
and a common base npn transistor 67 as well as a resistor 63 and a 
resistor 68 for factoring down the off-hook current of switch 32 by the 
same ratio as previously mentioned for the current mirror scaler 38. 
Resistor 35 in the current scaler 37 is chosen so that the current in 
diode connected transistor 66 will be in the range of the current expected 
in transistor 67 near the hook status detection threshold. Under this 
condition, the V.sub.BE 's of transistors 66 and 67 will be approximately 
equal which means that the voltage at the emitter of transistor 67 will be 
approximately equal to zero. As a result, the current in resistor 68 will 
be equal to the current through resistor 63 multiplied by the ratio of 
resistor 63 to resistor 68. Since the alpha of transistor 67 is 
advantageously approximately equal to unity, the collector current of 
transistor 67 is approximately equal to the current through resistor 68. 
By tying resistor 63 to ground and providing for a virtual ground at the 
emitter of transistor 67, the use of a diode biased transistor, such as 
the transistor 66, in series and between ground and resistor 63 is 
avoided. Avoiding connection of transistor 66 in the tip current path from 
ground is desirable since that transistor in that path would have to be 
implemented by a multiple-transistor set requiring substantial extra 
semiconductor area. 
Output currents of the current scaler 37 and current mirror 38 are summed 
at a node 39', and the combined current is drawn from comparing circuit 
40. In the latter circuit, a pnp transistor 69 conducts a certain 
threshold current. That transistor is a slave transistor in a current 
mirror connection including a master pnp transistor 70 and a helper pnp 
transistor 71 having its base-emitter junction connected across the base 
collector junction of transistor 70. Current in transistor 70, and hence 
the threshold current in transistor 69, is fixed in accordance with the 
state of the LS/GS signal on lead 34 as previously mentioned. Emitter 
resistors 72 and 73 for transistors 70 and 69 are selected to provide any 
suitable ratio between the emitter currents, and the resistor 72 is 
advantageously a trimmable resistor for convenient factory setting of that 
ratio to achieve the desired threshold current. When a subscriber station 
is on hook, the sum current at node 39' is less than the threshold current 
which can be supplied by transistor 69; and transistor 69 saturates, 
sinking the excess current through transistor 71. However, when the 
station is taken off hook, the summed current from node 39' exceeds the 
threshold current which can be supplied by transistor 69 and the excess 
current is drawn from the base terminal of a pnp transistor 76 which is 
arranged with another pnp transistor 77 in a Darlington connection. That 
action draws transistors 76 and 77 into conduction and increases the 
potential drop across the collector resistor 78 to ground thereby 
signaling the control logic 28 that the subscriber has gone off hook. 
Threshold current in comparing circuit 40 is advantageously set to a value 
corresponding to the minimum allowable loop off-hook current, i.e., that 
for the longest expected loop given worst case manufacturing tolerances 
and environmental conditions. In one loop start embodiment, that threshold 
current was about 300 microamperes. About half of that current is needed 
for ground start operation where only ring lead current flows initially 
when the station set is taken off hook. The LS/GS signal is utilized to 
control the appropriate threshold to accommodate the loop start or ground 
start character of the subscriber loop to which the origination scan 
circuit is to be connected. 
The LS/GS signal is applied through a coupling resistor 79; and when it is 
low, e.g., ground, it biases an npn transistor 80 off; so the collector 
current of another npn transistor 81 flows through an npn transistor 87 
instead of transistor 80. Transistor 81, and another npn transistor 82 are 
connected as current sources with emitter resistors 83 and 86 for 
determining the current in transistor 70. Bases of transistors 81 and 82 
are provided with a current rail type of bias, e.g., by connection to a 
base terminal of a master transistor in a current mirror arrangement not 
otherwise shown. The collector terminal of transistor 82 is directly 
connected to the collector of transistor 70; and an npn transistor 87, 
which is base biased the same as transistors 47 and 48, is provided for 
connecting the collectors of transistors 70 and 81. When transistor 80 is 
off, as just described, transistor 87 is on; and the currents from 
transistors 81 and 82 are summed and applied to the collector of 
transistor 70 to provide the full current needed to set the aforementioned 
larger threshold current needed for loop start operation. When the LS/GS 
signal on lead 34 is high, however, transistor 80 is on to supply all the 
current that transistor 81 can handle and transistor 87 is turned off. 
Therefore, only a portion of the prior threshold current is available as 
is required for the ground-start option. 
A current limiting function is provided in the circuit of FIG. 3 to protect 
the physical integrity of the switches 32 and 33 and their associated 
resistors in the event of a short loop condition, a power cross, a large 
current surge from the power supply leads, or other large voltage 
occurrence on the subscriber loop. This function is provided in the 
current scaler 37 and the current mirror scaler 38 by utilizing a fraction 
of the voltage drop across resistors 59 and 63 to initiate limiting of the 
circuit current that flows through switches 32 and 33. Thus, in the 
current mirror 38, occurrence of an excessive current flow in resistor 59 
develops a sufficient potential difference across a parallel connected 
potential divider, including resistors 88 and 89, to bias a normally 
nonconducting npn transistor 90 into conduction at a sufficient level to 
deprive switch 33 of sufficient drive current to bring its Darlington 
transistors out of saturation and increase the collector-emitter voltage 
difference of transistor 57. That collector-emitter voltage of transistor 
57 varies with the external load, so as to limit the voltage drop across 
resistor 59, and thereby limits the current in resistor 59 and switch 33. 
Resistors 88 and 89 are advantageously proportioned so that an equilibrium 
is achieved at a desired level of current in resistor 59. That 
proportioning is achieved by selecting those resistors so that the product 
of the limiting current level, resistor 59, and the ratio of the 
resistance of resistor 89 to the sum of the resistances of resistors 88 
and 89 is equal to the turn-on voltage for transistor 90. Resistor 91 is 
included in the emitter path of transistor 90 to suppress any tendency of 
the described limiting function to oscillate. A similar limiting function 
is provided on the tip side using a pnp transistor 84 to supply current to 
the slave transistor of current mirror 62 and thereby starve the 
Darlington connected transistors of switch 32. 
Although the present invention has been described in connection with a 
particular embodiment thereof, it is to be understood that additional 
embodiments, applications, and modifications thereof which will be obvious 
to those skilled in the art are included within the spirit and scope of 
the invention.