Interoffice loop supervision arrangement

An arrangement for preventing false reseizures of loop-signaling, repeat coil-coupled interoffice trunk circuits for telephone switching systems is disclosed. In the prior art, false reseizures of an incoming trunk circuit can be caused by discharge of the midpoint capacitor at the facility side of the repeat coil bridge of an associated outgoing trunk circuit at the opposite terminus of the interoffice trunk facility. During the talk state, this midpoint capacitor is charged approximately to the signaling battery supply level with polarity such that, upon termination of the talk state and accompanying tip-ring reversal, the capacitive discharge is sufficient to re-operate the supervisory relay in the distant office incoming trunk circuit, thereby generating a false reseizure attempt. By placing said midpoint capacitor within a diode bridge at the outgoing trunk circuit, the charge built up thereon is prevented from assuming a polarity necessary to initiate such false reseizures while simultaneously allowing for normal operation of said outgoing trunk circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to interoffice DC loop signaling arrangements for 
communication switching systems and more particularly to solutions of 
false reseizure problems attributable to facility-side midpoint capacitors 
associated with repeat coils utilized in certain loop-signaling 
interoffice telecommunication trunk circuits. 
2. Description of the Prior Art 
Known interoffice trunks providing for metallic voice circuits between 
communication switching systems commonly employ DC supervisory and address 
signaling methods. Such methods utilize open and closed DC current loops, 
as well as normal and reverse battery potential across the two-wire 
facility, in conjunction with trunk circuit supervisory relays and/or 
reverse battery detector circuits to convey signaling information between 
trunk circuits terminating the end points of the interoffice trunk 
facility. Such trunk circuits are commonly known in the art as 
loop-signaling or loop-dial trunk circuits. 
Additionally, as is well-known in the art, many of such trunk circuits 
employ impedance matching transformers commonly known as repeat coils. 
Repeat coils are characterized as having a pair of windings on both the 
primary and secondary of the impedance matching transformer, each pair 
interconnected by so-called midpoint capacitors. Rather than using the 
terms primary and secondary with reference to the transformer windings, 
the terms office-side and facility-side are used when discussing repeat 
coils. These designations flow naturally from considering that which 
happens to be connected to a particular side of a repeat coil under 
discussion -- i.e. the telephone switching office, or "office-side," or 
the interoffice transmission trunk facility, or "facility-side." 
Such prior art loop-signaling incoming trunk circuits with repeat coil 
coupling, may encounter false reseizure problems at the termination of the 
talk state due to charge build-up on the facility-side repeat coil 
midpoint capacitors in the outgoing trunk circuits. Under such a 
condition, the facility-side midpoint capacitor in the outgoing trunk 
circuit may discharge during the newly-initiated idle trunk state in a 
manner resulting in the incorrect operation of a supervisory relay in the 
incoming trunk circuit at the distant office. Indeed, such a false 
reseizure, under certain conditions, may become reiterative, resulting in 
a condition known as "flipping," wherein the distant office incoming trunk 
circuit is repetitively falsely seized over a prolonged time interval. 
Such false incoming trunk seizures are accompanied by false requests for 
common control circuit elements such as incoming registers, thus degrading 
service efficiency at the distant office associated with the incoming 
trunk circuit. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an arrangement in 
a loop-signaling, repeat coil-coupled trunk circuit for preventing false 
reseizures of the corresponding distant office trunk circuit. 
The trunk circuit's facility-side repeat coil midpoint capacitor is placed 
within a full-wave rectifier circuit whose input terminals are 
respectively connected to the repeat coil's facility-side windings. With 
this arrangement, said facility-side midpoint capacitor is prevented, 
during the trunk talk state, from charging with a DC polarity necessary to 
initiate reoperation of the supervisory relay of the distant office trunk 
circuit at the termination of the trunk talk state.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
The invention is best understood by initially considering a typical 
interoffice trunk signaling operation of the prior art. An exemplary prior 
art interoffice configuration of an outgoing trunk circuit at a local 
switching office connected to an incoming trunk circuit at a distant 
switching office is depicted in FIG. 1. 
It is to be noted with reference to all figures of the drawing that dashed 
lines are used at various points therein to indicate that the path shown 
proceeds via various elements such as logic relay contacts which are not 
pertinent to the invention and which are therefore not specifically shown. 
It is also to be noted with reference to the drawing that the well-known 
detached relay contact schematic symbology is employed throughout. Each 
relay contact so depicted is designated in the drawing in a manner which 
indicates the relay of which it is a part. For example, in FIG. 1, the 
make contact designated A1-1 in the outgoing trunk circuit 102 indicates 
that this is contact 1 of relay A1. 
Referring to FIG. 1, calling party 120 at switching office 100 is shown 
coupled to called party 130 at switching office 101. Calling party 120 is 
connected via the switching matrix (not specifically shown) of office 100 
to outgoing trunk circuit 102 via the normal two-wire pair commonly 
designated tip and ring, herein designated in FIG. 1 as T1, R1, to the 
office-side of repeat coil 105 of outgoing trunk circuit 102, namely 
office-side windings 105-1 and 105-3. The opposite end of winding 105-1 is 
connected to one terminal of office-side midpoint capacitor C1 and to a 
first terminal of the upper coil A1-A of relay A1. The opposite end of 
winding 105-3 is connected to the opposite terminal of office-side 
midpoint capacitor C1 and to a first terminal of the lower coil A1-B of 
relay A1. Coil A1-A has a second terminal connected to ground potential, 
while coil A1-B has a second terminal connected to office battery 
potential, V. 
The facility-side of repeat coil 105, namely facility-side windings 105-2 
and 105-4, is coupled to the interoffice transmission facility conductors 
T2 and R2. The opposite end of winding 105-2 is connected to one terminal 
of facility-side midpoint capacitor C2, one terminal of reverse battery 
detector 106, and input node 1 of a full-wave rectifier bridge comprised 
of diodes 107, 108, 109 and 110. The opposite end of winding 105-4 is 
connected to the opposite terminal of capacitor C2, a second terminal of 
reverse battery detector 106, and to input node 2 of the full-wave 
rectifier bridge via make contact A1-1 of relay A1. Loop signaling 
impedance means, comprising relay coil L1, is coupled between the output 
nodes 3 and 4 of the full-wave rectifier bridge. 
As used herein, the term "input nodes" used with reference to a full-wave 
rectifier bridge refers to those nodes of the bridge which will accept 
bidirectional current flow in the medium connected between said "input 
nodes," while the term "output nodes" refers to those nodes of the bridge 
which will permit only a predetermined unidirectional current flow in the 
medium connected between said "output nodes." Input nodes are sometimes 
designated "AC nodes," since bidirectional current flow is possible 
therebetween, while output nodes are sometimes designated "DC nodes," 
since only unidirectional current flow is possible therebetween. 
Interoffice transmission facility conductors T2 and R2 are terminated at 
their opposite extremes in incoming trunk circuit 103. As shown in FIG. 1, 
conductor T2 is coupled to conductor T3 of incoming trunk circuit 103 via 
the break contact portion of transfer contact B-1, and conductor T2 is 
also coupled to conductor R3 of incoming trunk circuit 103 via the make 
contact portion of transfer contact B2. In a symmetric manner, conductor 
R2 is coupled to conductor R3 of incoming trunk circuit 103 via the break 
contact portion of transfer contact B-2, and conductor R2 is also coupled 
to conductor T3 of incoming trunk circuit 103 via the make contact portion 
of transfer contact B-1. 
Conductors T3 and R3 are respectively coupled to first terminals of 
facility-side repeat coil windings 111-1 and 111-3, of repeat coil 111. 
The opposite end of winding 111-1 is connected to one terminal of 
facility-side midpoint capacitor C3 and to a first terminal of the upper 
coil A2-A of relay A-2. The opposite end of winding 111-3 is connected to 
the opposite terminal of facility-side midpoint capacitor C3 and to a 
first terminal of the lower coil A2-B of relay A2. Coil A2-A has a second 
terminal connected to ground potential, while coil A2-B has a second 
terminal connected to office battery potential, V. 
The office-side of repeat coil 111, namely office-side windings 111-2 and 
111-4, is coupled to conductors T4 and R4, thence through the switching 
matrix (not specifically shown) of distant office 101 to called party 130. 
The opposite end of winding 111-2 is connected to one terminal of 
office-side midpoint capacitor C4 and to a first terminal of upper coil 
B-A of relay B. The opposite end of winding 111-4 is connected to the 
opposite terminal of midpoint capacitor C4, a first terminal of lower coil 
B-B of relay B, and to make contact A2-1 of relay A2. The opposite side of 
contact A2-1 is coupled to ground potential via trunk relay logic not 
critical to the invention and hence not specifically shown. 
As seen from the arrangement of FIG. 1, an off-hook calling party 120 
requesting the communication connection, on being connected via office 100 
to outgoing trunk circuit 102, will cause the operation of supervisory 
relay A1. In turn, the closing of make contact A1-1 provides a signaling 
loop forward on leads T2 and R2 to incoming trunk circuit 103 thereby 
causing operation of supervisory relay A2. An inductive signaling loop 
forward to the distant office provided by relay L1 of outgoing trunk 
circuit 102 is utilized to provide for minimal AC transmission loss at 
repeat coil 105. The full-wave rectifying diode bridge comprised of diodes 
107, 108, 109 and 110 is placed around relay L1 to force DC inter-office 
loop current to flow in only one direction through the inductive winding 
of L1. This forced unidirectional DC current flow is required to prevent 
dropping of the A2 relay in incoming trunk circit 103 upon subsequent 
battery polarity reversals at leads T2 and R2. 
For the exemplary embodiment, assume that incoming trunk circuit 103 is 
equipped with the well-known dial-stop option, wherein a battery polarity 
reversal is returned to the calling office via the T2 and R2 conductors 
upon seizures of incoming trunk circuit 103 and is maintained thereon 
until a suitable registering device is attached for receipt of address 
signaling information (e.g. dial pulses). Such dial-stop operation is 
enabled as represented in FIG. 1 by make contact A2-1 closing an operating 
path to ground potential for the lower coil B-B of relay B whenever a 
seizure of incoming trunk circuit 103 is indicated by the operation of 
supervisory relay A2. Battery reversal back to outgoing trunk 102 is then 
provided by B relay transfer contacts B-1 and B-2. 
The reversal is recognized at outgoing trunk circuit 102 via reverse 
battery detector 106. Detector 106 may comprise one of several forms 
well-known in the art. For example, detector 106 could be comprised of a 
polar relay, operable only by a predetermined polarity of the potential 
appearing across conductors T2 and R2. 
When an appropriate register has been attached at incoming trunk circuit 
103, relay B is released (by logic contacts not specifically shown) to 
inform originating office 100, via transfer contacts B-1 and B-2 and 
detector 106, to begin outpulsing of address information required for 
completing the desired connection through office 101 to called party 130. 
When called party 130 answers, supervisory relay B of incoming trunk 
circuit 103 will reoperate over the subscriber loop via conductors T4 and 
R4. 
Hence, as seen from the arrangement of FIG. 1, when calling party 120 is in 
communication with called party 130 over the connection shown -- i.e., in 
the so-called talk state of the trunk, supervisory relay A1 of outgoing 
trunk circuit 102 and supervisory relays A2 and B, both of incoming trunk 
circuits 103, will all be operated. It is thus apparent from FIG. 1 that, 
as a result of the talk state, the facility-side midpoint capacitor C2 of 
outgoing trunk circuit 102 will charge approximately to the office battery 
potential V, with the side of C2 connected to winding 105-2 assuming an 
approximate voltage potential V, with respect to ground potential 
appearing at the side of C2 connected to winding 105-4. The potential 
across C2, as seen from FIG. 1, is derived from battery and ground supply 
at the coils A2-A and A2-B of supervisory relay A2 of incoming trunk 
circuit 103, fed back to C2 via operated transfer contacts B-1 and B-2 of 
incoming trunk circuit 103. 
The previously-discussed "flipping" problem with the A2 relay arises in 
connection with conditions immediately following a termination of the 
trunk talk state wherein calling party 120 initially terminates the 
communication by going on-hook at his subscriber set. The sequence of 
events pertinent to the problem of "flipping" is as follows. When calling 
party 120 goes on-hook, relay A1 of outgoing trunk circuit 102 releases 
thus removing via opening of contact A1-1, the loop forward to incoming 
trunk circuit 103 resulting in the release therein of relay A2. Now assume 
called party 130 goes on-hook thereby releasing supervisory relay B of 
incoming trunk circuit 103. Release of relay B restores transfer contacts 
B-1 and B-2 to the released state thus placing pre-charged midpoint 
capacitor C2 of outgoing trunk circuit 102 across relay A2 via the T2 and 
R2 conductors with charge polarity capable of reoperating relay A2 via the 
discharging of C2. However, A2 in reoperating will, in turn, reoperate 
relay B which will again reverse polarities at the T2, R2 conductors via 
contacts B-1 and B-2. The polarity reversal will now re-release relay A2 
and thence release relay B via opening of contact A2-1, and the "flipping" 
sequence will begin anew. This looped sequence will continue until the 
charge on midpoint capacitor C2 decreases to a level insufficient to 
falsely reoperate relay A2 of incoming trunk circuit 103. 
Hence, as seen from the discussion set forth hereinabove, the so-called 
"flipping" problem stems from the initial talk state charge condition 
placed on the facility-side midpoint capacitor C2 of the outgoing trunk 
circuit 102 with a reverse polarity via transfer contacts at the distant 
office incoming trunk circuit. The instant invention serves as a solution 
to the flipping problem by providing for the placement of the 
facility-side midpoint capacitor of the outgoing trunk circuit within a 
full-wave rectifying means, thus preventing said midpoint capacitor from 
ever being exposed to said reverse polarity. 
FIG. 2 depicts the outgoing trunk circuit of FIG. 1 modified in accordance 
with the principles of the invention. All elements carried over from FIG. 
1 have been given the same designation in FIG. 2. The circuit of FIG. 2 
represents the best embodiment of the invention from an economic 
standpoint, in that the diode bridge used in the prior art outgoing trunk 
circuit of FIG. 1 is, in accordance with the principles of the invention, 
retained in FIG. 2 for the additional purpose of isolating midpoint 
capacitor C2 from the consequences of signaling battery polarity reversal 
on the T2 and R2 conductors. As seen from FIG. 2, facility-side midpoint 
capacitor C2 has been placed across the output nodes 3 and 4 of the diode 
bridge in parallel with the loop-forward relay L1. Additionally, to ensure 
the normal operation of outgoing trunk circuit 102 with the structural 
alterations involving capacitor C2, make contact A1-1 of relay A1 has also 
been placed within the diode bridge in series with the coil of relay L1. 
Since the trunk circuit may, in situations such as coin control on 
paystation calls, be required to pass voiceband multifrequency signals 
back via the repeat coil 105 into office 102 (FIG. 1) even when relay A1 
is released, contact A1-1 of relay A1 is also placed within the diode 
bridge comprised of diodes 107, 108, 109 and 110 of FIG. 2. Resistor R1 of 
FIG. 2 is placed in parallel with contact A1-1 to maintain a low impedance 
path through the diode bridge to allow for passage of signals, such as 
voiceband coin control signals, through midpoint capacitor C2 even when 
relay A1 of trunk circuit 102 is released. R1 is selected to have a 
resistance value low enough to provide for appropriate diode forward bias, 
yet high enough to prevent false operation of relay A2 of incoming trunk 
circuit 103 of FIG. 1. For example, with typical central office battery of 
-48 volts supplied via supervisory relay A2 (FIG. 1), a suitable value for 
resistor R1 has been found to be 47,000 ohms. 
It will be apparent to those skilled in the telephony art that, with the 
arrangement of trunk circuit 102 depicted in FIG. 2, facility-side 
midpoint capacitor C2 is effectively prevented from charging with a 
polarity capable of falsely reoperating relay A2 of incoming trunk circuit 
103 of FIG. 1 upon the termination of the trunk talk state where the 
release of relay B (FIG. 1) reverses the polarity of conductors T2 and R2. 
At the same time, the arrangement of FIG. 2 provides for normal signaling 
and voice transmission operation of outgoing trunk circuit 102. Hence, the 
prior art problem of "flipping" is overcome using the illustrative 
embodiment of FIG. 2 at the additional cost over the prior art of a single 
resistor. 
It should be noted that the invention described herein has been illustrated 
with reference to a particular embodiment. It is to be understood that 
many details used to facilitate the description of said embodiment were 
chosen for convenience only and without limitation on the scope of the 
invention. Other embodiments may be devised by those skilled in the art 
without departing from the scope and spirit of the invention. For example, 
a separate and distinct diode bridge could be placed around facility-side 
midpoint capacitor C2 of FIG. 1 in a "brute force" approach to 
implementing the principles of the flip-preventing invention. Accordingly, 
the invention is intended to be limited only by the scope and spirit of 
the appended claims.