Method and apparatus for checking connection paths in a telephone exchange

Apparatus for checking electrical connections switched through an automatic switching machine. Idle circuits (P1, Pn) e.g. subscriber lines and outside lines, are connected in turn through the switching machine to the terminating set (Tr) thereof. Tone is applied via the normal tone-feed circuits (GT C1 to Cn, TL1 to TLn) to the idle line and the tone is detected at the terminating set. All lines at the terminating set are scanned (Y1 to Yn) in turn to check for any possible cross-connection and then the next idle circuit is tested. The apparatus forms an integral part of the switching machine.

The invention relates to the checking of the circuits for interconnecting 
subscriber lines and outside lines in an automatic telephone exchange in 
order to verify the state of a connection, and to make sure that the 
connection has good continuity and good isolation. Preferably the 
invention relates to an arrangement which also determines whether a 
connection is open circuit or whether it is short circuit with possible 
resulting cross-connection of a plurality of subscriber lines. The 
invention is intended principally for cyclic testing or routining of test 
connections set up over idle circuits of the exchange. Such testing may be 
performed on a continuous basis or during specific test periods. 
In known exchanges, subscriber circuits are usually tested during 
maintenance by means of test devices which are external to the switching 
machine proper, e.g. by routiners or from test desks. 
Preferred embodiments of the present invention provide for the testing of 
the interconnection circuits of a telephone exchange by means of a 
checking circuit which is integrated in the switching machine of the 
exchange. 
The present invention provides a checking apparatus for testing the state 
of electrical continuity of a connection chain in a telephone exchange, 
the apparatus being integrated in the switching machine of the exchange 
and being arranged to perform checks continuously on connections by 
applying an alternating signal to one end of the connection and picking up 
this signal at a measuring circuit placed at the end of the connection, 
the apparatus including a tone-feed circuit which is constituted by the 
tone circuits of the switching machine situated at the subscriber line and 
outside line equipments thereof, and in that each tested connection chain 
comprises a tone generator, at least one interrupter switch for giving a 
cadence to a tone in normal telephone operation, a coupling transformer 
connected to at least one primary switching matrix, a matrix cross-point 
feed transformer, and the measuring circuit which is connected to a 
secondary winding of the feed transformer. 
The invention also provides a method of checking the state of electrical 
continuity of a connection chain in an automatic telephone exchange; the 
method comprising using the tone-feed circuits of the exchange for 
applying test signals to one end of a connection chain under test in 
picking up these signals at the other end of the chain and in comparing 
the levels of the picked up signals with reference levels.

FIG. 1 shows the tone-feed circuit which is used to generate the various 
tones which are applied to a telephone line by an automatic telephone 
exchange. It includes a 575 Hz tone generator Gt connected to terminals T0 
and RT0 of a subscriber line or an outside line terminal Ta, and switches 
C1 to Cn, e.g. CMOS switches. Each of the switches C1 to Cn has a control 
input g1 to gn which receives periodic data from the exchange processor 
for interrupting the tone. When tone current is allowed to flow it is 
applied to coupling transformers TL1 to TLn. These transformers have three 
windings: 
a first winding 1 connected to a subscriber station P1 to Pn; 
a second winding 2 connected to the terminals T0 and RT0 via one of the 
switches C1 to Cn; and 
a third winding 3 connected to two horizontals of a primary switching 
matrix M1 which includes horizontals Hp1 to Hpn and verticals Vp1 to Vpn. 
The tone-feed circuit is associated with a primary terminating set Tr 
constituted, for example, by the terminating board of the switching 
machine. The terminating set Tr includes three-winding transformers SD1 to 
SDn whose first and second windings 1 and 2 have a common point connected 
to positive battery with their opposite ends being connected to terminals 
SA and SK, themselves connected to respective verticals of the matrix M1. 
The third windings 3 of the transformers SD1 to SDn are connected to a 
measuring circuit CM via inlets Y1 to Yn of the said measuring circuit 
selected by a multiplexer MX (see FIG. 3). The measuring circuit has a 
high threshold output SH and a low threshold output SB as will be seen 
with reference to FIG. 3. 
FIG. 2 is a diagram similar to that of FIG. 1, but in this case it relates 
to a two-stage connection. The tone circuit GT, C1 to Cn, TL1 to TLn are 
connected to the horizontals HP1 to HPn of a primary matrix M1 whose 
verticals VP1 to VPn are connected to the verticals VS1 to VSn of a 
secondary matrix M2 of a secondary terminating set TS. The horizontals SH1 
to SHn of the matrix M2 are connected to the first and second windings 1 
and 2 of the transformer TD1 to TDn which feed them via the common points 
connected to the positive battery. The third windings 3 of the these 
transformers are connected to the inlets Y1 to Yn of the measuring circuit 
CM. These transformers TD, while including the same windings as the 
transformers SD, further include a fourth winding 4 connected to the 
speech pair ab. 
FIG. 3 is a circuit diagram of the measuring circuit CM. The third winding 
3 of a transformer SD (TD) of the terminating set Tr of FIG. 1 or of FIG. 
2 has one end connected to an intermediate bias potential by a potential 
divider comprising a resistor R0, a Zener diode Z0 and a capacitor C0, and 
its other end connected to one inlet Y1 of the measuring circuit which 
includes n inlets Y1 to Yn which are constituted by the inlets of an 
analogue multiplexer MX which also includes addressing inlets A0, A1, A2 
and E connected to the exchange processor. The outlet Z of the multiplexer 
MX is connected to one input of an operational amplifier A1 whose output 
is connected to a point B which is connected to the first inputs of two 
comparators A2 and A3. 
The comparators A2 and A3 have respective second inputs connected to points 
C and D at intermediate potentials between the positive and negative 
potentials of the battery. The potentials of the points C and D are fixed 
by a chain of resistors R1, R2, R3 and R4 with the point C defining the 
high threshold and the point D the low threshold. 
The output of the comparator A2 is applied to a retriggerable monostable 
flip-flop B1 whose output is the output SH, and likewise the output of the 
comparator A3 is applied to a retriggerable monostable flip-flop B2 whose 
output is the output SB. The flip-flops B1 and B2 are constituted by 
monostables having a period T3 so chosen as to maintain the triggered 
position throughout the period of application of the current which sets up 
the measuring circuit. The waveform diagram of the measuring circuit is 
given in FIG. 4 where the voltages at the points Y1 to Yn, A,B, SH and SB 
are shown. 
Voltages n.sub.1 , n.sub.2 and n.sub.3 at the point A represent: 
n.sub.1 high level of a good connection; 
n.sub.2 short duration interference signal, not measured; and 
n.sub.3 low-level (as in the case of a short circuit, and interference 
signal or cross-talk). 
The outputs SH and SB are read at a time T1 after a connection to be 
measured has been set up in order to eliminate interference and transient 
effects which occur on switching current. 
T2 shows the delay between the application of a signal to the inlet of the 
measuring circuit and the arrival of a signal at one of its outputs SH or 
SB. 
T3 shows the delay caused by the monostable flip-flop B1 or B2. T1 is 
chosen to be greater than T3. 
The low threshold SB is fixed as a function of the interference signals 
encountered on the lines of the switching machine. 
The operation of the measuring circuit of FIG. 3 is as follows: 
The measuring signal is picked up on a winding 3 of a transformer SD or TD 
and applied to one of the inlets Y1 to Yn of the multiplexer MX. The 
channel to be measured is selected by means of the address inputs of the 
multiplexer MX. The signal A (see FIG. 4) at the output of the multiplexer 
MX is initially amplified and rectified by the operational amplifier A1 
(half-wave rectification at B, FIG. 4). The rectified current B is applied 
to the comparators A2 and A3. The comparator A2 delivers a logic level 1 
signal when the peak level of the signal at B is greater than the 
reference level C (high threshold). The comparator A3 delivers a logic 
level 1 signal when the peak level at B is greater than the reference 
level D (low threshold). Each of the comparators A2 and A3 is followed by 
a retriggerable monostable flip-flop B1 or B2 to obtain a constant level 
at SH or SB once the corresponding threshold C or D has been exceeded. The 
operation is made clear by the waveforms of FIG. 4. The data collected at 
the outputs SH and SB is processed by the exchange processor. 
The operation of a checking apparatus described above is as follows: 
FIG. 1 case 
The circuit through the switch C1 is closed, for example, and the tone 
generator GT injects a signal at a frequency of 575 Hz from the subscriber 
line or outside line to the terminating set Tr via the circuit T0, C1, 
HP1, VP1, SD1, and Y1. 
The signal is picked up by the third winding 3 of the transformer SD1 which 
applies it to the input Y1 of the measuring circuit which delivers two 
output signals SH and SB at high and low levels respectively. If the 
connection is good, the signal picked at Y1 is good and the output SH of 
the measuring circuit will be at level 1. If, on the contrary, the 
connection is bad, the output SH will not give a signal; which may mean 
that the line is open-circuit or that the line is crossed. The measuring 
circuit successively checks the level of the signals on the transformers 
SD2 to SDn, i.e. the signal picked up on the inlets Y2 to Yn of the 
measuring circuit CM. If the measuring circuit supplies a low level signal 
SB=1, e.g. on the inlet Y3, this indicates a cross connection between 
circuit n.degree. 1 and circuit n.degree. 3. If SB=0 for all the inlets Y2 
to Yn, and SH=0 this indicates that circuit n.degree. 1 is open circuit. 
The procedure is repeated with the following line by establishing the 
following circuit through the switch C2: 
T0, C2, HP2, VP1, VS1, HS2, TD2, Y2 
and the signals picked up at the inlets Y2 to Yn and Y1 are checked as 
before. 
The following lines are tested in an analogous manner. When the cycle is 
over a new cycle begins. 
In the FIG. 2 case the apparatus operates in the same way except that the 
established connections are as follows: 
T0, C1, HP1, VP1, VS1, HS1, T01, Y1 for the first line, 
T0, C2, HP2, VP1, VS1, HS2, TD2, Y2 for the second line, and 
so on for the following lines. 
The advantages of employing the apparatus in accordance with the invention 
are the folowing: 
The device enables proper operation of the switching machine to be detected 
by detecting whether a connection circuit is faulty, in particular whether 
it is short-circuited or whether a cross-point is faulty. The testing of 
circuits is performed continuously or in accordance with the determined 
program on those circuits and lines which are free by means of a device 
which is integral with the switching machine, the circuits and lines which 
are engaged being skipped. 
The measuring apparatus is common to a plurality of subscribers.