Note: Descriptions are shown in the official language in which they were submitted.
liS~33
Cross Reference To Related Application
The disclosure of the present invention includes a description of
a novel "hold control" circuit that is disclosed and claimed in a separate
copending Canadian application for "HOLD CONTROL FOR A KEY TELEPHONE
SYSTEM", SN 308,492, filed July 31, 1978 by Harry R. Rasmussen, one of the
joint inventors in the present application.
Background
In general the present invention relates to telephone station
equipment, and more particularly to multiple telephone set installations
serviced by multiple telephone lines from a central office.
Many telephone customers, especially businesses, need a telephone
installation in which calls can be placed or received at any one of a number
of telephone sets, and over any one of a plurality of available telephone
lines from a central office. Such installations are in general available
and are called key telephone systems (or KTS). One of the most commonly
used systems of this type is intended for meeting the needs of customers
that require a relatively large number of telephone lines, such as five or
more lines, and for these customers the cost of the service is not unreas-
onable on a per line basis. However, there is a need for a less complex,
lower cost key telephone system capable of efficiently meeting the needs of
customers that have phone traffic requiring less than five lines but more
than one line. For example, it is believed that there are many customers,
in small businesses for example, that have a definite need for two line, key
telephone service, but cannot justify the cost of such service because most
available key telephone systems are cost efficient when used with five or
more telephone lines.
One of the reasons for the prohibitively high cost of key telephone
systems when used for two line installations is that the system includes a
central control unit that must be set up and stored at a location remote
from the telephone sets. The central control unit in turn involves a sub-
l.~iiS83
stantial installation cost, both for the control unit itself and also forthe auxiliary wiring that must be strung between the control unit and each
of the multiple telephone sets. Also, the telephone sets themselves must
be specially made to accommodate the maximum number of key functions for
which the system has been designed. For example, a standard KTS desk set
may provide for five or more line select buttons, even though the customer
is only using the system for two lines and thus only two of the available
buttons are functional.
- la -
83
Although other telephone systems are available for
providing a form of two line s-ervice on a relatively low cost
basis, none of these existing two line systems have the capa-
bility or offer the convenience and flexibility of key telephone
systems.
Accordingly, it is an object of the invention to pro-
vide a novel low cost key telephone system that is particularly
tailored for cus-tomers needing less than five telephone lines
from a central office.
Another object is to provide such a key telephone
system on which the control circuitry, visual indicator lights
and manual control buttons for each telephone station are
capable of being packaged in a small, compact unit that is
readily attachahle to an existing, standard telephone desk set
of the type intended for single line use.
Another ob;ect is to provide such a key telephone
system having a conference call capability.
Another object is to provide a key telephone system
that eliminates the need for a central control unit and the
attendant disadvantages thereof including the loss of office
or storage space otherwis-e needed for the central control unit
and the high labor cost entailed in installing wiring between
the control unit and the various telephone sets.
A further object is to provide a key telephone
system which uses supervisory signals applied to the telephone
lines by the central office to display the operating conditions
of the lines at each set.
Additionally, it is an object of the invention to
provide a key telephone system that has one or more of the
foregoing advantages and is- capable of being manufactured at a
C - 2 -
t 83
relatively low cost per unit, and is durable, operationally
reliable, easily serviceable, and compatible with existing
central office equipment and signalling.
Summary
In accordance with the present invention there is
provided a control unit for each of a plurality of single-line
telephone sets for converting such sets into a multiple-line
key telephone system which has the capability of indicating
on-hook, off-hook and hold conditions on each of a plurality
of telephone lines connected to the system, comprising first
and second line input means adapted to be separately connected
to first and second telephone lines, respectively; output means
adapted to be connected to a telephone set; first and second
hold control means separately connected to said first and
second line input means, respectively, for selectively placing
the lines connected thereto in a hold condition; line condition
sensing circuit means having first and second line condition
sensing means separately coupled to said first and second line
input means, respectively, for sensing on-hook, off-hook and
hold conditions on the respective lines connected to said
first and second input means, each of said first and second
line condition sensing means having anoutput at which a control
signal is produced for indicating whether the associated tele-
phone line is in an on-hook condition, an off-hook condition
or a hold condition, and first and second electrically operated
indicating means, said first indicating means being responsive
to said control signal at the output of said first line condi-
tion sensing means for indicating on-hook, off-hook and hold
conditions on the telephone line connected to said first line
input means, and said second indicating means being responsive
- 2a -
111~583
to the control signal at the output of said second line condi-
tion s-ensing means for indicating on-hook, off-hook and hold
conditions on a telephone line connected to said second line
input means; line selection switching circuit means having
first and second line switching de~ices each having first and
second states, said first line switching device when in its
second state connecting said firs-t line input means to said
output means and when in its first state disconnecting said
first line input means from said output means, and said second
line switching device when in its second state connecting said
second line input means to said output means and when in its
first state disconnecting s-aid second line input means from
said output means, said first and second line switching devices
normally assuming their first states, first and second manually
operable line selector switches-, and first and second latching
means each having an output and a plurality of inputs, the out-
put of said first latching means controlling the state of said
first line switching device and the inputs of said first
latching means being responsive to operation of said first
manually operable line selector switch when said first line
condition sensing means senses an off-hook condition on the
line connected to said first input means to latch said first
line switching device in its second state so long as said
off-hook condition continues to be sensed by said first line
condition sensing means, the output of said second latching
means controlling the state of said second line switching device
and the inputs of s-aid second latching means being responsive
to the operation of said second manually operable selector
switch when said second line condition sensing means senses
an off-hook condition on t~e line connected to said second line
- 2b -
C
11115~3
input means to latch said second line switching device in its
second state so long as said second line condition sensing
means continues to sense such off-hook condition.
The key telephone system of the invention is intended
for use with at least two telephone lines that extend between
a central office and two or more telephone stations. Each
telephone station is equipped with a telephone set, which may
be a standard s-et intended ~or a s-ingle line installation.
~ plurality of control units are provided, one at each of the
telephone stations, to convert the single line sets into a
multiple line key telephone system. The telephone lines are
connected to separate inputs provided on each control unit
at each station so that telephone calls can be received or
initiated over any one of the multiple lines using the tele-
phone set at that station.
- 2c -
Each control unit includes line switching means, hold control means, line
condition sensing means, and visual indicating means. The line switching means is
responsive to manually-actuated line select switches to selectively couple one of the
central office lines to the telephone set for receiving or initiating a call thereover. The
hold control means, which is also responsive to a manually-actuated hold select switch,
provides for establishing a hold condition on any one of the lines over which a telephonic
connection has been made through the central office. The line condition sensing means
and visual indicating means of each control unit provide for sensing the status of each
line, whether idle (on hook), busy (off hook), ringing or holding.
Thus, the key telephone system of the present invention is seen to provide
many if not all of the features and advantages of a conventional key telephone system,
without requiring the expense of auxiliary wiring between the various stations, without
needin~ the storage space for a central control unit and without involving the complexity
and cost inefficiency of the latter (at least for installations needing less than five lines).
These and further features, objects and advantages of the invention will
become apparent to those skilled in the art from the following detailed description and
appended drawings.
Brief Description of the Drawings
Figure 1 is a generalized block diagram of the key telephone system of the
20 invention.
Figure 2 is a more detailed block diagram of one of the plurality of control
units that is provided at each telephone station in the system of Figure 1.
Figure 3 is a detailed block and schematic diagram of the control unit shown
in Figure 2 in accordance with one preferred embodiment of the invention.
Detailed Description
The key telephone system (KTS) 11 in which the invention is used is shown in
Figure 1 for a two line installation. First and second telephone lines, Ll and L2, extend
from a central office 12 to a customer's installation which includes KTS 11. Lines Ll and
L2 are standard, two conductor lines, each including a tip and a ring conductor, which
30 carry both central office supervisory signals and telephonic voice signals between central
office 12 and a customer's telephone set, installed at his business or residence.
!?s3
KTS 11 encompasses a plurality of telephone stations #l-N, each of which is
equipped with a telephone station set and a KTS control unit, such as illustrated by
telephone set 13 and control unit 14 for station #1. Low voltage DC power is supplied to
control unit 14 of station #1 and to the corresponding control units of the remaining
stations #2-N by means of either a common power supply 15 as shown in Figure 1, or by a
plurality of individual power supplies, one for each station. The illustrated power supply
15 operates from a source of dc at 8-16 volts which is applied to input 16. At the output of
supply 15 a low supply voltage +Ycc appears on line 17, while line 18 is reference ground.
Each of the control units, such as unit 14 for station 1, includes a control
circuit (Figures 2 and 3), a set of line select switches Sl and S2 (one for each of lines L1
and L2), a hold select switch S3, and a set of line condition indicator lights ILl and IL2
(one for each of lines Ll and L2). Switches Sl, S2 and S3 are pushbutton switches of the
momentary contact type. Switches Sl and S2, when separately depressed, cause theassociated one of lines Ll and L2 to be coupled to the telephone set 13. Switch S3, when
depressed, places the line over which a telephonic communication has been established in
a hold condition. Indicator lights ILl and IL2 at each control unit visually display the
instantaneous condition of the associated line. The various telephone line conditions are:
on-hook (sometimes called idle), off-hook (sometimes called busy), holding, and ringing.
Each of lines Ll and L2 includes a tip conductor indicated by a T and a ring
conductor indicated by an R. The tip and ring conductors of lines Ll and L2 are connected
in parallel to the control units of all of the available stations #l-N. Thus, as illustrated in
Figure 1, each station receives as inputs, to the station control unit, the tip and ring
conductors of every one of the available lines from central office 12.
The outputs of each control unit 14 include common output tip and ring
conductors, designated as r and R', respectively, which are connected to the tip and ring
terminals of the station's telephone set 13. Output tip and ring conductors T' and R' are
selectively connected to the tip and ring conductors of either one or both of lines Ll and
L2 by the station control unit 14 as described more fully below. The outputs of each
station control unit 14 also include a pair of bell ringing conductors represented as Bl and
B2. Conductors Bl and B2 extend from the control unit 14 to the station's telephone set 13
and provide for energizing the bell or other audible signalling device in the telephone set
in response to a ringing signal on either of lines Ll and L2.
~lliS83
The telephone set 13 at each of the stations may be a standard
single line telephone set. For example, a type 500 set (conventional dial-
ing) or a type 2500 set (Touch-Tone dialing - A service mark of AT~T) are
suitable.
The circuitry, indicator lights and selector switches for each
control unit 14 are constructed and assembled so as to form a small, compact
unit that can be mounted directly to the case of the telephone set 13. Al-
ternatively, the control unit may be packaged in a housing mounted adjacent
to but separate from the telephone set. The former packaging and mounting
of switches, Sl, S2, S3, indicator lights ILl and IL2, and the associated
circuitry are described in United States Patent No. 4,061,8~8 for CONTROL
UNIT MOUNTING AND INTERCONNECTING APPARATUS FOR TELEPHONE SETS, by Harry R.
Rasmussen, issued December 6, 1977.
Station Control Unit
The control units for stations #l-N are identical, and thus only
one control unit, namely unit 14, will be shown and described in detail.
With reference to Figure 2, control unit 14 includes a set of line relays 24
and 26 that serve as switching devices for selectively connecting the tip(T)
and ring(R) conductors of lines Ll and L2 to the common output tip(T') and
ring(Rt) conductors which extend to the telephone set 13.
A bell relay 28 is also connected to the tip and ring conductors
of Ll and L2 and serves as a switching device for selectively connecting a
ringing signal of an incoming call, whether on line Ll or L2, to the common
bell ringing conductors Bl and B2.
Lines Ll and L2 are also separately and respectively connected to
an Ll interface circuit 30 and an L2 interface circuit 40. Circuits 30 and
40 include electro-optical isolating devices that enable lines Ll and L2 to
be coupled to line condition sensing circuitry and hold signal generating
circuitry of control unit 14 without direct connection of such circuitry to
the telephone lines.
83
Similarly, separate sensing circuits 50 and 60 are provided,
one for each of lines Ll and L2. Sensing circuit 50 is connected to
interface circuit 30 by means of connection 38 to receive signals
developed by interface circuit 30 that indicate the operating condi-
tion, whether on-hook, off-hook, holding or ringing, of line Ll.
Sensing circuit 50 discriminates between the various signal condi-
tions and develops certain control signals that govern the operation
of other circuitry in control unit 14. Sensing circuit 60 is coupled
to interface circuit 40 over connection 42 and functions in the same
manner as
- 5a -
5~3
sensing circuit 50 except circuit 60 receives and interprets the line condition signals
appearing on L2.
Indicator lights ILl and IL2 are connected to outputs 52 and 64 of sensing
circuits 50 and 60, respectively, for visually displaying the operating condition of the
respective lines as sensed by circuits 30 and 40. The indicator lights ILl and IL2 operate
independently, each displaying the instantaneous operating condition of the associated one
of lines Ll and L2.
A line select circuit 80 of control unit 14 is responsive to outputs 52 and 64
of sensing circuits 40 and 60 and to line select switches Sl and S2 and to hold select switch
S3 for controlling the operation of line relays 24 and 26. Also, circuit 80 coordinates the
operation of line relays 24 and 26 for automatically dropping one of the lines when the
other line is to be picked up at the same station, and for concurrently operating both
relays 24 and 26 when a three-point conference call is to be established as more fully
described below.
The hold control circuitry is distributed among circuits 30, 40, 50, 60 and 80.
The hold condition is initiated by depressing switch S3 which causes line select circuit 80
to produce control signals on lines 86 and 88 which in turn cause sensing circuits 50 and 60
and interface circuits 30 and 40 to latch one of lines Ll and L2 in a hold condition.
Connections 59 and 61 between Sl, S2 and circuits 50, 60, respectively, insure that only the
20 line then connected to the telephone set, is placed on hold in response to actuation of the
single hold select switch S3.
During a hold condition on either one of lines Ll or L2, the associated
interface circuit 30 or 40 places a line terminating impedance across the applicable line so
as to simulate the internal impedance of a telephone set and thereby cause the central
office to hold the line in communication with the remote party. At the same time that
the line terminating impedance is connected across the line, a hold indicating signal is
applied to the line over connections 94 and 96 in response to a flash generator 90.
A bell transfer circuit 70 having a connection 71 to sensing circuit 60
operates bell relay 28 in a manner that will be described more fully below to insure that
30 the ringing signal is applied to the bell of the station's telephone set regardless of which
line receives an incoming call.
C~3
With reference to Figure 3, the circuit elements of control unit 14 and in
particular, the elements of the hold control circuitry of the present invention, are
described in greater detail as follows.
Line Relays
Line relays 24 and 26 include separate relay coils 24a and 26a schematically
located within the dotted line that circumscribes line select circuit 80 (Figure 3). A set of
normally closed relay contacts 24b and 24c of relay 24 connect the tip and ring conductors
of line Ll to the output tip and ring T' and R' of control unit 14. Although contacts 24b and
24c of relay 24 are normally closed, the coil 24a is normally energized as will be described
10 more fully herein so that functionally, contacts 24b and 24c are normally open and are
closed only when coil 24a is selectively deenergized in order to couple line Ll to T' and R'
and thus to the telephone set 13 (Figure 1).
Relay 26 includes a set of normally open contacts 26b and 26c separately
connected in series between the tip and ring conductors of line L2 and the output tip and
ring conductors T' and R'. Unlike relay 24, the coil 26a of relay 26 is normally
unenergized and is selectively energized by circuit 46 when line L2 is to be connected to
T' and R' and thus to the telephone set 13. The connection and operation of relays 24 and
26 in this manner insures that at least one telephone line will be connected to the
telephone set 13 in the event of a power failure. In such case both relay coils 24a and 24b
20 will be forced to their deenergized state and line Ll will in that case be connected to the
telephone set 13 through contacts 24b and 24c.
Bell Transfer Relay and Circuit
Bell transfer relay 28 includes a relay coil 28a located in bell transfer
circuit 70 (Figure 3) for selectively operating two sets of contacts including a normally
closed set of contacts 28b and 28c, and a normally open set of contacts 28d and 28e.
Contacts 28b and ~8c are separately and serially connected between the tip and ring
conductors of line Ll and the output bell ringing conductors Bl and B2 which extend to the
bell ringing device of the telephone set 13. Similarly, normally open contacts 28b and 28e
are separately serially connected between the tip and ring conductors of line L2 and the
30 output bell ringing conductors Bl and B2. An incoming ringing signal on line Ll will
automatically appear on output conductors Bl and B2 to cause the telephone set 13 at
station 14 to ring.
1111583
Bell transfer circuit 70 includes an input 71 connected to the L2 sensing
circuit 60, a DC blocking capacitor 782 serially connected between input 71 and the
junction of the anode of a blocking diode 783 and the cathode of a clamping diode 781.
The anode of diode 781 is connected to ground and the cathode of diode 783 is connected
through a parallel RC network of resistor 784 and capacitor 785 to ground. The cathode
of diode 783 is also connected to pins 2 and 6 of a linear integrated circuit 786 (described
below). The output of circuit 786 is provided at pin 7 and is connected through coil 28a to
+Vcc and a diode 787 is connected in shunt around coil 28a.
Circuit 786 is a commercially available device manufactured and sold by a
10 number of companies including National Semiconductor Corporation of Santa Clara,
California, and Raytheon Corporation of Boston, Massachusetts, and is commonly
designated in the electronic industry as a 555 timer. It is a multipurpose timer circuit
that can be adapted for performing a wide variety of timing and control functions
depending upon the external circuitry to which it is connected. The operational
characteristics of the 555 timer, in general, and its particular use as circuit 786 are
described below. The particular functioning of the other 555 timer circuits, used
throughout control unit 14, will be covered hereinafter.
Supply voltage for the 555 timer is applied between pin 1 and pin 8, with pin 1
being tied to ground and pin 8 to +Vcc. The active inputs and outputs of the timer, as used
20 in the circuitry disclosed herein, are: a first output pin 3, a second output pin 7 (which is
operationally similar to pin 3), a first input pin 2 and a second input pin 6. A reset input
pin 4 is also shown (connected to +Vcc) but the reset function is not used in the present
circuitry and the connection to +Vcc is merely to prevent false triggering of the reset
function.
Details of the construction and operation of 555 timer are available from
the abov~mentioned manufacturers. For the present disclosure, a brief, generalized
description of the construction of the 555 timer will suffice. It is comprised of first and
second comparator stages, a bistable flip-flop stage and a voltage divider impedance
network. The voltage divider impedance network is connected between pin 8 and ground
30 and thus serves to divide the supply voltage +Vcc into predetermined fractions, namely
1/3 +Vcc and 2/3 +Vcc. The first and second comparators are each connected to compare
the input voltage applied to an associated one of the first and second input pins 2 and 6,
S83
respectively, with the fractional 1/3 and 2/3 +Vcc voltage levels developed by the divider
network. The input impedances at the input pins 2 and 6 are characteristically very high.
The outputs of the first and second comparators are connected so as to set and reset,
respectively, the flip-flop stage. The flip-flop stage is normally reset and in this state
output pin 3 is high (at or near +Vcc) and output pin 7 is an open circuit. Pin 7 is
connected to the collector of a transistor which has its emitter grounded, and is switched
between a nonconducting state (when the flip-flop stage is reset) and a conducting state
(when the flip-flop stage is set).
When the input voltage at pin 2 rises above 1/3 +Vcc, the output of the first
comparator enables the flip-flop stage to be switched to its set state, but only after the
input voltage at pin 6 subsequently rises to 2/3 +Vcc. The flip-flop stage will remain in
the set state until both the voltage at pin 2 drops below 1/3 +Vcc and the voltage at pin 6
drops below 2/3 +Vcc. Unless both these voltage conditions are met, the flip-flop stage
will remain in its set state. In the set state, output pin 3 is low (grounded) and output pin
7 is also low (grounded through the collector-emitter path of the above-mentioned
transistor which has now been switched to its conducting state). The required concurrence
of voltage conditions at input pins 2 and 6 provides the 555 timer with an electronic AND
logic function.
Once the flip-flop stage is in its set state, it may be latched in such state bymaintaining the voltage at either one of pins 2 and 6 above its threshold switching level,
respectively, 1/3 +Vcc and 2/3 +Vcc, even though the voltage at the remaining input pin
goes low. Thus the 555 timer has the capability of performing as an electronic latch.
Additionally, the input pins 2 and 6 are adapted to be connected to external
resistive-capacitive delay networks in a manner that conditions the 555 timer to function
as a timing circuit. The timing intervals of the circuit are determined by the values of
the externally connected resistive and capacitive elements. When used as a timing circuit
input pins 2 and 6 may be connected together to receive a common input voltage, or they
may be connected separately in order to tailor the timing function to each partic~ar
application.
In still another use of the 555 timel, it may be operated as an inverter. In
such case, the input pins 2 and 6 are tied together for receiving an input voltage. Output
pin 3 swings high when the voltage at input pins 2 and 6 is low (below 1/3 +Vcc) and output
pin 3 swings low when the voltage at input pins 2 and 6 is high (above 2/3 +Vcc).
_g_
Accordingly, it will be appreciated that the 555 timer is a versatile,
integrated circuit capable of being connected so as to provide one or more of the
functions of an AND logic circuit, a latching circuit, a timing circuit and an inverting
circuit. Because its most common use is for timing, it is usually called a timer circuit and
is referred to as such herein.
For the purpose of illustrating control unit 14, timer circuit 786 and other
identical integrated circuits used throughout the control unit are shown as a single
integrated circuit unit known by the designation 555. In the actual manufacture of control
unit 14, it has been found preferable to use a dual integrated timer circuit known in the
industry as a 556 which packages two integrated circuits, each of which is functionally
identical to a 555 timer, into a single integrated component.
The 555 timer used for circuit 786 has its input pins 2 and 6 tied together to
respond to the voltage across the RC network of resistor 784 and capacitor 785. As the
voltage at pin 2 rises from ground potential in response to a ringing signal detected by
sensing circuit 60 as described herein, pin 2 becomes enabled at 1/3 +Vcc and thereafter
the flip-flop stage of circuit 786 switches when pin 6 reaches 2/3 +Vcc. The output pin 7
thereupon switches to ground and causes relay coil 28a to be energized. When the voltage
at pins 2 and 6 starts to drop from +Vcc toward ground, circuit 786 does not switch until
the voltage at pin 2 falls below 1/3 ~Vcc. The RC network of resistor 784 and capacitor
785 hold the voltage at pin 2 above 1/3 +Vcc during the silent intervals of a ringing signal
and thereby maintain relay coil 28a energized from one burst of AC ringing to the next.
Line Interface Circuits
(Including the Line Terminating Impedance and the
Hold Signal Generator of the Hold Control Circuitry)
Line interface circuits 30 and 40 are identical and therefore only circuit 30
for line 1 will be described in detail. The tip and ring conductors of line Ll are connected
directly to input terminals 302 and 304 so that the signal condition on line Ll is
continuously applied to circuit 30. The positive side of the tip and ring conductors (usually
tip is positive) is connected to terminal 302 since circuit 30 is polarity sensitive.
Alternatively, a full-wave rectification bridge may be connected and poled between the
tip and ring conductors and input terminals 302 and 304 to insure a proper application of
polarity to the input of circuit 30 regardless of the tip and ring polarity. Conneeted
across input terminals 302 and 304 is a first network 306 including an electro-optical
--10--
isolator that detects the voltage on Ll without interfering with the normal communication
of signals between the telephone set and the central office. Connected in parallel with
network 306 across input terminals 302 and 304 is a second network 308, constructed in
accordance with the present invention, that functions during the hold condition to apply a
line terminating impedance across Ll (that simulates an off-hook condition) and at the
same time apply a hold indicating signal across line Ll, again without interfering with the
normal signalling between the telephone line and its associated central office.
Network 306 includes a diode 310, a low impedance resistor 312 of 430 ohms,
integrated Darlington paired transistors 314 (although only one transistor symbol is shown,
the accompanying letter "D" designates the fact that two Darlington connected transistors
are used in an integrated package), a zener diode 316, a base bias resistor 318 of 470 K
ohms, an integrated electro-optical isolator 320, another resistor 322 of 2.2 K ohms and a
current limiting diode 324. Diode 310 and resistor 312 are connected in series to pass
positive current from the tip conductor of line Ll to a junction between the collectors of
Darlington transistors 314 and the cathode of zener diode 316. Resistor 312 is of relatively
low value so as to enable zener diode 316 to respond to the line voltage. Diode 324 limits
the amount of current flowing in the circuit loop formed by the connection of network 306
across the terminals 302 and 304 to a preselected maximum current. In this embodiment,
diode 324 limits the current to 0.5 milliamperes to prevent excessive current drain on the
central office voltage source that supplies line Ll.
Darlington transistors 314, zener diode 316, electro-optical isolator 320 and
the associated resistors 318 and 322 serve to sense the voltage on line Ll and cause an
output signal to be produced on output connection 38 that reflects the instantaneous signal
condition on line Ll. For this purpose the collector emitter paths of Darlington transistors
314 are connected in series with a light emitting diode 320a of isolator 320 so that when
the Darlington transistors 314 are conducting, diode 320a is energized and emits light that
impinges on the input base electrode of Darl;ngton paired phototransistors 320b and 32ûc
of isolator 320. The emitter of transistor 320c is connected to ground and its collector is
connected to the junction between output connection 38 and resistor 322. The collector of
transistor 320b is connected to ~Vcc and to the opposite end of resistor 322 from the
collector of transistor 320c. When transistors 320b and 320c are conducting as a result of
the light emitted by diode 320a, transistor 320c clamps the voltage on connection 38 to
ground, and when these transistors are nonconducting as a result of diode 320a being
unenergized, then the potential on connection 38 rises to +Vcc. Thus, for an o~hook
condition, the voltage on connection 38 is low and for on off-hook condition it is high.
Switching of isolator 320 is controlled by Darlington transistors 314 in
response to the voltage at a junction 326 between the anode of zener diode 316 and
resistor 318, which voltage is in turn responsive to the voltage condition appearing across
the tip and ring conductors of line Ll.
As described more fully below, the voltage across line Ll changes
significantly between on-hook and off-hook conditions. This change in voltage together
10 with a careful selection of the break-down voltage for zener diode 316, serve to cause the
voltage at junction 326 to change in an abrupt, discrete manner as the line voltage swings
between the on-hook and off-hook values. The discrete change in voltage at junction 326
serves to bias Darlington transistors 314 on when Ll is in an on-hook condition, and to bias
the Darlington transistors 314 off when Ll is in an off-hook condition. A ringing signal on
line Ll, after rectification by diode 310, also causes on/off switching of Darlington
transistors 314 in synchronization with each burst of ac ringing.
The breakdown voltage for zener diode 316 should be within the range of 15-
20 volts, and a breakdown voltage of 15 volts is preferred. When the line voltage across
the tip and ring conductors of Ll is below 15 volts, reflecting an off-hook condition on the
20 line, then zener diode 316 has a very high impedance relative to resistors 312 and 318 and
thus the entire line voltage is dropped across diode 316. Under these conditions, the input
base of Darlington transistors 314 is essentially at the same potential as the output
emitter of transistors 314 and therefore Darlington transistors 314 are biased off. When
the line voltage rises above the 15 volts break-down voltage of diode 316, reflecting an on-
hook condition, the excess voltage is dropped across resistor 318. The voltage drop across
resistor 318 causes the input base of transistors 314 to swing to a positive potential and
turn transistors 314 on.
Preferably, and as illustrated here, network 308 employs a unique hold
control circuit invented by Harry R. Rasmussen, and which is disclosed and claimed in the
C C 1l n ~ n ~ , 4~L
30 above-mentioned application SN 20,78~, for "HOLD CONT~OL FOR A KEY TELEPHONE
SYSTEM". As disclosed therein, network 308 is selectively connected to the inputterminals 3û2 and 304 by a hold control relay 29 that includes a relay coil 29a and a set of
--12--
83
normally open relay contacts 29b. Coil 29a of the hold relay is schematically located
within the dotted line that circumscribes sensing circuit 34, while the normally open
contacts 29b are shown in circuit 30 and are connected in series between input terminal
302 and network 308.
At the heart of network 308 lies a set of serially-connected, like-poled zener
diodes 330 and 332 with the cathode of diode 330 disposed for connection to terminal 302
through normally open hold relay contacts 29b and the anode of diode 332 connected to
terminal 304. Connected in shunt across diode 330 is the transistor output stage of
another photo-optical isolator 334. Isolator 334 includes a light emitting diode 334a
having its anode connected to +Vcc and its cathode joined to connection 94 that extends to
flash generator 90. Light produced by diode 334a when energized impinges on the base
electrode of one of a pair of Darlington paired phototransistors 334b and 334c. In the
absence of light from diode 334a, the collector-emitter paths through transistors 334b and
334c are at a high impedance such that a virtual open circuit appears across zener diode
330. When light is produced by diode 334a in response to a signal from flash generator 90
over connector 94, transistors 334b and 334c conduct providing a short circuit path in
shunt around zene~ diode 330.
As described more fully below, the serially connected zener diodes 330 and
332 when connected across line Ll by means of contacts 29b, cause a predetermined,
zener-regulated voltage to be presented across the tip and ring conductors of line Ll so as
to simulate the connection of the internal impedance of a telephone set across the
telephone line as occurs when the telephone set is off-hook. The switching of photo-
transistors 334b and 334c by diode 334a when zener diodes 330 and 332 are connected
across line Ll causes the upper zener diode 330 to be periodically shunted in
synchronization with the pulsating signal developed by generator 90. Thus the voltage of
line Ll due to the combination of the serially connected zener diodes 330 and 332
fluctuates between the sum of the break-down voltages of both zener diodes (when the
phototransistors of isolator 334 are nonconducting), and the voltage due solely to the
lower zener diode 332 (when the phototransistors of isolator 334 are conducting).
The values of zener diodes 330 and 332 are carefully selected in accordance
with the following design constraints. The voltage fluctuation appearing across the tip
and ring conductors of line Ll caused by the periodic shunting of zener diode 330 by
generator 90 must be sufficient to produce a hold indicating signal having sufficient
amplitude to enable reliable response by the sensing circuits of the other control units in
KTS 11. Secondly, the combined break-down voltages of zener diodes 330 and 332 which
occurs when diode 330 is unshunted, must be sufficiently low so as to enable a minimum
loop current to flow in the telephone line. The minimum loop current is needed to
maintain the connection at central office to the remote party. A combined break-down
voltage of approximately 20 volts meets this constraint, and will cause, a minimum of 20
milliamperes to flow in the line. Also, when diode 330 is shunted, a minimum residual
voltage should be maintained across the line, and this is provided by selecting zener diode
332 to have Q breakdown voltage of approximately 5 volts. The resulting voltage swing of
15 volts across the tip and ring conductors of the line during the hold flash condition has
been found adequate as a reliable hold indicating signal.
Sensing Circuit
Except for the difference described immediately below, sensing circuits 50
and 60 are identical and thus only circuit 50 will be described in detail. The one
difference that does exist is the provision of bell transfer circuit 70 in association with
sensing circuit 60. Only one bell transfer circuit is used and in this instance it cooperates
with sensing circuit 60 in a manner that is described more fully below.
Now with reference to Figure 3, sensing circuit 50 is composed of a line
select enable control and indicator light driver network 540, a hold latching network 542
and a hold enable network 544. All three of these networks are jointly connected to a
control signal bus 546 which in turn is connected to output connection 38 from line
interface circuit 30.
Network 540 responds to the signal condition on line Ll as represented by the
signal on output connection 38 and, when an off-hook condition appears on telephone line
Ll, network 540 enables line select circuit 80 to respond to actuation of line select switch
Sl to thereby operate line relay 29. For this purpose, network 54~ includes a timer circuit
548 that is identical to the above-described circuit 786. Input pins 2 and 6 of circuit 548
are connected through a nonlinear RC delay network including resistor 550, capacitor 552
and diode 554 to bus 546. Resistor 550 has a value of 150 K ohms and capacitor 552 has a
value of 1.0 microfarad and together they form an RC delay network that prevents the
response of network 540 to dial pulses appearing on line 11 after line Ll has gone off-hook.
--14--
583
Diode 554 connected in shunt across resistor 550 allows circuit 548 to respond
immediately to an off-hook signal on line Ll, but delays the response at pins 2 and 6 of
circuit 548 to the low going pulses on bus 546 caused by the dialing pulses on Ll so that
circuit 548 remains in a switched condition during the dialing phase following an off-hook
signal on line Ll.
The output of circuit 548 at pin 7 is joined to connection 52a for energizing
indicator light ILl, and the output of pin 3 is joined to connection 52b for applying a line
select enable signal to line select circuit 80. In particular, connection 52a extends
through a serial connection of resistor 556 and indicator light ILl to a terminal 558 to
which +Vcc is applied. Indicator light ILl is energized when the output at pin 7 of circuit
548 is clamped to a ground potential in response to the signal applied at input pins 2 and 6.
The function of output 52b at pin 3 is described below.
Network 542 is made up of another timer circuit 560 identical to circuit 786,
and having a first and second input pins 2 and 6 connected to a nonlinear RC delay
network including resistor 562 having a value of 470 K ohms capacitor 564 of 1 microfarad
and a diode 566. Input pins 2 and 6 of circuit 560 are connected to a junction between a
resistor 562 and a capacitor 564 which are in turn respectively connected at their opposite
ends to bus 546 and to ground. Circuit 560 responds to the voltage at the junction
between resistor 562 and capacitor 564 and switches when such voltage rises to 2/3 +Vcc
as bus 546 swings high. Diode 566 is connected in shunt about resistor 562 and becomes
forward biased so as to short circuit the resistive portion of the RC network and rapidly
discharge capacitor 564 when the voltage on bus 546 swings low and drops below 1/3 +Vcc.
When the voltage on bus 546 swings high, diode 566 is reverse biased and restores the
delaying effect of resistor 562 and capacitor 564 causing a time delay in the response of
circuit S60. As described more fully herein, this time delay operates to maintain hold
relay 29 energized during the application of the hold indicating signal to Ll by network 308
and to de-energize the hold relay 29 when Ll goes to an off-hook condition.
The output of circuit 560 is taken from pin 3 which is serially connected
through a blocking diode 568 and through normally open latching contacts 29c of hold
relay 29 ~nd thence to one side of hold relay coil 29a. The opposite side of coil 29a is
joined over connection 570 to the output pin 7 of still another timer circuit 572 of hold
enable network 544. Additionally, connection 86 from line select circuit 80 is joined to
-15--
S~3
the junction of hold contacts 29c and coil 29a. A current surge limiting diode 576 is
connected in shunt about coil 29a.
As described more fully hereinafter, coil 29a is initially energized over
connection 86 from circuit 80 and is thereafter latched in an energized state during the
hold condition through a latching circuit including hold relay contacts 29c, diode 568 and
circuit 560. The connection 570 to circuit 572 functions to enable the energization of hold
relay coil 29a only under certain operating conditions of network 544, which are described
below.
Hold enable network 544 includes, in addition to linear integrated circuit
572, a nonlinear RC input circuit composed of diode 578, resistor 584 and capacitor 586,
and a second nonlinear RC network of diode 582 and resistor 588 and capacitor 590. The
RC network of resistor 584 and capacitor 586 is joined to connection 59 from line select
switch Sl through blocking diode 578 and thence to input pin 6 of circuit 572. Bus 546 is
connected to pin 2 of circuit 572 via the RC network of resistor 588 and capacitor 590,
while diode 582 is connected in shunt about resistor 588 to selectively short circuit the
resistor when bus 546 is positive with respect to the voltage at pin 2.
Capacitor 586 is charged through diode 578 to maintain +Vcc at pin 6 for a
predetermined interval after release of Sl. The interval is set by the time constant of
resistor 584, being 1 megohm, and capacitor 586, being 1 microfarad. Before capacitor 586
discharges, line relay 24 is operated and control signal bus 546 goes to +Vcc in response to
Ll going off-hook. The +Vcc on bus 546 is applied to pin 2 of circuit 572 through diode 582
which, in accordance with the latching feature of circuit 572 damps output pin 7 of the
circuit at ground potential even though the voltage at input pin 6 thereafter goes low as
capacitor 586 discharges through resistor 584. The RC network of resistor 588 of 100 K
ohms and capacitor 590 of 10 microfarad maintains the voltage at pin 2 high to hold the
output pin 7 of circuit 572 at ground potential for a predetermined time after bus 546 goes
low so as to continue the hold enable during the hold condition when bus 546 is fluctuating
between +Vcc and ground. When the hold condition is terminated as described herein and
thereafter line Ll goes back on hook, then circuit 572 reverts to its normal state causing
output pin 7 to assume an open circuit condition.
Line Select Circuit
As shown in Figure 3, line select circuit 80 includes a line relay switching
network 8Q2 for operating coil 24a of relay 24 which in turn controls contacts 24b, 24c
--16--
~11~583
that connect Ll to the telephone set. Another line relay switching network 804 operates
coil 26a of relay 26 which has contacts 26b, 26c that connect L2 to the telephone set.
Additionally, circuit 80 includes first and second hold initiate transistor networks 806 and
808 connected between hold select switch S3 and connections 86 and 88, respectively that
extend to sensing circuits 50 and 60.
Network 802 includes a time circuit 810, identical to circuit 786 described
above. Output pin 3 of circuit 786 is connected to one end of coil 24a of relay 24. The
opposite end of coil 24a is grounded and a surge-current suppressing diode 812 is connected
in shunt about coil 24a. As briefly described above, coil 24a is normally energized so as to
maintain the set of normally closed contacts 24b and 24c in an open condition, closing
these contacts only when Ll is to be connected to the telephone set associated with
control unit 14. Thus, output pin 3 of circuit 810 is normally at +Vcc to energize coil 24a
and switches low when the voltages at inputs pins 2 and 6 swing toward +Vcc.
A diode 814 is serially connected between Ll select switch Sl and pin 2 of
circuit 810 and is poled to apply +Vcc to pin 2 when Sl is depressed to close the normally
open contacts thereof. An RC delay network of resistor 818 of 10 K ohms and capacitor
820 of 10.0 microfarads is joined to connection 816 to maintain pin 6 high for
approximately 100 milliseconds after Sl is released to allow time for circuit 810 to be
latched by a signal applied to pin 2 over a connection 822 as described below.
Network 802 includes an additional timer circuit 824, identical to circuit
786, best in this case connected to function as a polarity inverter. Output pin 3 of circuit
824 is coupled to pin 2 of circuit 81û through a 100 K ohm resistor 826 serially joined to
connection 822. Circuit 824 has its input pins 2 and 6 jointly connected through a 100 K
ohm input resistor 828 to connection 52b from sense circuit 50 to maintain these input
pins at +Vcc so long as Ll is on-hook and to drive these pins to ground potential when Ll
goes off-hook. As a result, the voltage at pin 3 of circuit 824 is the inverse of the voltage
at the input pins and is normally at ground potential and swings to +Vcc when Ll goes off-
hook.
Input pins 2 and 6 of circuit 824 are also connected through a coupling
capacitor 830 to the cathodes of a pair of diodes 832 and 834. The anode of diode 832 is
connected to the normally open contacts of hold select switch S3 which as described more
fully herein causes circuits 824 and 810 to energize coil 24a and thereby disconnect Ll
33
from the telephone set when S3 is depressed. The anode of diode 834 is connected to L2
~~ select switch S2 and also causes circuits~4 and 810 to energize relay coil 24a when S2 is
depressed. Momentary closure of S2 or S3 causes a positive voltage spike to be applied to
pins 2 and 6 of circuit 824 through diodes 834 or 832, respectively, and through coupling
capacitor 830, causing the output pin 3 to swing high thereby cancelling a latching signal
that otherwise holds pin 2 of circuit 810 high, allowing circuit 810 to revert to its normal,
unswitched condition, and causing relay coil 24a to be energized.
Line relay switching network 804 is similar to network 802 except that coil
26a of L2 line relay 26 is connected between pin 3 of a timer circuit 840 and +Vcc so that
coil 26a is normally unenergized as described above and is energized to close normally
open contacts 26b and 26c only when L2 is to be connected to the telephone set 13. The
remaining components of network 804 are identical to network 802. Thus a current-
suppressing diode 842 is connected in shunt about coil 26a. A diode 844 is connected
between S2 and input pin 2 of circuit 840 while S2 is connected directly to pin 6 of circuit
840 and is also connected to an RC delay network including resistor 846 and 848. An
additional timer circuit 850 corresponding to circuit 824 of network 802 and connected to
function as an inverter, has its output pin 3 coupled to pin 2 of circuit 840 over connection
852 that includes a series resistor 854, and has input pins 2 and 6 jointly connected through
an input resistor 856 to connection 64b from sensing circuit 60 (corresponding to
connection 52b of circuit 50). Diodes 858 and 860, corresponding to diodes 832 and 834
described above, apply signals from switches S3 and Sl to input pins 2 and 6 of circuit 850
through coupling capacitor 862.
Hold initiate transistor network 806 includes Darlington paired transistors
864 having the emitter output joined to connection 86 that extends to hold timer network
542 of sensing circuit 50. The collectors of transistors 864 are connected to +Vcc.
Transistors 864 are operated between their conducting and nonconducting states by a
signal from hold select switch S3~applied through an RC delay network of a 10 K ohm
resistor 872 and a 10 microfarad capacitor 870 joined to the input base of Darlington
transistors 864. When hold select switch S3 is depressed, Darlington transistors 864
conduct causing connection 86 to swing up to +Vcc and thereby energize coil 29a in
sensing circuit 50, assuming coil 29a has been enabled by network 544.
-18-
1~1~83
Hold initiate transistor network 808 is identical to network 806 and thus
includes Darlington paired transistors 874, an RC network including resistor 882 and a
capacitor 880. When S3 is depressed, Darlington transistors 874 conduct causing
connection 88 to swing to +Vcc for energizing the coil of an L2 hold relay corresponding to
the above described hold relay 29 for line Ll.
Hold Flash Generator
Generator 90 is a conventional free-running multivibrator connected
between +Vcc and ground and having a square wave output signal swinging between +Vcc
and ground at the rate of approximately .50 Hz to produce a 30 impulse per minute hold
indicating signal. The output of multivibrator 90 is extended over output connections 94
and 96 to the electr~optical isolators of line interface circuits 30 and 40 as described
above with respect to connection 94 and isolator 334 for Ll line interface circuit 30.
Operation of KTS
The operation of KTS 11 is best described by separately considering the
following operating modes: initiating an outgoing call at one of stations #l-N; receiving an
incoming call at one of the stations; ending a call; holding a call; and establishing a three-
point conference call.
Initiatin~ An Outgoin~ Call
Assuming that a call is to be placed at station #1 using telephone set 13
(Figure 1), it will be observed that lines Ll and L2 are both in an on-hook condition and the
telephone set 13 is disconnected from lines Ll and L2 by control unit 14. The telephone
sets at stations #2-N are also disconnected from lines Ll and L2 by their associated
control units.
The person using set 13 now depresses either Sl or S2 to connect set 13 to line
Ll or L2, respectively. Assuming that line I is selected, then Sl will be depressed causing
circuit 810 (Figure 3) to de-energize the Ll line relay 24 and thereby cause contacts 24b,
24c to connect set 13 to the T' and R' output leads from control unit 14. The internal
impedance of set 13 immediately causes the central office voltage on line 1 to drop from
an on-hook voltage of from 48 to 96 volts dc depending upon the type of central office, to
a lower off-hook voltage of from 3 to 20 volts dc. Within line interface circuit 30,
network 306 including zener diode 316 immediately senses the drop in voltage on Ll and
becomes nonconducting, turning Darlington transistors 314 off and causing output
_19_
583
connection 38 to rise to +Vcc via optical isolator 320. The control signal bus 546 of
sensing circuit 50 is now at or near +Vcc which results in the switching of the voltage at
output pins 3 and 7 of timer circuit 548 to ground potential turning on indicator light ILl
and causing timer circuit 824 of line select circuit 80 to maintain pin 2 of timer circuit
810 at +Vcc. As a result circuit 810 becomes latched in a condition that maintains the coil
24a of Ll line relay 24 in a deenergized state which in turn maintains the connection of
line Ll to telephone set 13 even though the contacts of Sl open as the Sl pushbutton is
released.
Concurrently with the above-described operation of control unit 14, each of
the other control units of stations #2-N respond to the off-hook condition on Ll and turn
on their respective indicator lights ILl in the same above-described manner that line
interface circuit 30 and sensing circuit 50 of unit 14 responded to the off-hook voltage on
line Ll. The illumination of ILl at each of the control staticns informs telephone users at
these stations that line Ll is busy.
Another related operating sequence occurs when the person at station #l has
just completed a call over L2 and wishes to place another call over L. In such case unit 14
automatically operates to disconnect L2 from set 13 when Sl is depressed. The momentary
closure of the contacts of Sl cause a positive voltage spike to be applied to pins 2 and 6 of
timer circuit 850 of line select circuit 80 via diode 860 and capacitor 862. Responsively,
circuit 850, which is part of network 804 that controls the L2 line relay 26, goes to ground
potential, causing pin 2 of integrated circuit 840 to also go to ground potential. Pin 6 of
circuit 840 is already at ground potential, since any voltage previously applied to pin 6 and
capacitor 848 by the momentary closure of the contacts of S2 has been discharged through
resistor 846 to ground. Timer circuit 840 thus switchesits output pin 3 from ground
potential to +Vcc thereby de-energizing coil 26a of L2 line relay 26, disconnecting L2
from the station set. Thus, when either one of switches Sl or S2 is individually operated to
select one of the available lines, the opposite line is automatically disconnected
("dumped"), unless the opposite line has been previously placed in a hold condition in the
manner described hereinafter.
It is now assumed that telephone set 13 has been connected by control unit 14
to line Ll and a dial tone is received at set 13. The user now signals central office using
either standard dialing or Touch-Tone (service mark of AT~T~ signalling to reach the
-2~-
583
called party via the central office. The tone frequencies associated with Touch-Tone
dialing do not have any appreciable effect on the operation of control circuit 14. Dial
pulse signalling produces a series of high-going voltage pulses on Ll, the peaks of which
exceed approximately 36 volts dc. Line interface circuit 30 and sensing circuit 50 respond
by producing a series of low going transitions on output connection 38 and on control
signal bus 546. These low going pulse transistions do not register at indicator light ILl
because diode 554 of network 540 becomes reverse biased during the low going pulses on
bus 546 and forces any change in the voltage at pin 2 of circuit 548 to be slowly
discharged through the RC circuit of resistor 550 and capacitor 552. Thus, circuit 548
remains switched and indicator light ILl remains continuously energized, discriminating
against any response due to the pulse dialing sequence. Thereafter, central office 12
(Figure 12) couples Ll to the called party's line to establish telephonic communication
therebetween.
While a call is in progress over line 1 at station #1, the remaining stations
#2-N are available for receiving an incoming call or initiating an outgoing call over L2 by
operating L2 select switch S2 of the control unit at the associated station.
Receiving an Incoming Call
The ringing signal of an incoming call that appears on lines Ll or L2 is
coupled by control unit 14 to telephone set 13 by means of bell transfer relay 28. As shown
in Figure 3, contacts 28b and 28c of the bell transfer relay are normally closed to connect
the T and R conductors of Ll to Bl and B2 that are in turn joined to the bell or other
audible signalling device of the telephone set. Thus, the ringing signal of a call coming in
on line Ll will be immediately applied to the bell of telephone set 13 without requiring any
responsive change of the circuitry or relays in control unit 14. Similarly, the telephone
sets associated with each of the other stations #2-N will sound the ringing signal through
the normally closed bell transfer relay contacts of their respective control units.
Additionally, the ringing signal is applied to input terminals 302 and 304 of
line interface network 30. Although the ringing signal may vary somewhat in voltage and
frequency, it is typically a 90 volt peak-to-peak ac signal at 20 or 30 Hz superimposed on
the on-hook dc voltage, typically 48 volts, developed at central office and appearing on
the line during an on-hook condition. Thus, the ringing signal causes the T conductor of
line Ll to swing between approximately +138 volts and -42 volts relative to the R
--21--
conductor. A burst of these ac cycles will be spaced in time by intervals of silence during
which the voltage on line Ll returns to the 48 volt dc level representing an on-hook
condition. The superimposed ac signal is rectified by diode 310 and the resulting positive
voltage swings, varying from zero volts to +138 volts are applied across network 306
including zener diode 316.
Zener diode 316 is thus periodically switched off as the voltage drops below
the breakdown voltage of diode 316. Darlington transistors 314 and electro-optical isolator
320 are thus similarly, periodically switched off, causing the voltage on output connection
38 to fluctuate between ground potential and +Vcc at the pulsating rate of the ac ringing
10 signal.
Sensing circuit 50 and in particular, network 540 thereof, receives the
pulsating signal from bus 546 and detects each burst of ac ringing to turn on ILl for the
duration of each such burst. This is accomplished by setting the time constant of the RC
network of resistor 550 and capacitor 552 to hold the charge on capacitor 552 so that pins
2 and 6 of timer circuit 548 are maintained above 1/3 +Vcc for the duration of each burst
of closely spaced positive voltage swings on bus 546, with capacitor 552 discharging slowly
through resistor 550 at the end of each burst. In this way indicator light ILl is maintained
on for the duration of each burst. The interviewing silent intervals between ringing signal
bursts are long enough to allow capacitor 552 to discharge and thereby force timer circuit
20 548 to turn ILl off. Indicator light ILl thus flashes a visual signal to alert the user at the
station of a ringing signal representing an incoming call, and can be used by itself or in
conjunction with the audible ringing signal device of the telephone set.
The ringing signal of an incoming call that arrives on line L2 does not
immediately reach Bl and B2 because of the intervening, normally open contacts 28d and
28e of relay 28. Rather, the ringing signal is applied to the T and R inputs of line
interface circuit 40 for line L2, corresponding to the input terminals 302 and 304 of line
interface circuit 30 for line 1, and circuit 40 in conjunction with sensing circuit 60
responds to the ringing signal and causes bell transfer circuit 70 to close contacts 28d and
28e and open contacts 28b and 28c. The ringing signal on L2 is thereby transfered to the
30 Bl and B2 outputs for sounding the bell of set 13.
In particular, circuit 40 responds to the ringing signal, as described above for
Ll line interface circuit 30, to produce a fluctuating signal on output connection 42 in
synchroni~ation with the burst of ac ringing.
--22--
q~
The fluctuating signal on connection 42 due to the ringing signal bursts has
two functions. First, the pulsating signal is applied via bus 646 (corresponding to bus 546
of circuit 50) and hence to input 71 of bell transfer circuit 70 such that each positive
~ ~ 7/ 7~
voltage swing at input terminal 70 is coupled through capacitor ~ and forward biased
diode 783 to capacitor 78S, charging the capacitor and causing the voltage thereacross to
rise toward +Vcc. Pins 2 and 6 of timer circuit 786 thus receive +Vcc, causing output pin
7 to be clamped to ground potential and causing the energization of bell transfer relay coil
28a. Energization of coil 28a causes the above mentioned opening of contacts 28b and 28c
and the correlative closing of contacts 28d and 28e, so as to apply the ringing signal on L2
10 to Bl, B2 shortly after the ringing signal is received at control unit 14. The time constant
of the RC network formed by resistor 784 and capacitor 785 is long enough to hold the
voltage charge on capacitor 785 resulting from the positive pulses developed by line
interface circuit 40 on output connection 42 for spanning the interval of silence between
bursts of ac ringing. Pins 2 and 6 of circuit 786 thus remain at a level greater than 1/3
+Vcc during the silent intervals between rings thereby maintaining coil 28a energized. At
approximately 3 seconds after the last burst of ac ringing voltage, capacitor 785 is
discharged to drop the potential at pins 2 and 6 of circuit 786 below the 1/3 +Vcc switching
threshold of the circuit, allowing pin 7 to assume an open circuit and thereby deenergize
coil 28a and restore contacts 28d~e to their conditions as shown in Figure 3.
Secondly, the fluctuating positive going signal on connection 42 from line
interface circuit 40 is effective via circuit 60 to turn indicator light IL2 on during each
burst of the ac ringing signal. IL2 is energized through a resistor 656 that serially
connects IL2 to output connection 64a of circuit 60 (corresponding to connection 52a of
circuie 50) which in turn is joined to the timer circuit corresponding to circuit 548 of
network 540 of the above-described Ll sensing circuit 50. The functioning of sensing
circuit 60 in this respect is identical to the above-described operation of sensing circuit
50.
It is noted that the contacts of the bell transfer relay 28 are arranged so
that line Ll is normally connected to Bl, B2 of the station's set and will remain so
connected in the event of a power failure in the supply voltage for control unit 14. With
the above described arrangement of Ll line relay 24 in which the normally closed contacts
24b, 24c automatically connect line Ll to the T' and R' terminals of the station's telephone
--23--
583
set, and in the event of a power failure, a fully operable telephone line and set is ensured
at each of the available stations.
Ending a Call
Aæuming that a call over line Ll at station 1 has been completed and the
handset of telephone set 13 is returned to its cradle, this action disconnects the internal
impedance of the phone set from Ll causing Ll to assume an on-hook condition. The
voltage on the tip and ring conductors of Ll rises to the on-hook voltage of 48 to 96 volts
dc depending upon the type of central office and as a consequence zener diode 316 begins
conducting. Network 306 including Darlington transistors 314 and photo-optical isolator
320 are turned on to clamp connection 38 to ground potential. The signal bus 546 swings
low to ground potential and after a 150 millisecond delay caused by the RC network 550
and capacitor 552, output pin 7 of timer circuit 548 assumes an open circuit extinguishing
indicator light ILl.
At the same time, output pin 3 of integrated circuit 548 swings to +Vcc
causing pin 3 of circuit 824 of line select circuit 80 to be switched to ground potential,
thereby cancelling the latching signal theretofore applied to input pin 2 of timer circuit
810. Circuit 810 now energ~zes line relay coil 24a to disconnect Ll from T', R'. The call
has been completed and the telephone line has been disconnected from the station's set,
and control unit 14 is restored to an idle condition. The idle or on-hook condition is
indicated by the fact that ILl is off.
To end a call established over line L2, line interface circuit 40 and sensing
circuit 60 function in the same manner described above for circuits 30 and 50 to cause L2
line relay 26 to disconnect the r and R' conductors from the T and R conductors of L2 and
return unit 14 to an idle condition.
Telephone calls are also ended automatically when a person at any particular
station depreæes the line select switch of the opposite line from that on which a call is in
progress. This feature is described in detail above under the section dealing with the
initiation of an outgoing call.
Holding a Call
When a call is in progress over Ll or L2 and the station's set is connected to
the applicaMe line, then that line may be placed in a hold condition in accordance with the
following operation. To illustrate this operation, it will be assumed that a call is in
--24--
~ill583
progress over Ll at station #1 and thus telephone set 13 is connected to the tip and ring
conductors of Ll through line relay contacts 24b and 24c. Now the party at station #l
wishes to place the call on Ll in a hold condition so as to enable the telephone set 13 to be
disconnected from Ll (so as, for example, to answer an incoming call on L2) while stiU
maintaining the telephonic connection of line Ll through the central office to the remote
party on L2.
To accomplish this, the hold select switch S3 is depressed. This applies a
positive voltage spike to pins 2 and 6 of timer circuit 824 of line select circuit 80 thereby
overriding the latching voltage applied to pin 2 of timer circuit 810 and causing Ll line
relay 24 to be energized. Contacts 24b and 24c open and thus disconnect r and R' from
line Ll.
Concurrently therewith, the closure of the S3 contacts applies a positive
voltage to the base of Darlington transistors 864 of hold initiate transistor network 806
which in turn energizes hold relay coil 29a of hold relay 29.
At this time, hold enable network 544 of sensing circuit 50 has been
preconditioned by the initial operation of Ll select switch Sl (when the telephone call over
line 1 was first established) so that the output pin 7 of timer circuit 572 is at ground
potential enabling the energization of relay coil 29a over connection 570. The enabling
signal applied to the lower end of hold relay coil 29a over connection 570 insures that only
20 the party at the particular station that received or made the call can place Ll in a hold
status.
Coil 29a is now energized and remains energized for a delay interval
4 ~
established by the RC network of resistor ~and capacitor 870 of network 806 even after
the contacts of S3 open following the release of S3. The hold relay contacts 29b within
line interface circuit 30 are now closed, connecting network 308 of circuit 30 across the
tip and ring conductors of Ll.
Zener diodes 330 and 332 of line interface circuit 30 are now connected
across Ll to limit the maximum voltage that can appear across the tip and ring conductors
of Ll to l9.7 volts, which is the sum of the combined breakdown voltages of zener diodes
30 330 and 332. From this maximum, the voltage across Ll drops to 4.7 volts each time zener
diode 330 is shunted by the photo-optical isolator 334 in response to hold flash generator
90. The change in voltage level across Ll between the maximum of 19.7 volts and the
--25--
~a~8~
minimum of 4.7 volts at the 30 impulses per minute rate of generator 90 creates the hold
indicating signal that is issued over Ll to the control units at the other stations of the
KTS. Moreover, the maximum voltage allowed to exist across line Ll during the hold
status is 19.7 volts, which is a low enough voltage to simulate the presence of a line
terminating impedance that is equivalent to the internal impedance of a telephone set
when off-hook and that enables an adequate minimual loop current to flow in the line to
hold the central office connection to the remote party. Thus, so long as the voltage
across Ll is maintained at or below the 19.7 volts established by the zener diodes 330 and
332, central office continues to sense an off-hook condition at Ll.
As mentioned above, the breakdown voltage of zener diode 316 is selected to
lie within the range of 15 - 20 volts in order to detect the voltage change on the line
between an on-hook condition and an off-hook condition. In addition to this constraint, the
breakdown voltage of zener diode 316 is selected to lie below the combined voltages of
zener diodes 330 and 332 and above the voltage due solely to diode 332, so that diode 316
is switched between its conducting and nonconducting states each time the voltage on line
Ll swings between the maximum of 19.7 volts and the minimum of 4.7 volts at the rate of
flash generator 90.
Line interface circuit 30 responds to the fluctuating hold indicating signal on
Ll and causes the voltage on connection 38 to fluctuate between ground potential and
20 +Vcc as the zener diode 316 is switched between its conducting and nonconducting states.
The control signal bus 546 of sensing circuit 50 receives this fluctuating hold indicating
signal and applies it to networks 540, 542 and 544.
In network 540 the RC network of resistor 550 and capacitor 552 are
selected to provide a short enough delay to enable timer circuit 548 to respond to the hold
indicating signal on bus 546 and thereby cause indicator light ILl to flash on and off at
appro~imately the rate of generator 90.
In network 542, the associated ~C components are selected such that timer
circuit 560 does not respond to the fluctuating signal on control bus 546. In particular,
capacitor 564 is charged slowly through resistor 562 when bus 546 swings high toward +Vcc
30 and is quickly discharged through diode 566 when the bus swings low to ground potential.
The time constant of resistor 562 and capacitor 564 is such that the frequency of the hold
indicating signal is too rapid to allow the voltage on pins 2 and 6 of timer circuit 560 to
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rise to the 2/3 ~Vcc threshold switching level of circuit 560 and thus pin 3 of circuit 560 is
maintained high at +Vcc during the presence of the hold indicating signal on line Ll. With
pin 3 of circuit 560 at +Vcc, coil 29a of hold relay 29 is latched in the energized state
through the normally open hold relay contacts 29c. When thus latched, the hold relay 29
maintains the line interface circuit 30 and sensing circuit 50 in the hold condition, so as to
continuously generate and apply the hold indicating signal to Ll and maintain the
simulated line terminating impedance across L. Similarly, in network 544 the nonlinear
RC network of resistor 588, capacitor 590 and diode 582 provide a time constant when
diode 582 is reverse biased that maintains pin 2 of timer circuit 527 high throughout the
/0~
~, 10 hold condition even though bus 546 periodically swings~eg.
The hold indicating signal on Ll is received at each of the other control
units, causing their respective line interface and sensing circuits, corresponding to circuits
30 and 50 to pulse the associated indicator light, corresponding to light ILl, on and off in
synchronization with the hold indicating signal. A visual signal is thereby received at each
station indicating that the line is in a hold condition. Note that only the hold relay 29
associated with the line and station at which a call has been made or received will be
operated and held energized by the timer network 542 because of the required enabling
signal from network 544 as described above.
The time constant of resistor 562 and capacitor 564 of network 542 is
20 sufficiently long so that circuit 560 does not switch in response to dial pulses which might
otherwise enable the latching of the hold relay 29 by an inadvertent operation of hold
switch S3 during dialing.
Additionally, the hold enable signal from network 544 over connection 570
prevents the inadvertent operation of the hold relay under the following circumstances.
When line Ll is picked up by operating Ll select switch Sl, the hold relay 29 associated
with line Ll is enabled by network 544 as above described. Now assume that while the
party at the station is talking over L1, a call comes in over L2. Intending to hold Ll while
L2 is answered, the person at the station depresses hold select switch S3. Because of the
presence of the incoming ringing signal concurrently on line L2 when S3 is depressed,
30 there is the possibility (in the absence of a hold enable network such as network 544) that
the fluctuating signal condition on bus 464 of sensing circuit 60 will cause the hold timer
of sensing circuit 60 to latch the associated hold relay and thereby inadvertently place L2
in a hold mode. The hold enable network 544 in each of the sensing circuits prevents this
occurrence by enabling only the hold latching network or networks associated with the line
or lines that have been selected by the select switches Sl and S2 at the subject station.
As described in greater detail in the above-mentioned copending application
SN 820,785, for "HOLD CONTROL FOR KEY TELEPHONE SYSTEM", zener diodes 330
and 332 function during the hold condition to maintain a certain minimum current flow
through the loop associated with line Ll and it is that current flow that the central office
senses in determining whether the station is on-hook or off-hook. The minimum current
flow will be established by the source voltage at the central office, less the breakdown
voltages of the zener diodes 330 and 332, divided by the loop impedance which includes
the line resistance. So long as the loop current flow in the line remains greater than
approximately 20 milliampheres, it usually varies within the range of 20 to 65
milliamperes after seizure of the line at central office, then the central office will sense
on off-hook condition and will remain seized on the line. There is a certain initial
threshold current level (in excess of the minimum sustaining current) required to cause the
central office to switch from an unseized to a seized state, however, once the line has
been seized, it will continue to hold it so long as the minimum sustaining current is
maintained. For most all central offices and most all operating conditions, a minimum
substaining current of 20 milliamps is sufficient. Under certain "best case" conditions as
20 little as 13 milliamps may be adequate.
To place L2 in a hold condition, line interface circuit 40, sensing circuit 60
and hold initiating transistor network 808 of line select circuit 80 function in an identical
manner to that described above for line Ll to cause a hold indicating signal to be
generated and applied to line L2 at the output signal rate of hold flash generator 90. In
this instance, indicator light IL2 and its counterparts in the control units associated with
stations #2-N will display the on/off hold indicating signal to indicate that line L2 rather
than line Ll is on hold.
The hold condition existing on either of the lines is terminated by the same
procedure that is described above for initiating an outgoing call or receiving an incoming
30 call. Thus, to terminate a hold condition on line Ll, the handset of the telephone at any of
the available stations is lifted off the cradle and Ll select switch Sl is depressed. The line
relay 24 hereupon operates to connect Ll to the T' and R' terminals of the telephone set 13
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83
thereby connecting the internal impedance of the telephone set across line Ll and forcing
the dc voltage on Ll to assume the off-hook level which lies below the breakdown voltage
of zener diode 316. In other words, the internal impedance of the telephone set is such as
to mask the maximum voltage produced by zener diodes 330 and 332, thereby preventing
the voltage from reaching the breakdown threshold of zener diode 316.
Connection 38 and signal bus 546 thereupon assume a steady +Vcc voltage
level. The disappearance of the low-going fluctuations on bus 546 allows capacitor 564 of
hold latching network 542 to accumulate a steady positive charge that raises the voltage
at pins 2 and 6 of timer circuit 560 to its switching threshold, forcing output pin 3 to go
10 low to ground potential and thereby deenergizing hold relay coil 29a. Contacts 29c of the
hold relay open and thus unlatch the hold relay terminating the hold mode and allowing
circuit 810 of line select switching network 802 to latch line relay 24 in a condition that
maintains Ll connected to the station's set.
Establishing a Conference Call
Many times it will be desirable to join both lines Ll and L2 to the station set
13 in order to permit a conference call between the party at the station and two remote
parties connected separately over lines Ll and L2. Usually, the conference call will be
established after a call between the station and a remote party has been effected over one
of the lines and that line is placed in a hold condition in order to receive or place a second
20 call over the opposite line.
Taking each of these examples in succession, first assume that a call is in
progress over Ll and control unit 14 of Figure 3 has connected Ll to the station's telephone
set 13. Now a ringing signal appears on L2 representing an incoming call. The party at
station #l tells the remote party on Ll that he is going to place the latter on hold which he
does by depressing hold select switch S3. Line Ll is thereby released from the T' and R'
output conductors of unit 14 by the opening of line relay contacts 24b and 24c. Set 13 is
thus available for connection to L2 and the party at station #1 accomplishes this by
depressing line select switch S2. The control unit responds by energizing relay 26 and thus
closing 26b and 26c coupling the telephone set to L2 and the party at station #1 answers
30 the caller on L2 who is to be the third party to the three-point conference.
Now with Ll on hold and L2 connected to the station's set, the conference
call is established by simultaneously depressing both line select switches Sl and S2. The
--29-
13,~1583
closure of the Sl contacts applies +Vcc to both pins 2 and 6 of circuit 810 of network 802
forcing pin 3 of circuit 810 to deenergize the line relay 24 and connect Ll through contacts
24b and 24c to the station's set. L2 is already connected to the station's set through the
contacts 24b and 24c which have been previously closed by circuit 840 and continue to
remain closed when line select switch S2 applies ~Vcc to both input pins 2 and 6 of circuit
840.
By simultaneously depressing Sl and S2, the automatic dumping of the
opposite telephone line through the cross coupled circuits formed by serially connected
diode 860 and capacitor 862 and serially connected diode 834 and capacitor 830 is negated
by the overriding, positive voltages applied to the input pins of circuits 810 and 840 from
the simultaneously closed contacts of Sl and S2. For example, when Sl is depressed, a
positive voltage spike is applied to circuit 850 via diode 860 and capacitor 862 which tries
to force input pin 2 of circuit 840 to go to ground potential via resistor 854. However, at
the same time S2 overrides the low going output of circuit 850 and maintains input pin 2
at +Vcc to hold circuit 840 in a state that continues to energize L2 line relay 26.
Similarly, the low going signal normally applied to input pin 2 of circuit 810
through resistor 826 when S2 is depressed is overridden by the direct application of +Vcc
to input pin 2 by the simultaneous closure of the Sl contacts.
Both lines Ll and L2 are thereby jointly connected to the telephone set and
20 will remain so until the three-point conference call has been completed and one or both of
lines Ll and L2 are to be disconnected.
At the end of the conference call, if both callers are to be disconnected,
then the party at station #l simply hangs up station's set 13. Both lines Ll and L2 assume
their respective on-hook conditions and force the circuitry of control unit 14 to switch line
relays 24 and 26 so as to restore the system to an idle state.
If only one of the remote callers is to be disconnected, then the party at
station #l simply presses the opposite line select switch. Thus, if the conference caller on
Ll is to be disconnected and the call on L2 is to be continued, then line select switch S2 is
depressed thereby automatically dumping line Ll in the manner described above under the
30 section on terminating a call.
It is also possible to place both of the remote parties on hold in order to split
the calls for talking to the remote parties individually, or allowing another station to talk
-3n-
`` llllS83
to one of the remote callers while station #l resumes its conversation with the remaining
remote party. When hold select switch S3 is depressed, the above described hold control
circuitry associated with both Ll and L2 is activated placing both lines Ll and L2 in a hold
status. Now any one of stations #l-N may pick up one of the lines by depressing the
appropriate select switch to connect the associated telephone set to the selected line.
The other line remains on hold and may be picked up at another station.
Installation of KTS 11
In installing KTS 11 at a particular location, consideration should be given to
the relationship between the ability of line interface circuit 30 to discern between on-
10 hook and off-hook signal conditions and the type of central office involved and distance of
the KTS stations from the central office. For central offices that require approximately
45 milliamperes of initial current flow to seize a line in response to an off-hook condition
at the customer station, reliable operation of KTS 11 has been achieved for line distances
of up to 1 mile from the central office. For central offices that need or~y 36
milliampheres of initial current flow to seize a line when it goes off-hook, then the system
has operated reliably at distances of as much as 1 to 2 miles.
Furthermore, the capability of the various control units to reliably detect a
fluctuating hold indicating signal generated by a control unit at another station (and to
consistently supress such signal when the telephone set is taken off-hook at another
20 station) depends, among other factors, on the line distance or distances between stations,
and on the line distances from such stations to the central office. In particular, for a KTS
11 installation that is at a relatively long distance from the central office such that the
control units are separated from the central office voltage source by a substantial amount
of loop impedance, then the fluctuating hold signal can be consistently detected (and
eonsistently suppressed by the off-hook impedance of a telephone set) over interstation
distances of more than one mile.
When, however, KTS 11 is installed relatively close to the central office,
then there is a lesser amount of loop impedance between the central office voltage source
and the control units. Consequently, the interstation distance over which the fluctuating
30 signal can be reliably detected (and supressed) is diminished because of the greater
Influence of the central office source voltage on the varying line voltage at the control
units, and the limits of the permissible interstation distance, for reliable operation, are
--31--
583
reduced in proportion to the proximity of the installation to the central office. For
example, with the embodiment disclosed herein, at an installation distance of 1000 feet or
less from the central office, the interstation separation should be limited to a maximum
of one mile.
Although the above installation considerations are stated with respect to
line distances, it will be recognized that there are other variables, including wire size and
type, which to a lesser extent affect the loop impedance and thus will alter somewhat the
permissible interstation distances.
While only a limited number of embodiments of the present invention have
been disclosed, it will be readily apparent to persons skilled in the art that numerous
changes and modifications may be made to these embodiments without departing from the
spirit of the invention. For example, KTS 11 has been described as being connected to
telephone lines extending directly from a central office, but it will be apparent that the
system may also be used with lines that extend from an equivalent source such as a PBX
(private branch exchange) or an ESS (electronic switching system). In most cases, the PBX
or ESS merely functions to route a call between a customers local telephone line and a
central office and thus when KTS 11 is coupled to such local line it is still connected,
albeit indirectly, to a central office. In other cases, the calls on PBX or ESS lines may
originate in the PBX or ESS system, without passing through a central office. In this
latter situation, the PBX or ESS generated signals that appear on the local telephone lines
are equivalent to those originating at a central office and KTS 11 operates in essentially
the same manner as described hereinabove.
In certain PBX and ESS systems, a type of switching function is employed
which may require minor adaptation of the KTS 11 circuitry in order for it to properly
function in all situations. For example, in an ESS unit and some PBX systems, calls may
be transferred to another line by clicking the off-hook switch of a telephone set and then
disling in a pre-designated code which instructs the ESS (or P8X) to transfer the call to
another station. In using the KTS 11 with such systems, it is desirable to provide a delay
network in series with connection 52b between timer circuit 548 of network 54û and
circuit 824 of the line select circuit 80 (and an identical delay network in series with
connections 64b between sensing circuit 60 and line select circuit 80). Such delay
networks are designed to provide an approximately 1.5 second switching delay when line ~1
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lill5~33
goes on-hook and output connection 52b of timer circuit 548 swings to +Vcc. The delay
occurs only in response to a switching signal on connection 52b that swings from ground
potential to +Vcc (not vice versa) and serves to delay the alltomatic disconnect response
of the Ll line relay switching network 802 in response to the appearance of an on-hook
signal on Ll, so that momentary actuation of the off-hook switch ~to signal an ESS or PBX
unit) does not cause network 802 to disconnect Ll from the station's telephone set. The
automatic disconnect still occurs after a continuous on-hook condition of Ll for 1.5
seconds or more which allows the serial delay network to time out and cause network 802
to respond to the switched condition of timer circuit 548. The added serial delay network
10 in connection 64b operates in the identical fashion to delay the automatic disconnect of
L2. in response to sensing circuit 60. The added delays do not, however, delay the
automatic disconnect that occurs when the opposite line select switch is operated (as
provided by the cross coupled paths of diodes 834 and capacitor 830, and diode 860 and
capacitor 862).
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