Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to telephone intercom sys-
tems, and more particularly to a system for allowing several
self-contained station controller circuits adapted for use
with relatively few links to be opera~ed together to expand
the number o links in the intercom system.
Description of the Prior Art
. _
Intercom systems have long been used for selectively
connecting a calling telephone station to a called telephone
station to allow two-way communication. Generally, these sys-
tems connect the stations to each other through individual
audio links, the number of which is far less than the number
of stations in the system. Consequently, link sharing by the
stations is necessary. A call is initiated when a calling
station sequentially scans the audio links in search of an un-
used link and, when it finds an available link, connects it-
self ~o that link. The calling station then generates the
address of the called station by dialing with either a dual-
tone multi-frequency device or a rotary dialing device. Link
controllers associated with individual audio links decode the
address of the called station and cause the called station to
connect itself to the audio link to which the calling station
is connected.
Recently, a multi-link intercom system of this type
has been available from Tone Commander Systems, Inc., of Red-
mond, Washington, under the model deslgnation ML8000. This
system employs four audio links, and all of the station con-
trollers and link controllers for the system operate on a
four-link access period basis. The ML8000 systern features a
self-contained large-scale integrated station controller
7~
which is only capable of connecting its station to one of four
audio links and which, like the remainder of the system, in-
herently operates on a four-link access period basis.
Although the number of audio links required in an
intercom system for a given number of stations varies
depending upon the extent to which each station utilizes the
intercom, it i~ generally necessary to increase the number of
audio links as the number of stations accessing those links
increases. Thus, the need has developed for intercom systems
having eight or more audio links. Expansion of four~link
intercom systems employing self-contained station controllers
to allow a greater number of links to be accessed requires
that the self-contained controller circuits properly interface
with each other. The interfacing requirement could
undoubtedly be met by merely increasing the number of called
station address lines/ control lines and other circuitry in
proportion to the increase of audio linXs. However, this
solution would result in an unacceptable proliferation of the
interconnection wiring in the system.
Intercom systems having a substantially larger num-
ber of audio links are also more difficult to test than sys-
tems having fewer links. Thus, it is desirable to simplify
testing by allowing the system to selectively make some of the
links busy to facilitate testing of other links.
SUMMARY OF THE INVENTION
.
It is an object of the invention to provide a tele-
phone intercom system which is easlly expandable in increments
of M links by parallel operation of self~contained station
controller~ each adapted to interface with M links.
It is another object of the invention to expand the
number of audio links in an intercom system by operating sev-
~3S7~
eral self-contained ~tation controller circuits in parallel
without re~uiring a corresponding increase in interconnection
wiring.
It is a further object of the in~ention to facili-
tate testing of an intercom system having a r~latively large
number of links by preventing some of the links from being
used.
These and other objects of the invention are pro-
vided by a telephone intercom system having a large number of
station controllers each adapted to connect a telephone sta-
tion to one of M links and each operating in M time multi-
plexed link available periods. The system allows N self-con-
tained station controller circuits for each station to operate
in parallel to collectively connect the station to M x N audio
links, each of which are controlled by an individual link con-
troller. The link controllers receive the address of the
called station from the calliny station, decode the address
and apply the address as a station controller circuit enabling
signal in time multiplexed form to the called station control-
ler during one of N address valid periods of each of M link
available periods corresponding to the link to which the call-
ing station is connected. When the controller circuit in the
called station controller is enabled, it connects itself to
the audio link corresponding to the one of N portions of the '
M operating periods during ~hich the enabling signal was re-
ceived. All station controller circuits for a given station
operate in the same one o~ M operating periods at the same
time, and the N controller circuits Eor a called station con-
troller must thus receive an enabling signal at the same time.
Since the enabling signal for one controller circuit is re-
ceived at a different time than enabling signals for other N-l
1~13b7~;~, o
controller circuits (since only the one controller circuit is
associated with the links to which the calling station is con-
nected), the system retains the enabling signal received dur-
ing each operating period for any of the controller circuits
and then applies the enabling signal to the selected one of
the N controller circuits at the same time that an enabling
signal would be applied to any of the other controller cir-
cuits. The station addresses and internal control lines, be-
ing time multiplexed, convey the increased quantity of inform-
ation resulting from increasing the number of links yet re-
quire relatively few additional interconnecting wires. The
system also includes circuitry for selectively generating
signals indicating that specific audio links are unavailable
for use so that the remaining links, which may be far fewer in
number than the total number of links in the system, can be
easily tested.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of an intercom system em-
ploying individual, self-contained station controller circuits
in expandable form.
Fig. 2 is a schematic of a station controller utili-
zing a pair of self-contained station controller circuits
operating in parallel.
Fig. 3 is a schematic of an audio coupling and off-
hook detection circuit of the station controller of Fig. 2.
~ ig. 4 is a schematic of the address decoder and an
"A" link detect and delay circuit o~ the ~tation controller
which allows the .self-contained station controller circuits to
operate in parallel.
Fig. 5 is a timing diagram illustrating the addres-
sing sequence for the station controllers.
,;,7~gA),
Fig. 6 iS a synchronous state logic diayram and
state assiynment chart of a~ hook state control circuit for
determining if an off-hook or dial pulse condition has
occurred.
Fig. 7 is a schematic of a call progress timer used
with the hook state control circuit of Fig~ 6.
Fig. 8 is a schematic of one implementation of the
synchronous state logic diagram of Fig. 6.
Fig. 9 is a synchronous state logic diagram and
state assignment chart of a station sequencer for controlling
the operation sequence of the station.
Fig. 10 is a schematic of a circuit for determining
when all of the audio links in the intercom system are busy.
Fig. 11 is a schematic of a circuit for controlling
the state of link available and rlng acknowledge busses for
the station controllers.
Fig. 12 is a schematic of a circuit for controlling
analog switches to selectively connect a station to an audio
link.
Fig. 13 is a schematic of a ring out circuit used by
the station controller for actuating a ringing device in a
called telephone station.
Fig. 14 is a schematic of one implementation of the
synchronous state logic diagram of Fig~ 9.
Fig. 15 is a block diagram of four link controllers.
Fig. 16 is a schematic of portions of the link
controllers for four audio links.
Fig. 17 is a timing diagram of signals used by the
link controllers.
Fig. 18 is a flow chart summary for the program il-
lustrating the operatiny sequence of a microprocessor used by
each link controller.
~i3~41-
Fig. 19 is a flow chart of the cornputer program for
the link controller microprocessor as illustrated in Fig. 18.
Fig. 2C is a block diagram of the signal generator
for decoding station addresses and generatiny various signals
utilized by the station controllers and link controllers.
Fig. 21 is a schematic of the signal generator of
Fig. 20.
DETAILED DESCRIPTION OF_T~E INVENTION
A multi~link telephone intercom system employing the
inventive link expanding circuitry is illustrated in Fiy. 1.
The system includes a large number of conventional key tele-
phone stations lO each having a conventional ring device 12,
a hand set 16 and a dialing mechanism of either the rotary
dial pulse generator or dual-tone, multifrequency (DTMF)
variety associated hook switch and inductive coupling network
18 connected to tip and ring lines 20, 22, respectively. A
dual pulse is the signal resulting from breaking loop current
by the rotary dial of a telephone set. A number of dial
pulses in sequence constitute a certain digit. The duration
of dial pulse and the time between dial pulses have certain
specified limits. Dual-tone multifrequency is an alternative
method of signalling. Two sinusoidal tones are mixed and
transmitted together. The frequency of one of these tones is
from the low group: 697, 770, 852, 941. The frequency of the
other tone is from the high group: 1209, 1336, 1477, 1633. A
particular combination of two frequencles is interpreted as a
certain digit. The tip and ring lines 20, 22 are normally
unconnected by any direct current path when the handset 16 i5
on hook, but are connected to each other through the inductive
coupling network 18 when the handset 16 is raised off hook.
Off-hook, as used herein, in~ic~tes the condition of a tele-
phone set where the han~set is removed from the cradle; that
is, the telephone set is in use. In use, the audio signals
received and generated by the handset 16 are present across
the tip and ring lines 20, 22. A called sta~ion is dialed by
either generating appropriate dual-tone multi-frequency sig-
nals across the tip and ring lines 20, 22 by a conventional
tone dialing mechanism or by intermittently opening the direct
current connection between the tip and ring lines 20, 22 a
number of times corresponding to the number being dialed by a
conventional rotary dialing mechanism.
The intercom system includes a large number of tele-
phone stations 10, only one of which is illustrated in detail.
Each of the telephone stations 10 is connected to a station
controller 30 which selectively connects the audio output from
the tip and ring lines 20, 22, respectively, to one of eight
audio links 32 when the handset 16a is raised off hook. Basic-
ally, as explained in greater detail hereinafter, when the
handset 16a is raised off hook, the station controller 30a
scans the status of the audio links 32 in search of an unused
link 32. When the station controller 30a finds an unused link
32, it connects the audio output from the telephone station lOa
to the unused link and generates an appropriate signal on a
partially bidirectional control bus 34 to prevent other sta-
tion controllers 30 from connecting to that link 32. When one
of the other telephone stations, lOh for example, is called,
the station controller 30h associated with called station .lOh
causes the ringing device 12h of the called station lOh to be
actuated and causes an appropriate ring acknowledgement con-
trol signal to be generated to the calling link controller 38
which responds by applying an audio ~INGBACK signal to the
~L~3~4~L
link 32 which is then heard by calling station lOa. The
RIN~ACK signal is one of the call progress signals. It is
formed by the modulation of a 440 signal by a 40 Hz signal~
This, then, is transmit~ed for a duration of 0.8 seconds and
off for 2.4 seconds. The ringback tone is transmitted to the
calling party to indicate that the called party's telephone
set is ringing. When the called station lOh answers the call
by establishing an off-hook condition, its station controller
30h ceases actuation of the ringing device 12h and connects
the audio output from ~he station lOh to link 32l thereby com~
pleting an audio link between stations lOa and lOh. If the
called station lOh is busy, the station controller 30h will
not return a ring acknowledgement control signal to the call-
ing link controller 38 and an audio busy signal will be ap-
plied to the link 32 which is heard by the calling station
lOa.
Each of the station controllers 30 contains two
self-contained controller circuits 36 which are preferably
large-scale integrated circuits. These circuits 36 were
initially developed for a four-link intercom system. Conse-
quently, the circuits are capable of controlling only four
links and they inherently operate and receive control signals
in a four time division multiplex mode corresponding to the
four links. Use of this circuit 36 with intercom systems hav-
ing eight or more audio links 32 presents the problem of al-
lowing the circuits 36 to operate in parallel without adding a
correspondingly larger number of interconnecting lines to the
various signal buses and also necessitates the interconnection
of control signals between the four link station controller
circuits 36 to allow a priority scheme which will prevent the
two independent station controllers 36 from simultaneously
connecting a single station 1~ to two audio links. A princi-
pal feature of the invention, then, is circuitry explained in
detail hereinafter for allowing the integrated circuits 36 to
operate in parallel while interfacing with each other and
the remainder of the system.
Each of the audio links 32 is connected to a link
controller 38. The link controllers 38 decode the station
number being dialed by each of the calling stations 10 and
generate appropriate coded signals on a called station address
bus 40. Link scanners 39 sequentially scan four links in
search of a link ready to receive dual-tone multi-frequency
dialing signals and decode the dialing signals when received
on that link. Thus four links share a single tone decoder.
The link controllers 38 also generate appropriate four time
division multiplexed signals on the control bus 34 and called
station address bus 40 as explained in greater detail herein-
after. The coded called station address signals on bus 40 are
decoded by a signal generator 42 which generates appropriate
signals on bus 44 to cause the station controller 30 for the
called station 10 to actuate its ringing device 12 and to con-
nect itself to the proper audio link 32 when the called
station 10 answers. The signal generator 42 also generates
various signals for the remainder of the system, such as
system clock signals, ringback tones and busy tones.
An understanding of the operation o~ the multi link
intercom system necessitates an understanding o~ the time
multiplex scheme used to transfer a large amount of informa-
tion on relatively few lines. Basically, a number of link
access periods exist corresponding to the number of audio
links 32 connected to each station controller circuit 36.
Thus, the system illustrated in Fig. 1 utilizes four link
access periods. During each link access period, a siynal is
generated on a link available bus (part of control bus 34) to
indicate the status of the corresponding audio link. The link
available bus consists of a number of separate lines corres-
ponding to the number of circuits 36 employed in each station
controller 30. A single link available line will then be
required to indicate the status of each group of four links to
be controlled by a set of station controller circuits 36
dedicated to that four link group. Thus, the link available
bus for the system illustrated in Fig. 1 consists of two
lines, designated LKAHA and LKAHB. An LKAHA signal is genera-
ted by circuit 42 during each link access period that the cor-
responding link of the first four links is available; i.e.,
not already connected to another station controller 30. Sim-
ilarly, a LKAHB signal is generated by circuit 42 during each
link access period that the corresponding link of the remain-
ing four links is available.
When the telephone station lOa goes off-hook, the
station controller 30a for that station lOa sequentially ex-
amines each of the link available lines LKAHA and LKAHB during
each of the four link access periods. When an available link
is found, the integrated circuit 36a or 36b associated with
that link couples the telephone station 10 to that available
audio link 32 and removes a link available signal on LKAHA or
LKAHB otherwise present during the link access period corres-
ponding to that link. The link available signal LKAHA or
LKAHB then indicates to the remainder oE the ~ystem that the
link is unavailable for use by any other telephone station 10.
After the telephone station 10 has been connected to a given
audio link 32, the station 10, hereinafter referred to as the
calling station, dials another telephone station 10, hereinaf-
~3~i7~
ter referred to as the called station. The manner in ~7hichthe called station i5 dialed depends upon the nature of the
telephone station 10. Telephone stations 10 having conven-
tional rotary dialing mechanisms open the connection between
the tip and ring lines 20, 22 a number of times corresponding
to the number being dialed. This dial pulse information is
then time division multiplexed by the station ~ontroller and
transmitted to the link controller 38. q~elephone stations 10
employing a dual-tone multi-frequency dialing mechanism gener-
ate appropriate identifying audio tones across the tip and
ring lines 20, 22, respectively, which are applied by the sta-
tion controller 30 to the audio link 32 to which the control-
ler 30 is connected. Generally, the calling station 10 will
dial two digits in order to designate the called station. Re-
gardless of which dialing mechanism is used, the dial pulses
or audio tones are conveyed to the link controller 38 for the
link 32 to which the telephone station 10 is connected. The
link controller 38 then generates a time multiplexed code de-
signating the called station on bus 40. The station address
signal on the bus 40 is generated during the link access peri-
od corresponding to the link to which the calling station 10
is connected. The called station 10 can determine which link
its station controller 30 should connect to by synchronizing
with the access period in which the address signal is re-
ceived. However, there are eight audio links but only four
link access periods. Thus, if a station address signal is re-
ceived by a station controller 30 during the first access per-
iod, the controller 30 must determine whether it should con-
nect to link lA or lB. This function is accomplished by fur-
ther multiplexing the station addre5s signals on bus 40 into
two address valid periods corresponding to an "A" groove link
11
~3~q~
address and a "B" yroove link address. Thus, two address per-
iods exist for each link access period providing ~ total of
eight address valid periods uniquely identifying the eight
audio links 32. Thus, if the leftmost telephone station 10a
is connected to the link 3B, the addre~s of the called station
designated by the dialing mechanism of the calling station 10a
is generated on the address bus 40 during the second address
valid period of the third link access period. Other stations
10 connected to other links may also generate called station
address signals on bus 40 during the address valid period of
the link access periods corresponding to the links to which
they are connected. The called station address signals on bus
40, which are in binary form, are decoded by the signal gener-
ator 42 and applied to the designated station controller 30.
A detection circuit in the station controller 30 of
the called station 10 determines whether the called station
address should be connected to either an "A" group link or a
"B" group link corresponding to the link access period in
which the address is received. Since the "A" group links are
accessed by circuit 36a and the "B'l group links are accessed
by circuit 36b, the detection circuit determines which circuit
36a or 36b is active. This function is accomplished by stor-
ing the address for the first address valid period of the link
access period and presenting it to controller circuit 36a
while the address for the second address period of that link
access period is applied directly to controller circuit 36b.
Either of the circuits 36a or 36b can then be enabled at the
same time, but only one circult will be enabled since the sta-
tion was only designated during one address valid period of
the link access period.
- 12
~3~
If the station controller 30 for the called station
10 is not already connected to another audio link 32, it gen-
erates a ring acknowledge signal during the link access period
corresponding to ~he link to which the called station control-
ler 30 is connected. The station controller 30 is capable of
identifying the proper time for generating RAKL since it
should occur during the same link access period that it has
been enabled by receiving its station address. In actuality,
a ring acknowledge line connected to all of the station con-
trollers 30 is provided for each circuit 36 in the station
controller 30. Thus, for the system illustrated in Fig. l,
two ring acknowledge lines exist forming part of control bus
34 designated RAKLA and RAKLB. The non~busy called station
controller 30a generates a signal on the RAKLA line during the
appropriate link access period whenever the calling station lO
is connected to one of the "A" links. Similarly, the non-busy
called station controller 36b generates a signal on the RAKLB
line during the appropriate link access period whenever the
calling station lO is connected to one of the "B" links.
When called station lO goes off-hook (i.e. answersl,
the ring acknowledge signal RAKLA or RAKLB terminates. The
station controller 30 for the called station lO then prevents
a link available signal ~rom being generated on the line LKAHA
or LKA~B during the link access period corresponding to the
link to which the calling station is connected. The station
controller 30 for the called station al~o couples the tip and
ring lines 20, 22 for the called ~tation lO to the same audio
link that the station controller 30 for the calling station is
connected. Two-way conversation is thus effected between the
calling station lO and the called station lO. If the called
station lO is already connected to an audio link 32, a ring
acknowledge signal RAKLA or RAKLB is not produced during the
13
7~L
appropriate link access period so that the link controller 38
for the link to which the calling station 10 is connected ap-
plies a busy audio slgnal Erom the signal generator 42 to the
audio link 32 which is then received by the calling station
10~ It should be mentioned that the above is a basic descrip-
tion of the entire multi-link intercom system. A complete ex-
planation of the subcircuits and subsidiary features of the
system are explained in greater detail hereinafter.
A block diagram of a station controller 30 is illus-
trated in Fig. 2. The system includes an audio coupling and
off-hook detector 50 connected to the tip and ring lines 20,
22 of the telephone 10. The audio coupling portion of circuit
50 couples the audio signal from the tip and ring lines to an
audio output which is applied to an audio switch 52~ The aud-
io switch 52 connects the audio output from circuit 50 to one
of the eight audio links 32 la-4b as designated by the corres-
ponding one of eight switch control lines on input terminals A
and B. Signals appearing on the Eour switch control lines
from the integrated circuit 36a switch the audio output of
coupling circuit 50 to the "A" group links, lA, 2A, 3A, 4A.
Similarly, signals appearing on the four switch control lines
from integrated circuit 36b switch the audio signal from coup-
ling circuit 50 to the "B" group audio links lB, 2B, 3B, 4B.
Thus, the integrated circuits 36a,b select which of the audio
links 32 the audio output of coupling circuit 50 is connec-
ted.
The off-hook detection portion of circuit 50 causes
an OFFHOOK signal to go low (logic "O" or -12 V) when the
handset 16 of the telephone 10 goe~ off-hook. This low i8
applied to NOR 54 which, it is assumed, is enabled by a low
at its other output, thereby generating a high (logic "l" or
14
0V) at the YHKSWT input to integrated circuit 36a. As ex-
plained in detail hereinafter, integrated circuit 36a then se-
quentially examines its LKAHA input during the four link ac-
cess periods to determine whether any of the "A" group audio
links are available. If one of the "A" group links is found
to be available, the circuit 36a pulls the LKAHA bus 10~7 dur~
ing that link access period so that no other stations will
find that link to be available. The circuit 31a also trans-
mits a signal to the audio switch 52 connecting the audio out-
put of circuit 50 to that audio link.
If none of the "A" group links are available, the
BUSY output of circuit 36a goes low, which places a high on
the input to NOR gate 56 through inverter 58. NOR gate 56
then applies a low to NOR gate 58 which, since OFFEIOOK is low,
produces a high at the YHKSWT input to integrated circuit 36b.
Circuit 36b then sequentially examines its LKAHB input during
the four link access periods. If an available "B" group link
is found, circuit 36b pulls LKAHB low during the access period
corresponding to the available link and it transmits a signal
to audio switch 52 to connect the audio line from circuit 50
to the appropriate "B" group link, lB, 2B, 3B or 4B. It will
be noted that the availability of a given link is determined
by detecting wh~ther LKAHA or LKAHB is high during the corres-
ponding link access period~ Since the circuits 36a,b pull the
LKAHA or LKAHB 1 ines low during the link access period corres-
ponding to any link to which they are connected, none o~ the
other station controllers 30 will detect a high state on the
LKAHA or LXAHB lines during that link access period. Whenever
either of the circuits 36a,b have been connected to a link,
their HBI output goes high thereby preventing further opera-
tion of the other integrated circuit because of the high sig-
~3~
nal at its YKSWR inp~t as explained hereinafter. Also~ wheninput as explained hereinafter. Also, when none of the "A"
group links are available but an available "B" link has been
ound, the HBI output of integrated circuit of 36b goe~ high,
thereby disabling NOR gate 54 so that a high is not applied to
the YHKSWT input to integrated circuit 36a which would cause
the circuit 36a to connect the calling station to an "A" yroup
link if an "A" group link subsequently became availahle.
If all of the "A" group links are found to be un-
available and the "B" group links are then likewi~e found to
be unavailable, the BUSY output of integrated circuit 36b goes
low, thereby enabling NOR gate 60 which gates a busy signal E~B
from the signal generator 42 through a capacitor 62 and a re-
sistor 64 to the audio terminal of coupling circuit 50. A
busy signal is then heard in the receiver of the handset 16 to
indicate that none of the audio links 32 are available for
useO Station busy signals; i.e., signals generated when a
link is available but the called station is busy, are normally
transmitted to the coupling circuit 50 via the audio link to
which the circuit 50 is connected. However, where the system
is busy; i.e., no audio links are available, and there is no
audio link over which to transmit the busy signal to the coup-
ling circuit 50. Consequently, the busy signal must be inter-
nally generated at the station controller 30.
After an audio link 32 has been found to be avail-
able, the audio terminal of the coupling circuit 50 is then
connected to the available link by audio switch 52 by one of
the four-line switch control output~ rom either circuit 36a
or 36b. The calling station then dials a number, typically two
digits, corresponding to the number of a called station. If
the calling station is equipped with a dialing mechanism which
16
~ ~3~7~
generates dual~tone multi-frequency dialing signalc~ the sig-
nals are applied directly to the link through the coupling
circuit 50 and the audio switch 52. The link controller 38
for the link to which the audio switch 52 is connected then
decodes the first and second digits of the c~lled station and
generates appropriate time multiplexed signals on bus 44 which
are applied to an address decoder 70 as SlH, S2H...S5H, S~X~
and SXXH. The Signals S+XH and SXXH act as enabling signals
for the particular group of five stations. One of the other
five lines, SlH, S2~1..., S5H then selects which of the partic-
ular group of five stations is being addressed. If the call-
ing station telephone l0 is equipped with a rotary dialing
mechanism, the OFFHOOK output of off-hook detector 50 pulses
low a number of times corresponding to the number dialed.
These pulses are coupled through either NOR gate 54 if the
calling station is connected to an "A" group audio link or NOR
gate 58 if the calling station is connected to a "B" group
audio link. The integrated circuit 36a or 36b then determines
whether valid dial pulses have been produced and, if so, gen-
erates a corresponding number of time division multiplexed
pulses at the DPLA output of circuit 36a if an "A" group link
has been accessed or the DPLB output of circuit 36b is a "B"
group link has been accessed. These pulses are demultiplexed
and counted by the link controller 38 for the link to which
the calling station is connected and the link controller 38
generates appropriate signals on bws 40 which are decoded by
the signal generator 42 and applied to the addresa decoder of
the called station aa SlH, S2H,...,S5M, S~XM, SYX~I. It should
be remembered, at this point, that these station address valid
signals 5lH...SXXH are time multiplexed so that they are only
present at the called station address decoder 70 during the
17
~3~
address period of the link access period corresponding to the
link to which the calling station is connected~ As mentioned
above but explained in greater detail hereinafter, the cir-
cuits 36a, 36b are basically four-link devices operating with
four-link access periods. In order to allow the circuits 36a,
36b to operate in parallel, the station address valid period
for a station being dialed by a station connected to an "A"
group link is interleaved with the station address period for
that station being dialed by a station connected to a "B"
group link. Since the calling station will be connected to
either an "A" group link or a "B" group link, but not both, a
station address for the called station is generated during
only one of the two address periods of the link access period.
Thus the station address periods in which an address signal is
presented to the address decoder 70 for every station control-
ler 30 in the entire system are interleaved as follows: link
lA, lB, 2~, 2B, 3A, 3B, 4A, 4B. However, since the circuits
36a are basically four link access period devices and are
clocked by the same FOH signal, the address of the called sta-
tion must be operated on at the same time regardless of
whether the calling station is connected to an "A" group link
(thus addressing controller circuit 36a) or a "B" group link
(thus addressing controller circuit 36b). Consequently, it is
necessary to delay and hold the "A" group link address signals
by an "A" link detect and delay circuitry 72, so that the cir-
cuit 72 can apply any "A" group link address signals to the
circuit 36a at the same time that the "B" link address signals
from the address decoder 70 would be applied to the circuit
36b. Thus, if a given station is being called by a calling
station connected to an "A" group link, the YCALL input to
circuit 36a will be low at the same data sample point that the
18
~13674~
YCALL input to integratecl circuit 36b would be low if the
calling station was connected to a "B" group link. Proper op-
eration of the system requires that the enabling signal YCALL
to the appropriate circuit 36 occurs at the same time wit~P-re~
spect to the clock signal F0H regardless of whether the call-
ing station is connected to an "A" group link or a "B"~group
link.
After the appropriate controller circuit 35 receives
its enabling signal YCALL, it generates a ring acknowledge
signal RAKL during the link access period corresponding to the
link to which the calling station is now connected. The link
controller 38 for that link detects the RAKL signal during the
proper link access period and applies a ring-back signal to
the link which is transmitted to the ear piece of the handset
for the calling station as explained hereinafter.
The controller circuit 36a or 36b for the called
station asserts its BELL output which is applied to OR gate 74
which drives a relay coil 76 through inverter 7B. Thus, cur-
rent flows through relay coil 76 whenever the BELL output of
either integrated circuits 36a, 36b is asserted. As explained
hereinafter, current flowing through the relay coil 76 closes
a pair of contacts in the audio coupling and off-hook detector
50 which applies a high voltage AC signal to the tip and ring
lines of the called telephone station 10 to actuate an inter-
nal ringer.
One of two events occur after the enabling signal
YCALL for the circuit 36 of the called station i~ received.
If the calling station 10 goe~ on-hook to di~continue the
call, the link controller 38 remove8 the address for the call-
ing station thereby terminating the enabling signal YCALL for
the called station. If the called station goes off-hook (i.e.
lg
~13~
answers), the ring acknowledge signal RAKLA or R~KLB generated
by the circuit 36a or 36b for the called station term:inates so
that the ring back tone is removed from the audio link 32 and
the LKAHA or LKAHB output of the circuit 36a or 36b, respec-
tively, for the called station is pulled lo~ during the link
access period corresponding to the link to which the calling
station is connected so that the called station will remain
connected to the link even if the calling station goes on-
hook. Also, of course, the controller circuit 36a or 36b for
the called station connects the audio coupling circuit 50 for
the called station to the proper link through audio switch 52.
Two-way conversation is then effected between the calling sta-
tion and the called station.
A schematic of the audio coupling and off-hook de-
tector 40 of the station controllers 30 is illustrated in
Fig. 3. The tip and ring lines are connected to split wind-
ings of a transformer 90 through relay contacts 92. Current
flow then occurs from the audio ground AG through windings
90a, the telephone station 10, the windings 90b to audio
battery AB, normally -24 volts, through resistors 9~, 96.
When the handset 16 for a telephone station lO is on-hook, the
tip and ring lines are not connected to each other so that no
current flows from audio ground to audio battery. When the
handset 16 goes off-hook, the impedance between the tip and
ring lines drops substantially so that sufficient current
flows from the audio ground AG to audio batter~ AB to generate
enough voltage across re~istor 96 to forward bias the base
emitter junction of transistor 98. Current then flows from
audio ground to audio battery through resistor lO0 and a light
emitting diode 102. Light from the light emitting diode 102
is directly coupled to a phototransistor 104 which then con-
ducts current from ground to a -12 volt supply to resistor
104. Under these circumstances OFFHOOK goes low, and it is
this OFFHOOK signal that is applied to the NCR gates 54, 58 of
the circuit of Fig. 2~ After the audio output of the circuit
is connected to an audio link by switch 52, audio communica-
tion from the tip and ring lines is effected through the tran-
sformer 90~ A pair of series connected Zener diodes 106 are
connected across winding 90c to protect other circuitry in the
system from excessively large voltage transients.
~ith reference also, now, to Fig. 2, when the enab-
__
ling signal YCALL is received by an integrated circuit 36 for
a called station as explained above, the BELL output of the
circuit 36 goes high. Current then flows through relay coils
76. The relay coil 76 switches the relay contacts 92 between
lamp ground LG and a high voltage AC signal A.S which actuates
the ringer 12 for the called telephone station 10. When the
handset 16 for the called station goes off hook, the BELL
signal terminates causing the relay contacts 92 to return to
the position illustrated in Fig. 3, thereby allowing two-way
communication through the transformer 90.
A schematic of the address decoder 70 and "A" group
link detect and delay circuitry 72 is illustrated in Fig. 4.
As explained above, the function of the address decoder 70 and
"A" group link detect and delay circuitry 72 is to present an
___
enabling signal YCALL to the appropriate integrated circuit
36a,b at the same time during a link access period whether or
not the calling station is connected to an "A" group link or a
"B" group link. Since the address for a called s~ation is re-
ceived at different times depending upon whether the calling
station is connected to an "A" group link or a "B" group link,
it is necessary to retain the address signal from an "A" group
~13~4~
link calling station until the address iynal from a "B" group
link calling station would be received.
The address of the called station ~rom the bus 44 is
applied to the decoder 70 as SlH, S2H.~.S5H, S~XH and SXXH.
The SXX~ input designates the first digit of the called sta-
tion so that ten sequentially numbered stations having an
identical first digit will be interconnected to the sarne SXXH
bus. Thus, stations 20-29 will all be connected to the S20H
address bus. The S+XH address line designates either the low
order five numbers for the second digit or the high order five
numbers for the second digit. Thus, stations having a second
digit of from 1 to 5 are interconnected by the S~OH bus while
stations having a second digit of from 6 to O are interconnec-
ted by the S+5~ address bus. The remaining address buses,
SlH, S2H...S5H, designate the higher or lower order numbers
for the second digit. Station 23 would thus have its SXXH
input connected to the S20H bus, its S+XH input connected to
the S+OH bus and is S3H input connected to the S3H bus. Dur-
ing the link access period of the link to which the calling
station was connected, the S3H input as well as the S+20H and
S+OH inputs would go high. If the calling station is connec-
ted to an "A" group link, these buses would go high during the
first address valid period of the link access period. If the
calling station is connected to a "B" group link, these busses
would go high during the second address valid period of the
link access period. Consequently, the output of NAND gate 120
would go low, thus enabling NAND gates 122-133 through NAND
gate 132. Since NAND gate 12B is enabled, the high at its S3H
input would produce a low at the YCALL 2B input to the con-
troller circuit 36b during the link access period to which the
calling station is connected which is clocked into circuit 36b
22
by the next clock pulse F0f1. The outputs of the NA~JD gates
122~130 during an "A" group link access period are clocked to
the respective outputs of flip- flops 134 so that the outputs
presented to the circui~ 36a and they can be clocked into cir-
cuit 36a by the same clock pulse that would have clocked ~CALL
into circuit 36b. If the calling station is connected to a
"B" group link during the next clock cycle, the station ad-
dress for the called station is decoded by NAND gates 120-130
_
and applied the YCALL output.~ to the integrated circuit 36b
for connecting the called station to the "B" group audio
links. If the calling station is connected to an "A" group
audio link and dials 28, the S20H and S~5~ inputs to NAND gate
120 go high and the S3H input to NAND gate 126 goes high dur-
ing the first address valid period of the link access period
for the "A" group link to which the calling station is connec-
ted At the end of that clock cycle, the high at the output
of NAND gate 126 and the lows at the output of NAND gates 122,
124, 128, 130 are clocked to the respective outputs of flip-
flop 134. During the next address valid period, neither S20H,
S+SH nor S3H go high since the calling station is not connec-
ted to a "B" group link. If the calling station was connected
to a "B" group link, S20H, S~5H and S3H would be low during
the second address valid period of the link access period cor-
responding to the "B" group link to which the calling station
is connected, but they would be high during the following ad-
dress valid period of that link access period~ As explained
hereinafter and as illustrated in Fig. 5, the decoded called
station address is placed on bus 44 slightly in advance of
when the circuits 36a are clocked by FOH. The clock pulses,
FOH, are delayed one-quarter oE a link access period, however,
so that the circuits 36a or 36b are clocked during the middle
23
7~i
of the period that the station address is valic~ when the call-
ing station i6 connected to a link. During the previous clock
pulse, FOH, a valid station address, when the callinc~ station
is connected to an "A" group link, is clocked to the output of
flip-flops 134. Consequently, valid called station address
signals generated by a calling station connected to an "A"
group link apply an enable signal YCALL to the "A" group con-
troller circuit 36a by the flip-flops 134 at the same time
that the controller circuit 36b would receive an enable signal
. _
YCALL generated by a calling station connected to a "B" group
link.
The operation of ~he self-contained controller cir-
cuit 36, which is preferably a large-scale integrated circuit,
can best be understood by reference to the synchronous state
logic diagrams of Figs. 6 and 9~ Basically, a synchronous
state logic diagram identifies a number of system states by
rectangular blocks, decision points by diamonds and condi-
tional outputs by ovals. Each state is given a designating
letter and identified by a binary number. State assignments
are then listed on a state assignment chart in matrix form
with the corresponding state designator placed in the proper
location. The state assignment chart is then examinated to
ensure that only one binary digit changes at a time as the
system moves from one state to any other state. This assures
that the system will not enter any transient, unanticipated
state. Once the synchronous state logic diagram is generated
and states are properly assigned, logic components implement-
ing the synchronoua state logic diagram can be easily genera-
ted. In the synchronous state logic diagrams of Figs. 6 and 9
input variables are preceded by a Y or N lndicating that the
variables are either asserted high or low, respectively.
24
11.3~'~'4.~
Thus, YCALL indicates that a YCALL input will be high if the
CALL variable is asserted. Outputs bear an H or L prefix
designating whether the output is asserted by either a high or
low voltage level, respectively. Outputs are generated when-
ever the system is in a state in which the block for that
state lists the output.
A synchronous state logic diagram for determining
whether the handset 16 for a station 10 is off-hook is illus-
trated in Fig. 6. Initially, the system is in state a and all
of the flip-flops implementing the logic diagram and defining
the states corresponding to the state variables XYZ are reset.
In state "a" a RESET output is produced to prevent an internal
timer 220 illustrated in Fig. 7 (shown hereinafter) from in-
crementing and to retain all of the flip-flops in a reset con-
dition. The hook switch input YHKSWT is continuously examined
at 210 and as long as Y~KSWT is low or logic "0"~ the system
remains in a state "a" loop. When YHKSWT goes high, the sys-
tem shifts to state "b" at 202, thereby removing the RESET
output and allowing the internal timer to start counting. As
illustrated in Fig. 7, the call progress timer 220 includes a
number of cascaded flip-flops designated generally at 222
which are toggled by FLS pulses occurring once every four link
access periods. A YT1 is generated at the output of NAND gate
224 five milliseconds after the RESET input to the flip-flops
222 i5 removed, a YT2 signal is produced at the output of NAND
gate 226 one second after the RE5ET input to the flip-flops
222 is removed and a YT3 signal is produced at the output of
one of the flip-flops 222 73 milliseconds after the RESET in-
put to the flip-Elops 222 i8 removed. These timing signals
are utilized to debounce the YHKSWT input to ensure that it is
not simulated by a transient voltage level and to determine
whether a high~to-low transition of YHKSWT is either a dial
pulse the handset 16 going on-hook~
As the call progress timer ~20 continues to incre-
ment in state "b", the YT3 output of the counter 220 is con-
tinuously examined at 240. After 73 milliseconds, YT3 goes
high and YHKSWT is therl reexamined at 242. If YHKSWT is still
high, thus indicating that the original YHKSWT was not produc-
ed by a transient, the system enters state "c" at 244 in which
the call progress timer 220 is reset by the RESET output and
an off-hook tip output HOFHKT is produced. If YHKSWT was
found to be no longer high at 242, the system returns to state
"a" in which a ~ESET output is produced to reset the call
progress timer 220. The system then continues to examine the
YHRSWT outpu t at 200.
Assuming that YHKSWT is still found to be high at
242, the timer 220 is reset and the HOFHKT output is produced
in state "c" at 244, thereby indicating a valid off-hook con-
dition exists. In state "c" the YHKSWT input is continuously
examined at 246. If YHKSWT goes low again, the system enters
state "b" at 248 in which the RESET output is removed, thereby
starting the call p~ogress timer 220 and continuing to gener-
ate an HOFHKT output indicating that a valid off-hook condi-
tion continues to exist. YHKSWT can go low responsive to
either the station going on-hookt a dial pulse produced by a
rotary dialing mechanism or a transient pulse of noise on a
line. While the call progress timer 220 continues to incre-
ment, YTl is continuously examined at 250. After five milli-
seconds, ~Tl goes high, thereby causing the state of the
YHKSWT input to be reexamined ~t 252. If Y~]KSWT is still
high, the system returns to state 244 at which the timer is
reset and YHKSWT i~ reexamined at 246. YHKSWT, being high at
113~
252, indicates that the original YHKSWT low was produced by a
transient noise pulse since neither a dial pulse nor an on-
hook condition would produce YHKSWT low for less than five
milliseconds. If, after five milliseconds, YHKSWT is still
found to be low at 252, the system switches to state "e" at
254 in which a dial pulse output HDP is yenerated and a valid
off-hook output HOFHKI' continues. The call progress timer
220, which began incrementing in state d at 248, has continued
to thereafter increment since a RESET output has not been pro-
duced during this time~ In state "e" the YT2 input is contin-
uously examined at 256. Before YT2 goes high after one sec-
ond, Y~RSWT is continuously examined at 258. If, at any time
during the one-second interval YHKSWT goes high, the counter
220 is reset at 260 and the system enters state "f" at 262.
Normally, YHKSWT will go high during the one-second interval
if YHKSWT went low at 252 by either a dial pulse or a trans-
ient noise pulse. If, however, YHKSWT went low at 252 because
the handset 16 went on-hook, YHRSWT will continue to be low
after one second as determined at 256. The system then swit-
ches to a transition state "x" at 264 in which a RESET output
is produced to reset the call progress timer 220. The system
then reverts to original state "a". The transition state "x"
is required so that only a one-state variable will change at a
time when transitioning from state "e" to state "a" as illus-
trated by the state assignment chart of Fig. 6.
When the system is in state "f" at 262 a dial pulse
output HDP and a valid off-hook HOFHKT are produced. The YTl
input is continuously examined at 266 during this period and
after five milliseconds it goes high. YHKSWT is then reexam-
ined at 268. If YHI<SWT is now low, the system reverts to
state "e" at 254. A logic high YHKSWT at 258 indicates that
27
YHKSWT must have have been produced by a transient noise pulse
since a dial pulse would last longer than five millisecond.s so
that YHKSWT would still be low after five milliseconds. In
summary, when the handset 16 goes off-hook, the system shifts
to state "c" at 244 in wait of a dial pulse or an on-hook con~
dition. Either a dial pulse, an on-hook condition or a trans-
ient noise pulse causes the system to enter state "e" at 254
in which it is determined which of these conditions produced
YHKSWT. If ~he YHRSWT low was an on-hook condi~ion, the sys-
tem reverts to state "a'1 via transition state "x" at 264; and
if YHKSWT low was a dial pulse, a dial pulse output HDP is
produced and a valid off-hook condition HOFHKT is produced for
the duration of the dial pulse generated by the dialing mech-
anism after which the system returns to state "c" at 244 in
wait of another dial pulse or an off-hook condition.
One implementation of the synchronous state logic
diagram utilizing standard logic circuits is shown in Fig. 8.
A synchronous state logic diagram for a station se-
quencer portion of circuit 36 is illustrated in Figc 9. Each
circuit 36a or 36b controls access to only four audio links,
either the "A" group links or the "B" group links. The system
is initially in state "a" at 310. Assuming that the station
is idle and no calls are coming in, YCALL is found to be low
at 312, YFLAS~ is found to be low at 314, YOFHKT is found to
be low at 316 and YHKSWR is found to be low at 318 so that the
system remains in state "a". When the station handset 16 is
taken off-hook, YOFHKT goes high, as explained in reference to
the synchronous state logic diagram of Fig. 5, causing the
system to examine YSYBS at 320. As explained hereinafter,
YSYBS is generated whenever all of the audio links are busy.
It operates independently and as~ynchronously of the station
28
~13~
sequencer of Fig. 9 and examines the status of all of the
links to determine if any links are available for access by
the station controller. As explained above, a high is genera-
ted on the LKAHA line during any link access period in which
its corresponding "A" group link is available. Similarly, a
high is produced on the LKAHB line during any link access per-
iod when its corresponding "B" link is available.
A circuit 330 for generating YSBYS whenever none of
the audio links in the system are available is illustrated in
Fig. 10. A logic high present on the data input to flip-flop
332 is clocked to its ~ output by an FLS pulse which occurs
during each link access period for link lA and lB. If one of
the four audio links to which the controller circuit 36 is
connected is available, YLKA will be high for at least one of
the link access periods, causing flip-flop 332 to be reset be-
fore the high on the data input is clocked to its Q output by
the next FLS pulse. Thus at the end of four link access peri-
ods the output of the flip-flop 332 is clocked to the output
of flip-flop 334 by the subsequent FLS pulse. If YLKA reset
the flip-flop 332 during any of the link access periods before
the FLS pulse clocks flip-flop 334, a logic low at the output
of flip-flop 332 is clocked to the output of 334. If YLKA has
not gone low during any of the four link access periods prior
to the FLS pulse clocking the flip-flop 334, the logic high
which was previously clocked to the output of flip- flop 332
is clocked to the output of flip~flop 334. It should be re-
membered that one system busy circuit 330 is provided for each
circuit 36 controlling either the "A" audio links or the "B"
group audio links. As explained above, if YSYBS exists for
the circuit 36 controlling the "A" group audio links, the cir-
cuit 36b examines LKAHB in search of an available "B" group
29
- 1~3~i7~
link. If system busy circuit 334 circuit 36b controlling the
~'s" group audio links is unable to f ind an av~ ble "B " group
link, a system BUS~ signal is produced which causes a busy
signal to be tramsitted to the ear piece of the calling sta~
tion.
With reference back to the station sequencer syn-
chronous logic diagram of Fig~ 9, if all of the audio links
are busy, the sys~em enters state "k" at 340 in which a ~USY
output is produced. YOFHKT is continuously examined at 342 in
state "k", causing ~he system to remain in state "k" as long
as the handset for the calling station remains off-hookO When
YHKSWT (Fig. 6) goes low responsive to hanging up the handset,
a logic low YOFHKT occur causing the system to return to state
"a" at 310.
If that at least one audio link is available when
the YOFHKT goes high, the system examines YLKA at 344 until
all the link access period corresponding to the available link
is reached at which time YLKA (the LKAHA or LKAHB inputs to
circuits 36a,b, respectively) goes high. The system then
sequences through states "f", "g", "h" and ~i" in blocks 346,
348, 350, 352, respectively. In all of these states 346-352
an HBI output is produced to indicate that the controller cir-
cuit 36 is busy. In state "i", ~OFHKT is examined at 354. If
YOFHKT is high, the system produces an HSTAB output at 356 and
continues to sequence through states "f", "g", "h" and "i"~
The HSTAB output is connected to the gate of a field
effect tran~istor (FET) 360 illustrated in Fig. 11. The drain
of the FET is connected to the LKAHA output of the controller
36 and, in its open drain condition, is normally held at
ground by resistor 362. Whenever the ~ISTAB output is produced,
the LKAHA bus is pulled low during the link access period to
~3~'74~
indicate that the station controller is connected to the audio
link corresponding to that link access period. A different
circuit of the same configuration is used to yenerate a ring
acknowledge signal RAKL from an HRAK signal.
Referring back to Fig. 9, an internal flag is also
reset at 356 whenever the station sequencer has followed the
above-described loop, thereby indicating that the station has
been off-hook and connected to an audio link. The system, in
states "f", "g", "h" or "i", also produces a LINK signal which
enables an analog s~itch control illustrated in Fig~ 12. With
reference now, also, to Fig. 12, two state variables "A" and
"Ct for the four states "f", "g", "h" and "i" are decoded by
NOR gates 370-376 that feed the data inputs of respective
flip flops 378-384. Thus, a high is applied to flip-flop 384
in state "f" since state variable "A" and state variable "C"
are both logic low during this state but not in states "g",
"h" or "i". Similarly, flip-flop 378 receives a logic high
from NOR gate 370 in state i ~ince state variable "A" is low
and state variable "C" is high during this state. The flip
flops 378-384 are clocked by the FLS pulse which marks the
beginning of the link scannin~ cycle. Thus the Q outputs of
the flip~flops 378 indicate which of the four sta~es "f", "g",
"h" or "i" that the station sequencer was in at the time the
link scanning cycle began. This provides demultiplexed in-
formation identifying which audio link the station is connect-
ed to in order to connect the output of audio coupling circuit
50 to the proper link by switch 52 (Fig. 2). The outputs of
flip-flops 378-384 are connected to the switch outputs of cir-
cuit 36 through respective inverters 386.
To understand the manner in which the switch control
circuit of Fig. 12 identifies which link has been accessed, it
31
~13~; ~4~L
is important to follow the timing seq~ence of the system as it
cycles through states "f", "g", "h" and llj,11. Assume for pur-
poses of illustration that YLKA is determined to be high at
334 during the link access period for the third audio link
(either link 3A or link 3B). The system then enters state "f"
at 346 during the fourth link access period, state "y" at 348
during the first link access period, state "h" at 350 during
the second link access period and state "i" at 352 during the
third access period. As the system remains in the loop con-
tinuing to sequence ~hrough states 'If-i" the sequencer always
enters state i during the third link access period. Conse-
quently, when the flip-flops 378-384 of Fig. 12 are clocked by
the FLS pulse occurring during the first link access period,
the system will be in state "g" at 348 so that only NOR gate
374 will be producing a high output. This high output is
clocked to the output of flip flop 382 to drive analog switch
No. 3.
Referring back to the synchronous state logic dia-
gram of Fig. 9 for the station sequencer, the station contin-
ues to sequence through states "f", "g", "h" and "i" as long
as YOFHKT remains high as determined at 354, thereby indica-
ting an off-hook condition. When the station goes on-hook
again, YOFHKT goes low. The station sequencer, in state "i",
then examines an internal flag at 400. Since the station has
been off-hook and on a link, the flag was reset at 356 so that
the station shifts to an OFF condition, state "a".
The sequence for receiving an incoming call as ex-
plained with reference to Fig. 9. If a called station is
idle, it will be in state "a" at 310. An incoming call is in
dicated by a high logic level for YCALL multiplexed in the
link access period corresponding to the link to which the
32
calling station is connected (it being remembered that the
decoder 70 and "A" link delay and detect circui~ 72 of Fig. 2
caused the enabling signal YCALL to be presented to the cir-
cuit 36 at the same time regardless of whether the station
address was valid during the first part of the link access
period because the callng station was connected to an "A" link
or during the second part of the link access period because
the calling station was connected to a "B" link). The station
sequencer recognizes the YCALL high signal at 312 causing the
system to sequence through states "b", "c", "d" and "e" at
402, 404, 406 and 408, respectively. In state "e", the se-
quencer will be synchronized with the incoming call so that a
ring acknowledge pulse HRAK is generated during the link ac-
cess period corresponding to the link to which the calling
station is connected. In this regard sequencing through the
states "b-e" to synchronize the HRAK output with the link ac-
cess period of the calling station is similar to the manner in
which sequencing through states "f-i" synchronizes the LINK
output in state "i" to link access period for the first
available link. Thus, if a call is received on link 2 the
system enters state "b" during the third link access period,
state "c" during the fourth link access period, state "b"
during the first link access period and state "e" during the
second link access period.
The HRAK output produced at 408 controls an FET 360
(E'ig. 11) having an open drain which is normally pulled high
through resistor 362. However, the drain of the FET 360 is
pulled low during the link access period corresponding to the
link to which the calling station is connected by the HSTAB
signal which is produced during that time. ~s explained
above, the ~AKL signal informs the calling link controller 38
3~3~;r~f~
that the called station is not busy and causes a ring-back
signal to be transmitted to the calling station.
Referring back ~o Fig. 9, the sequencer continues to
recirculate through states l'bl', I'c", "d" and "e" at 402-408 as
long as the called station remains on-hook and the calling
station remains off-hook (assuming that the called station is
not in the auto-answering mode as explained hereinafter). The
system, in states "b-e", also generates a RING output which is
applied to a ring-out circuit 420 to produce the BELL output
for use as explained in reference to Fig. 2.
As illustrated in Fig. 13, the rinq-out circuit 420
provides the option of single burst ringing depending on the
state of RC. If the station sequencer is in states "b", "c"
"d" or "e", RING is high thereby removing the reset frorn
flip-flop 422. On the next negative transition of FR, the Q
output of flip-flop 422 goes high enabling the output NAND
gate 422 causing BELL to go high. If RC is high, the high
output of flip-flop 422 is clocked into flip-flop 426 on the
next positive transition of FR which disables the output gate
so that BELL is produced for only one ringing pulse. Because
FR is used to used to clock the flip-flops 422-426, the ring
control circuit 420 will not ring out until a full FR cycle
begins, thereby disallowing the possibility of a short ring-
out pulse. If RC is low, flip-flop 426 is held reset and the
gate will be enabled as long as RING is high, thereby produc-
ing multiple ringing. As soon as RING goes low, flip-flop 422
is reset, gate 424 is disabled and BELI. drops low. Also,
flip-flop 426 is reset through NOR gate 428 and inverter 430.
When Y~KSWR i8 high, yate 424 is also disabled so that no
ringing occurs after a call0d station goe~ from on-hook ring-
ing to off-hook on an outside line not connected with the in-
tercom.
34
7~1
Returning, once again to the synchronous state logic
diagram of Fig. 9, the sequencer Will continue to recirculate
through states "b-e" with the state of YOFHKT ~eing checked at
440 each cycleO If the called station does not answer and the
calling station discon~inues the call, YCALL will go low which
will be detected at 442 to cause the sequencer to return to
the idle state, state "a". If the s~ation ans~lers, the trans-
ition of YOFHKT from low to high will be detected at 440,
causing the sequencer to sequence through states "f-i" as
explained above. The called station will now be synchronized
to the link to which the calling station is connected and its
analog switch will connect the audio of the called station to
that link. The sequencer then recirculates through states "f-
i" in the same manner as described above when the link was
accessed from the idle state "a" by the calling station.
The station sequencer also includes an auto-answer
capability which causes the called station to automatically
answer if it is not already busy. The auto-answer capability
is enabled by setting the AUTO input high by manipulating a
manually actuated switch. When the station sequencer is in
~tate "a" and the station is called, the sequencer will step
through states "b-e" at 402-408. When the sequencer is in
state "e", a HRAK pulse is sent to the link controller 38 for
the link to which the calling station is connected. Since the
called station is on-hook, J~OFHKT for the called station is
low. However, because the auto~answer feature has been enab-
led, AUTO is high as determined at 444 causing the sequencer
to transition from state "e" to state "f" even though YOFHKT
is still low. The sequencer then steps through steps "f-i" in
synchronism with the link access period for the link to which
4~L
the calling station is connected in the same manner as if the
called station went off~hook causing YOFHKT to go high. How-
ever, since the called station is still on-hook when the se-
quencer reaches state "i", YOFHKT is low as determined at 354.
Since the internal flag has not been reset at 356, the sequen-
cer then follows a different recirculation path from the re-
circulation path followed if the called station goes off-hook.
Since YOFHRT is low and YFL~G is still high as detected at
400, AUTO is high as detected at 446 and, since the calling
party is connected to the link, YLKA is low as detected at
448, causing the sequencer to recirculate to state "f" at 346.
The sequencer will continue recirculating through states "f-i"
as long as YLKA is at zero, meaning that the calling party is
still on the link. If the called station goes off-hook, the
sequencer branches from the previous recirculation path at
354, thereby resetting the internal flag at 356 and causing
LKAH to be pulled low through the FET 360 (Fig. 11). The
called station now has control of the link to which the call-
ing and called stations are connected. If the called station
subsequently goes on-hook/ YOFHKT goes low. However, since
the flag has been reset at 356, the station then transitions
through 354 and 400 to state "a", the idle state.
The YHKSWR input at 318 allows the station to be put
into a state that will not produce a ring acknowledge. Start-
ing from state "a" at 310, if the calling station is not off-
hook on the intercom line, YOFHKT is low and the YHKSWR input
is also low. However, if a station goes off-hook but is not
connected to an intercom line, YOFHKT remains low but YHKSWR
goes high. Since YOFHKT has been determined to be low at 316
and YHKSWR has been determined to be high at 318, the system
shifts to state ";" at 450. The sequencer remains in this
36
JA~
inactive state as long as YHKSWR is found to be high a~ 452,
resulting from the station remaining off-hook, and YOFHKT is
found to be low at 454, resulting from the station not being
connected to an intercom line. If the station goes on-hook, a
YHKSWR low is detected at 452 to return the sequencer to state
"ai' at 310. Similarly, if the station becomes connected to an
intercom line while ofE-hook, a YOFHKT high is detected at 454
to transition the sequencer first through state "a" at 310 and
then immediately to 344 through 312, 314, 316 and 320. The
inactive state "j" at 450 prevents the station from re~urning
a ring acknowledge if the station happens to be addressed by a
calling station. Since a ring acknowledge RAKL is not produc-
ed by the called station, the calling station will not receive
a ring-back. Thus, when a station is connected to an outside
line it will not produce an audible ring when the station is
selected by a calling station.
As mentioned above, implementation of the station
sequencer once a synchronous state logic diagram and a state
assignment chart has been prepared is fairly straightforward.
One implementation of the station sequencer of Fig. 9 is shown
in the scbematic of Fig. 14. The efficiency of a logic cir-
cuit produced from a synchronous state logic diagram is rela-
tively high but, unfortunately, the apparent complexity of the
logic circuit implementation of the synchronous state logic
diagram resulting from that efficiency limits the ability to
easily and quickly understand a system by reference to the
logic circuit implementation. For this reason the system has
been explained with reEerence to the synchronous state logic
diagrams of E'igs. ~ and 9 instead oE by reference to the logic
circuit implementations of E'igs. 8 and 14.
74~
The primary purpose of the link controllers 38 is to
facilitate and control the process of making an intercom call.
It decodes dialing signals from a dual-tone multi-frequency
dialing mechanism or dial pulses from a rotary dialing mechan-
ism, sends station address signals to the signal generator 42
via a time division multiplexed bus 40 to designate a called
station and applies call progress tones to its audio link.
A block diagram for four link controllers 38 which
control the operation o~ audio links lA, 2A, lB and 2B is il-
lustrated in Fig. 15. Four additional link controllers 38
control the operation of audio links 3A, 4A, 3B and 4B. When
the station controller 30 for a calling station finds an
available link, it drives the LKAH bus low. Assuming that an
"A" link is found to be available, L~AHA goes low during the
link access period corresponding to the available link. Assum-
ing that the available link is lA, LKAH high during the first
link access period is demultiplexed at 510 to continuously
present a LKAH signal to a link controller circuit for link lA
which, as explained hereinafter, is primarily a microproces-
sor. The link demultiplexing signals C1 and C2 are generated
by synchronizing clock generator 512 which is clocked by the
clock frequency 2FO~ and is timed by the signal Fl-2H as
illustrated in Fig. 17. Cl and C2 demultiplex DPH, LHAH and
RAKL by clocking the demultiplexer 510 at Tl and T5. The same
synchronized clock generator also generates the multiplexer
control signal, A and B. These are binary control signals
which control the station address multiplexer 570, 572, etc.
The multiplexers serve to place the ~tation address generated
by the link controller circuit~, 520, on the ~tation address
bus, 40, during the ~tation addre~s time slots as indicated in
Fig. 17. Another ~ynchronlzed clock generator 512 is enabled
by F3-4H, the inverse of Fl-2H, to control the demultiplexers
38
~L~36~1
510 for links 3A-4B. The microprocessor 520 then generate~ a
DTRlA output to indicate that a station is connected to the lA
link and is ready to dial a called station. A scanner control
522 driven by Fl-2H causes a number of single pole, 5-thro~
switches 524, 526, 528, 530 and 532 to sequentially connect a
single input or output line to one of several input or output
lines. The primary purpose of the scanner is to allow four
link controllers to share a single dual-tone multi-frequency
deeoder. Switch 528 sequentially scans the DTR outputs of the
link controller circuits 520. As scanning continues, the
switch 528 ultimately connects the DTRlA output of the link
controller 520 to the scanner control 522 to stop the scan at
a position corresponding to audio link lA. Audio link lA is
then connected to a conventional multi-frequency tone decoder
540 through switch 524 which generates BCD data on four line
bus 554 identifying the number corresponding to the tone com-
bination on audio link lA. Link controller circuit 520 then
records the first digit in memory and waits for the second
digit to be decoded by the decoder 540 and conveyed to the
circuit 520 through bus 540. When the data are presented to
the output of the decoder 540, the decoder 540 also generates
a data valid strobe which is seleetively coupled to only the
link eontroller circuit 520 for the lA link by the switch 526.
Thus, even though the data from tone decoder 540 are trans-
mitted to all link controllers lA-2B, only the link controller
eircuit for the lA link 520 responds to these date. This
strobe signal also resets a call progress timer whieh frees
the decoder 540 from a link if a number is not dialed within a
predetermined period.
If the ealling station i~ equipped with a conven-
tional rotary dial meehanism, dial pulses DPLA or DPLB are
39
113~41
applied to the link controllers 520, 521 or 523, 525, respec~
tively, which counts the number of pulses for each digit to
determine the address of the called stati~n.
When the scanner control 522 stops scanning, a DTRlA
signal is applied to a switch 527 ~hrough analog switch 529 to
connect a dial tone to the audio link 1~ indicating to the
calling station that a station should be selected on its dial-
ing mechanism.
As mentioned above, the link controller circuits
520, 521, 523, 525 for four links lA-2B all utilize a single
tone receiver 540. Consequently, it is desirable for the sys-
tem to free the tone receiver 540 for use by other link con-
troller circuits after it has been used by each link control-
ler circuit to decode a called station address. Whenever the
last digit has been dialed on a station connected to link lA,
lALDR goes high which is detected by OR gate 560 and applied
to the scanner 522 through switch 530 to restart the scanning
operation. Similarly, whenever a dial pulse is generated by a
station controller, indicating that the station is not
equipped with a dual-tone multifrequency dialing mechanism,
lADPH goes high thereby applying a restart signal to scanner
control 522 through OR gate 560 and switch 530.
The first and second digit addresses for the called
station are presented to respective multiplexers 570, 572
which apply the address signals to bus 40 when the A and B
outputs of the synchronous clock generator are both logic "O",
which corresponds to link lA. As explained above, the signal
generator 42 then places appropriate signals on bus 44 to en-
able the station controller 30 for the called station during
so that the called station connects itself to the lA link.
Although the link controller 3~ has been explained only for
~3~1
link lA, it will be unders~ood that links 2A, lB and 2e opera-
te in the same manner. Links 3A, 4A, 3B and 4B also operate
in a similar manner except that synchronized clock generator
512 and the scanner controller 522 receive ~3-4H which i5 the
inverse of Fl-2H.
With reference now to Figs. 16 and 17, it will be
noted that there are eight links. Multiplexing for half of
the links, lA-2B, is done by four interconnected link control-
lers while multiplexing for the remaining four links is accom-
plished by four other interconnected link controllers. When
Fl-2~ goes high, flip-flop 570 is enabled through resistor 572
which is clocked to the Q output by a 2FOH pulse at Tl (Fig.
17). Counter 574 is then enabled by the low at the Q output
of 570 so that it is incremented by the 2FOH pulse train.
Counter 574 is thus clocked through four states. After Fl-?H
goes low, flip-flop 570 is clocked low thus resetting the
counter 574. The outputs of the counter 574 drive four multi-
plexers 576, 578, 580, 582 which apply the called station ad-
dresses at the outputs of link controllers 520 to the bus 40
during the station address valid periods of each link access
period corresponding to the eight links. It should be men-
tioned that the outputs of counter 574 are the A and B outputs
of the synchronized clock generator, 512, of Fig. 15. Note
that the system clock FOH leading edge at T4 occurs in the
middle of the pulse from the QO output of flip-flop 574. This
signal coincides with the station address valid periods shown
in Fig. 5 since it is this signal which gates the station ad-
dresses onto the bus 40.
LDRH signals from the signal generator 42 are multi-
plexed in the same time slots as the address signals for the
called station. Thus, LDRH for link lA occurs during the link
~1
~ ~L3~r,~4~L
access period for link lA. The mul~iplexed LDRH bus is ap-
plied to the data input of latch 590 ~hrough resistor 592.
Sampling pulses developed by I~AND gate 5g4 are applied to the
write disable input of the latch 590. The multiplexed LDRH
signal for link access periods lA, lB, 2A, 2B are applied to
the outputs of latcn 590 at Q0, Ql, Q2, Q3~ It should be
mentioned that latch 590 and associated circuitry correspond
to the demultiplexer 532 of the block diagram of Fig. 15.
The DPL, LKAH and RAKL signals are multiplexed on
their respectives buses so that they only occur during the
link access period corresponding to the link to which the
calls or calling station is connected. These periods are Fl
and F2 in Fig. 17. These signals for the "Al' audio links are
demultiplexed at 620, 622 and for the "B" audio links by
demultiplexers 624, 626. Thus, whenever, for example, a ring
acknowledgment is produced hy a call station connected to the
lA bus a demultiplexed RAKLA signal from demultiplexer 620 is
continuously presented to the microprocessor 520.
As explained above, the link scanner looks for a
dial tone request from the link controller circuits 520, 521,
523, 525 (Fig. 15). When it notes a DTR high, it connects the
audio link which is associated with the link processor circuit
generating the DTR signal to the tone decoder 540. The rone
receiver is disconnected and scanning is resumed if 10 seconds
pass without the calling station dialingl rotary dial pulsing
or hook flash is received, the calling ~tation hangs up or a
dual tone multi-frequency dialing sequence is completed. A
scan counter 700 (corresponding to scanner control 522 of Fig~
15) is clocked by Fl-2 from enabled NAND gate 702. It then
causes the switches 524, $29, 530, S26, S28, 710, 712 to se-
quence through each of their ~our positions. Thus, if a dial
42
~ 3~4~
tone request ~TR is genera~ed by link 2A, flip-flop 71S is
clocked high. When counter 700 is in the one state, switch
528 passes the high from flip-flop 716 to the data input of
flip flop 718. Since counter 700 is clocked on the negative
edge while flip-flop 718 is clocked on the positive edge,
flip-flop 718 is clocked high one-half of a clock cycle later.
The low at the Q output of flip-flop 718 thus disables NAMD
gate 702 so that counter 718 no longer increments from being
clocked by Fl-2H. The high at the Q output of flip-flop 718
is fed through switch 710 to OR gate 720 which resets flip-
flop 716. However, the logic low at the ~ output of flip-flop
718 continues to disable NAND gate 702. Thus, the scanner
stays locked in counter 700 01 position until flip-flop 718 is
subsequently reset as explained hereinafter. Flip-flop 718
causes switch 524 to connect the tone receiver to audio link
2a. Dial tone is also applied to audio link 2A by switch 529
and 730 whenever a dial tone request DTRA is received.
The logic high clocked to the Q output of flip-flop
718 produces a logic low at the reset terminals to counter
724, 726 through enabled NAND gate 728 permitting the counters
724, 726 to increment. After 10 seconds the Q6 output of
counter 726 goes high thereby resetting flip-flop 718 to re-
sume scanning, and set flip-flop 84- through switch 712. If
flip-flop 740 is set, a busy signal is applied to the audio
link as explained hereinafter and future dial tone requests
would be inhibited by a logic high applied to OR gate 720
which would maintain flip-flop 716 reset.
Each tlme the tone receiver 540 (E'ig. lS) decodes a
dual tone multifrequency dialing input, a strobe signal is
generated which is applied to the appropriate link controller
circuit 520 throuyh switch 526~ The strobe pulse is also in-
43
~3~
verted at 750 and applied ~o the reset terminals of the coun-
ters 724, 726 through NAMD gate 728, thereby reinicializing
the 10-second timeout. Thus, as long as the calling station
dials a number within 10 seconds after a dial tone request is
received at flip~flop 716, the scanning circuit remains set to
the 2A link. If, however, a number is not dialed within 10
second, scanning resumesO
After the dialing sequence is completed, a 2ALD~H
signal is generated by the signal generator which resets flip-
-flop 718 through OR gate 560b and switch 530. The reset ter-
minal to flip-flop 718 is normally held at logic high since
resistor 746 is connected to ground. Scanning thus continues
until another dial tone request DTRH is received. If the
scanner is stopped, a hook flash, hang-up or rotary dialing
pulse will also reset flip-flop 718 and restore scanning res-
ponsive to a signal generated by the microprocessor. The
circuitry for audio links lA, lB and 2B operate in the same
manner as the circuitry for audio link 2A.
NOR gates 760, 762 control the application of a busy
tone to link "A", and identical circuits to other links apply
the busy signal to those links. NOR gate 760 enables NOR gate
762 whenever 10 seconds have elapsed since a number is dialed
by a calling station. NOR gate 762 is also enabled by an out-
put of processor 520 through NOR gate 760 whenever a station
connected to a given link is busy. Analog switch 770 connects
the RINGBACK signal to the audio link under the control of
processor 520 whenever a ring acknowledge is transmitted by
the called station. Other equivalent groups of components
have performed similar functions for other links.
The link controller microprocessor 520 is reset by
the demultiplexed LKAHA signal on its P0 pin. Thus, the cir-
~4
~ ~ ~t~7 ~
cuit 520 is re~et when the link with which it is associated isnot accessed.
The system al50 includes circuitry for causing a
given link to simulate busy to facilitate testing of other
links. Opening a busy out switch 780 when the link is not be-
ing accessed by a station generates a logic high at the output
of NOR gate 782 ~hich saturates transistor 784 through resis-
tor 78Ç during the link access period corresponding to that
link. Since the collector of transis~or 584 is connected to
the LKAHA buss, LKA~A will be pulled low durng the link access
period, thus simulating that the link is already connected to
a station. Circuit 520 then assumes that its link is busy and
it eauses transistor 788 to periodically saturate through re-
sistor 790 thus periodically illuminating LE~ 792 through re-
sistor 794. Thus, if one desires to test the link controller
for link lB, all busy out switches except one for link lB are
opened so that the only available link is link lB.
A summary flow chart for the operation of the link
circuit microprocessor 520 is illustrated in Fig. 18. When a
station accesses the link, the microprocessor comes out of
reset and is initially in the dial tone state "1" at 812.
The system can leave the dial tone state "lil in a
number of modes. The systen, can be flashed from the dial tone
state "1" to the inactive state ll0n. Generally, however, the
system leaves state "1" by either actuating the rotary dialing
mechanism or the tone dialing mechanism. After each digit is
dialed, the system determines whether that digit was the last
to be dialed at 814. On all digits but the last digit the
system then enters state "2" at 816 in which it waits for
another digit to be dialed. When the next diyit is dialed,
the system determines whether that digit wa~ the la.st digit to
3~741
be dialed at 818 and, if not, remains in state "2" at 816.
When the last digit is dialed, ~he systern determines
whether the called station is busy at 820. If the called sta-
tion is not busy, the system enters the ringback state "4" at
822 in which a ringback tone is transmitted to the calling
station and a ring is produced at the called station. If the
called station is busy~ the system enters BUSY state "3" at
824. If a "camp-on" switch has been actuated, the system wiL1
automatically transition to the ringback state "4" at 822 when
the called station is no longer busy. If the camp-on feature
has not been enabled, the system returns to state "3". There-
after, state "3" can be left only by hanging up or by genera-
ting a hook flash which either returns the system to state "0"
or state "1" depending upon whether more than one station is
found to be on the link at 828. Thus, if two stations are
conversing on the same link in state "0" and one of the sta-
tions flashes to enter the dial tone state "1", and, then,
after the called station is dialed, the station is busy, the
system can be flashed out of state "3" back to state "0" so
that two-way communication between the two stations can once
again occur. However, if only one station is on the line, the
system transitions to dial tone state "1" so that either an-
other station can be dialed or the calling station can go
on-hook to return to state IlOn.
The system leaves ringback state "4" in one of three
modes. First, the called station can answer, in which case
the system returns to the inactive state "0" to allow two-way
communication between the calllng and caLled stations. Alter-
natively, the calling station can produce a hook fla~h. If it
is determined at 828 that there ar0 two stations on the link,
the system returns to the inactive state "0" 80 that two-wa~
46
, .
~:3~
communication between the two stations can occur~ If only one
station is on the link, the system transitions to dial tone
state "1" at 812 so that the calling station can call a dif-
ferent station.
Operation in the inactive state "~" is described as
follows: If a conference enable switch has been activated,
the system will transition to a dial tone state at 81~ when-
ever a hook flash occurs in state 810 If the conference en-
able feature has not been enabled and there is more than one
station connected to the link, the system returns to the in-
active state at 810. This is because conference enable has
not been actuated indicating that only two stations may be on
a link at the same time. If it is determined that two sta-
tions are, in fact, on the link at 816, then the system should
return to the inactive state 810 to allow two-way communica-
tion between the two stations. If, however, only one station
is on thé link, 816 transitions the system from state "0' to
state "1" at 812~ Thus, if two stations are communicating in
state "0" and conference enable is set, the dial tone state
may be accessed by generating a hook flash. Otherwise, the
dial tone state can only be entered at 812 when one station is
on the link.
If a station hangs up in any state, the system will
remain in that same state. If, however, that station was the
last to hang up, LKAH will go high, thus resetting the micro-
processor, so that when the link is again accessed, the system
will begin operation in the dial tone "1l' state, B12.
A flow chart for implementing the flow chart summary
of Fig. 8 is illustrated in Fig. 19. The flow chart requires
fairly straicJhtforward progra~ing to generate appropriate in-
structions dependinc3 upon the specific model microprocessor
47
~L~L3~
employed. Such programminy task can easily be accomplished in
under six man-months.
A block diagram of the signal generator 42 (Fig. 1)
is illustrated in Fig. 20. Basically, the signal generator
decodes dual-tone multi-frequency dialing signals from the
link controllers 38 and it generates a number of signals which
are used by the remainder of the system. The address of the
called station for the first digit is conveyed to a decoder
900 via a four line bus 40 (Fig. 1), and the second digit of
the called station is connected to a second decoder 902 and to
an OR gate 904 through four line bus 40. As explained above
in reference to Fig. 15, the addresses of the called station
for the fixst and second digits are time multiplexed. Thus
the sigals on bus 40 during a given link access period are
generated by the station connected to the link corresponding
to that access period. Decoder 900 converts its binary input
to turn on one of eight output lines designated S20H-S9OH and
generalized to be SXXH. Decoder 902 is a bi-quinary decoder
in which its BCD input actuates one of five lines SlH to S5H
and either S+OH or S+5H depending upon whether the outputs SlH
-S5H designate the low order numbers of a digit or the high
order numbers of a digit. Thus, if the second digit is 8,
S+5H and S3H go high whereas if the second digit is 2, S+OH
and S2H go high. The S+OH and S+5H lines are designated in
the station controller schematics as S+XH whereas the SlH-S5H
outputs are specifically identified. OR gate 904 determines
when a second digit is received to generate an LDR pulse which
informs the link controllers that the last digit has been re-
ceived by the signal generator. rrhe LDR signal is, of course,
time multiplexed so that it provides last digit information to
all of the link controllers. LDR could be generated after any
48
7~
number of digits to provide for special dial cocles or gr~ater
length codes for equipment sizes greater than 80 stations.
I~he signal generator 42 also includes a tone and
clock generator 910 which has an internal clock and generates
BUSY, RINGBACK and DIAL TONE signals which are selectively
applied to the audio links as explained above, various clock
signals FLS, 2FO~I, FOH, Fl-2H and F3-4H which are shown in the
timing diagrams of Figs. 5 and 17. rrhe circuit glO also gen-
erates a ring frequency signal FR which is utilized as ex-
plained above.
Additional details of the signal generator are il-
lustrated in the schematic of Fig. 21. The system includes a
3.59 mHz clock circuit 940 which drives a 12-stage, divided by
4094 counter 942. Various outputs of the counter 942 are used
to generate 14, 56 and 112 kHz outputs. 2FOH is taken
directly from the 112 kHz output and FOH is produced directly
from the 56 kHz output. FLS is generated by decoding other
outputs by AND gate 944 through inverters 946, 948. FLS is a
relatively short synchronizing pulse which occurs once every
FOH pulse. Fl-2~ is produced directly from the 14-kHz output
of counter 942 while F3-4H is produced by inverter 950 as its
inverse.
As explained above, the time multiplexed first digit
signals from the link controllers are applied to decoder 900
through resistors 960. The bus 40 over which the signals are
received is normally held low by resistors 962.
Similarly, time rnultiplexed second digits are coup-
led to decoder 964 through resiskors 966~ These lines are
floating, but normally held low by re~istors 968. Decoder 964
is identical to decoder 900, but other circuitry causes it to
act as a by-quinary decoder. Accordingly, SlH is produced by
49
ANn gate 960 whenever decoder 96~ deco~es either 1 or 6 (1 +
5) the remaining OR gates 972-978 function in a similar manner
except that OR gate 978 produces an S5H output whenever a bin-
ary O or 5 is decoded by NOR gate 960 and NAND gate 982. S~5H
high is produced by OR gate 984 and 986 whenever the high or-
der bit of the binary input to decoder 964 is high or the six
or seven outputs of decoder 964 are high. Otherwlse, a low
S+5H causes NOR gate 988 to generate S+OH high whenever de-
coder 964 does not decode a zero.
The signal generator also produces a BUSY 1 output
which is applied to a link when a station is busy. A BUSY 2
output which is applied directly to the station controllers
whenever all of the links are busy. Accordingly, a 440 Hz
output of counter 1000 clocks counter 1002 which generates a
40 Hz output since the Ql, Q2 and Q4 outputs are applied to
AND gate 1004, the outputs of which reset counter 1002. The
40 Hz output of counter 1002 is further divided by counter
1006 and specific outputs of counter 1006 are combined by ~OR
gate 1008, 1010 and inverted by inverter 1012 to produce FR
which actuates a ringer 12 in the called station 10. Outputs
from counter 1006 are also applied to NAND gate 1014 to gen-
erate a system busy output FB if system busy tone enable
switch 1016 is closed. The system busy output FB is a 350 Hz
tone having a 2 Hz interruption rate. The busy signal BUSYl
for a station busy is also a 350 Hz tone generated by NAND
gate 1018, but its interruption rat~ is 1 ~z, half that of the
system busy signal E'B. A 2FE'L signal is also generated by in-
verter 1020 which has the same requency as the system busy
repetition rate.
The 0.8 second on, 2.4 seconds off signal generated
by NOR gate 1010 also gates a 440 Hz ignal through NAND gate
~L~L3~7~
,, , .. ~
1022 which is combined with a 40 E~ signal by NO~ gate 1024
and amplified and filtered at 1026 to produce a RI~lGBACK sig-
nal which is heard by the calling station when the called
station is not busy. Under some circurnstances it is not desi-
rable for the calling station to receive a RINGBACK in which
case ringback enable switch 1028 is closed.
The signal generator also produces a dia:L tone by
combining a 350 Hæ signal from counter 941 and the 440 Hz
signal from counter 1000. These signals are applied to a dial
tone amplifier and filtering circuit 1030 which produces a
DIALTONE which is applied to the link to which a calling
station is connected when an available link is siezed. Under
some circumstances it is not desirable for the calling station
to receive a dial tone, in which a dial tone disable switch
1032 is closed.
The inventive system has been described herein as
utilizing two self-contained controller circuits for each sta-
tion, each connectable to four audio linXs, in order to access
eight links. It will be understoodr however, that the inven-
tive concept applies to the use of more than two controller
circuits in each station controller to expand the number of
links in the system. Also, two or more self-contained con-
troller circuits which are connectable to more than four links
may be used to expand the number of links in the system.
51
