U.S. patent number 3,825,897 [Application Number 05/399,833] was granted by the patent office on 1974-07-23 for transmitter control circuit for alarm system.
This patent grant is currently assigned to Electronic Surveillance Corporation Limited. Invention is credited to Richard A. Lawton.
United States Patent |
3,825,897 |
Lawton |
July 23, 1974 |
TRANSMITTER CONTROL CIRCUIT FOR ALARM SYSTEM
Abstract
In an alarm system having a plurality of stations connected to a
single line, where each station has a transmitter which can
transmit a pulsed signal to the line which is indicative of an
alarm event at that station, it is important that "collisions"
between signals from different stations on the same line be
avoided. Line lock-out means are provided at each station to
preclude transmission of a pulsed signal from that station when a
pulsed signal from another station connected to the line is sensed;
and the line lock-out means is provided with a timing circuit which
operates to permit transmission from that station after a
predetermined period of time following the last sensed pulse from
another station on the line. Thus, the station may immediately
transmit on the event of an alarm if sufficient time has passed
since the last time a pulse from another station was sensed on the
line; or the transmission is held off until that predetermined
period of time follows the last sensed pulse on the line from
another station. However, in the event of continual sensing of a
line busy condition because of an intermittent fault on the line or
a "runaway" transmitter at another station, each station will seize
the line after a predetermined length of time has passed since the
event of an alarm at that station, regardless of the condition of
the line. Thus, the line lock-out circuitry will preclude
transmission of a signal from a station for about 5 seconds after
sensing any pulse from another station, but the line may be seized
after 5 seconds from the last sensing of a pulse from another
station. A 2 minute "override" timer operates to seize the line
regardless of its condition, two minutes after an alarm event at
that station, if the alarm signal has not already been
transmitted.
Inventors: |
Lawton; Richard A. (Don Mills,
Ontario, CA) |
Assignee: |
Electronic Surveillance Corporation
Limited (Willowdale, Ontario, CA)
|
Family
ID: |
23581143 |
Appl.
No.: |
05/399,833 |
Filed: |
September 24, 1973 |
Current U.S.
Class: |
340/536 |
Current CPC
Class: |
G08B
25/04 (20130101) |
Current International
Class: |
G08B
25/04 (20060101); G08B 25/01 (20060101); H04g
003/00 () |
Field of
Search: |
;340/147LP,147R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pitts; Harold I.
Claims
What I claim is:
1. In an alarm system having a plurality of stations connected to a
single line; each said station having alarm sensing means
responsive to an alarm event, transmitter means for transmitting a
pulsed signal to said line indicative of an alarm event at that
station, means for sensing the presence on said line of a pulsed
signal from any station connected thereto, and means to preclude
transmission of a pulsed signal to said line when a pulsed signal
from another station connected thereto is sensed; the improvement
wherein:
said means to preclude transmission of a pulsed signal to said line
when a pulsed signal from another station connected thereto is
sensed, is adapted to permit operation of said transmitter means
after a first predetermined period of time following the last
sensed pulse from another station connected to said line;
and each said station further includes control and override means
which is operative for a second predetermined period of time from
the occurrence of an alarm event at a station to preclude operation
of said transmitter means at that station in the event that said
means for sensing continues to sense the presence of pulsed signals
on said line throughout said second predetermined period of time,
after which said override means is adapted to permit operation of
said transmitter means at that station after said second
predetermined period of time irregardless of the sensed presence of
pulsed signals on said line.
2. The alarm system of claim 1 where said means to preclude
transmission of a pulsed signal to said line when a pulsed signal
from another station connected thereto is sensed, includes first
gate means having a first alarm input and a second trigger input,
at least said second input having a predetermined voltage level at
which said first gate may become operative to initiate operation of
said transmitter means;
said means for sensing the presence on said line of a pulsed signal
from any station connected thereto, being connected to said second
trigger input of said first gate and to an RC timer circuit to
preclude acquisition of said predetermined voltage level by said
second input for said first predetermined period of time following
the last sensed pulse from another station connected to said
line.
3. The alarm system of claim 2 where said control and override
means includes second gate means having a first alarm input and a
second trigger input, at least said second input having a
predetermined voltage level at which said gate may become operative
to initiate operation of said transmitter means; and an RC timer
circuit connected to said second input and adapted to start timing
said second predetermined period of time upon the occurrence of an
alarm input signal at the first input of said second gate.
4. The alarm system of claim 3 where each first input of each of
said first and second gates is connected to the other, and each is
connected to a bistable latch adapted to switch to a first state
and to pass an alarm input signal to said first inputs upon the
occurrence of an alarm event at that station; and said bistable
latch is connected to a circuit associated with said transmitter
means to switch said latch to a second state and to remove said
alarm input signal from said first inputs after said transmitter
has transmitted a pulsed signal to said line indicative of said
alarm event.
5. The alarm system of claim 1 where said means for sensing the
presence on said line of a pulsed signal from any station connected
thereto, includes RC filter networks to form a passband only for
signals on said line of the pulse duration and frequency which are
indicative of an alarm event at a station connected thereto.
6. The alarm system of claim 4 where said second input of said
first gate is connected to the output of each said first and second
gated through inverter means so that said first gate is operative
to permit continued operation of said transmitter means when
signals therefrom are sensed on said line, while an alarm signal
continues to appear at the first input of said first and second
gates.
Description
FIELD OF THE INVENTION
This invention relates to an alarm system, and particularly a
control circuit for an alarm system which is used in multistation
systems where a plurality of stations is connected to a single
line. The invention is particularly adapted to alarm and
supervisory scanning transmitters of the sort which are installed
in buildings such as offices, warehouses and factories and which
may supervise or scan one of a plurality of conditions within that
building and be such as to generate an alarm condition in the event
of an abnormality of any condition being supervised or scanned. The
invention provides means for precluding collision of signals from
one station with signals from another station connected to the same
line; and the invention further provides override means by which
the line may be seized by a station having an alarm status after a
predetermined passage of time.
BACKGROUND OF THE INVENTION
There are many systems of supervising and scanning alarm systems
which may have a number of stations or subscribers all connected to
a single line. For example, a number of subscribers in a particular
geographic area of a city may be connected to a single telephone
line which is terminated at a central office. The central office
may have a considerable number of lines under its supervision, and
would be of the sort operated by businesses which operate
supervisory or surveillance of other businesses, private homes etc.
In any one of the subscriber stations which is connected to a line,
there may be supervisory and scanning apparatus which maintains an
electrical or electronic surveillance of conditions with respect to
many possible sets of alarm circumstances -- such as pressure drop
in a sprinkler system, burglar intrusion, water temperature drop in
a sprinkler system or temperature drop within a building such that
the sprinkler system might freeze, unauthorized entry into security
areas of a building, etc. Generally speaking, each of these sets of
circumstances may be electronically or electrically supervised by a
master board which scans the steady state condition of electrical
signals from a plurality of sensors which would be installed and
connected for that purpose. In the event of an interruption of the
steady state condition of any one of those signals, an alarm event
would be assumed, the master board would initiate an alarm status
signal and an alarm and supervisory scanning transmitter would
generate the signal and transmit it to the line. It should be
remarked, of course, that the above description and sequence may
vary from one type of installation of an alarm system to another,
but in any event a transmitter which is installed at the premises
of the subscriber acts to transmit a signal to the line which is
shared with a number of other subscribers so that the supervising
central office can be made aware as promptly as possible of the
alarm event.
It may happen, however, that at the time that an alarm event -- or
an assumed alarm event which may be predicated by such as a
temporary drop of water pressure in a sprinkler system -- occurs,
another station or suscriber may be in the process of transmitting
a signal on the line indicative of an alarm event at that other
station. Such signals are pulsed, generally square wave, and may be
at a rate of approximately 5 to 10 pulses per second where each
pulse width is characteristic of a running pulse train of square
waves at, say, 40 cycles per second. In all such alarm systems as
the sort described above, each subscriber or station transmits
various numbers of pulses and blank spaces between pulses so as to
code the identification of the station, and in some cases so as to
code the type of alarm event that may have been noted at that
station. In any event the coded, pulsed signal from any station is
transmitted to the line by intermittently causing a short circuit
of the line to ground, each of the intermittent short circuits to
ground being distinguished as a pulse at the central station
because of the temporary collapse of the line voltage for each
pulse. On any one line, a maximum blank period between two pulses
in the same code train is set, and that maximum period of time is
the determinate factor which is sensed at each station and which
controls the access of a station to the line when an alarm event
happens at a station.
Thus, means are provided in accordance with this invention to sense
and continuously monitor the line to which the station is connected
so as to determine at any time whether the line is busy because it
has been seized by a station for the purpose of transmitting a
pulsed signal indicative of an alarm event at that station, or if
the line is free so that it might be instantaneously seized by any
station in the event of an alarm at that station. The maximum blank
time between pulses in a single pulse train is normally set so as
to be a fairly short period of time -- say in the order of 5
seconds.
It should also be noted that there is a maximum period of time over
which any station would normally transmit a coded, pulses signal
indicative of an alarm event at that station; and such pulsed
signal would normally be repeated three or four times in a single
transmission. Still further, when an alarm condition is removed or
restores to normal at a station, a reset signal would be sent to
the central office by a coded, pulsed signal, usually once or twice
in a single transmission after the station has identified itself,
so that the central office can determine the present status of the
station and any alarm condition that may have existed thereat. A
"rounds counter" is usually to be found in the alarm and
supervisory scanning transmitter at any subscriber station, which
controls the number of times that a coded, pulsed signal train is
transmitted to the line, so that the determination of alarm or
reset condition can be made. There is a maximum amount of time that
would normally be taken for transmission of any given number of
rounds of a pulsed signal in a signal alarm transmission, and that
would be the maximum amount of time that the line would normally be
busy by having been seized by a station connected thereto. Such a
period of time would not normally exceed two minutes and is usually
much shorter.
If a fault has occurred in the transmitter at one subscriber
station which is connected to a line such that the transmitter
seizes the line and continues to repeat its coded, pulsed signal
train without stopping after the appointed number or rounds -- in
other words, a "runaway" transmitter -- or because of other,
intermittent line problems, the line monitor or line sensing
circuits at each other station connected to that line would all
continue to sense a "line busy" condition. In the known alarm
systems of the type described herein, when the line is busy, each
of the other stations is held off or locked out by the station line
lock-out circuits, and no other station has access to the line.
However, because it is known that any normal transmission of an
alarm event at a station connected to a line would take a maximum
period of time, this invention provides that an override timer will
act to seize a line and cause alarm status transmission to be made
to the line even if it is continuously sensed to be busy for a
predetermined period of time after an alarm event at that station.
The override transmission control is predicated on the fact that,
even with a runaway transmitter at a first station, with several
rounds of a pulsed signal train from another station which is
indicative of an alarm event at that other station, there is not
likely to continually be collisions between the pulses from the two
stations transmitting to the line at the same time, and because of
the repetitive nature of the signal train from the runaway
transmitter, the real alarm pulses from the second or subsequent
station can be recognized and read. Also, in the event of a number
of stations all transmitting bona fide alarm signals to a single
line, at the same time, it is evident that there is very serious
trouble with respect to a number of subscribers on that line.
Therefore, the override transmission control provides a high degree
of certainty that an alarm signal from a station can be recognized
and determined at the central supervisory office even though the
line may have been seized by a faulty transmitter or be
continuously sensed to be busy because of other intermittent line
faults.
It is important to note that previous alarm systems have been
unable to override a busy line, and therefore a real alarm
situation at a subscriber station may be unable to be transmitted
to the supervisory central office under certain circumstances.
Still further, the previously known alarm systems have been such
that access to the line by a station at which an alarm has occurred
is not permitted until after a certain predetermined period of time
has elapsed from the time that the transmission control at the
station wishing to transmit an alarm signal determines two
conditions, namely that the line is truely clear of all
transmission from other stations and that an alarm condition has
existed at the station since the time when the transmission control
at that station has initiated its scan of a line to determine that
it is truely clear. Thus, with prior alarm systems -- particularly
such as those taught in Falck U.S. Pat. No. 3,484,771, dated Dec.
16, 1969 -- when an alarm condition occurs and the line is busy
such that immediate access to the line cannot be obtained for
transmission of an alarm event signal from that station, access to
the line is delayed for a period of time so that the line can be
tested to be truely clear of transmission following the last pulse
of the transmission from another station, and only when an alarm
event has happened and the fact of that alarm event is stored in a
memory element at the subscriber station wishing to transmit.
Falck teaches that if the line is occupied upon the occurrence of
an event at a given station, a circuit operates to delay access to
the station for a minimum period of Y seconds by inhibiting the
start-up of a transmission control access for a period of Y -X
seconds and then when the line is clear, initiating the line access
which takes a period of X seconds, thereby resulting in a minimum
time delay of access to the line of Y seconds.
This invention, on the other hand, provides a transmission control
circuit and line lock-out circuit including means for continuously
monitoring the line and for enabling transmission to the line from
any station after the passage of a predetermined period of time
from the last sensing of a pulse from any station on the line,
regardless of whether or not the sensing station has an alarm event
noted or in its memory at the beginning of the timing cycle from
the last sensing of the line busy condition. That is to say, the
invention provides a timer which is independent of the fact that an
alarm event may or may not have occurred at that station, but which
precludes transmission of a pulsed signal indicative of an alarm
event in the event that the alarm has happened at that station in a
predetermined period of time since the last sensed line busy
condition; thereby precluding any possibility of collision in the
normal circumstances while assuring the soonest possible access to
the line from a station which requires to transmit an alarm event.
The invention further provides that, in the event of a runaway
transmitter at another station or other intermittent line faults
such that the line is continuously sensed as being busy, after a
predetermined period of time from the occurrence of an alarm event
at a station, that station will transmit to the line its coded,
pulsed signal indicative of that alarm, irregardless of the fact
that the line is continuously sensed as being busy.
In contradistinction to most of the known alarm systems,
particularly including the Falck alarm system referred to above,
the present invention provides a line lock-out and transmission
control circuitry, including timers aand transmitters means, that
may be entirely solid state, thereby providing relatively
maintenance and service free operation over a long period of time
and without high costs. In addition, the present invention provides
line lock-out and transmission control circuitry which utilizes
simple logic elements, particularly NAND gates, and simple RC timer
and integrator networks.
BRIEF SUMMARY OF THE INVENTION
It is a major feature of this invention that a line lock-out
circuit is provided which permits access to an alarm transmitting
line by a transmitter at a subscriber station in the event of an
alarm event at that subscriber station, at the soonest possible
moment after that alarm event if the line was sensed to be busy at
the time of the alarm event.
A further object of this invention is to provide a transmission
control circuit for multi-subscriber alarm systems where a
plurality of subscriber stations are connected to a single line,
having override means in each station to provide that a coded,
pulsed signal indicative of an alarm at any station may be
transmitted to the line after a predetermined period of time
following the occurrence of that alarm event, regardless of whether
or not the line has continuously been sensed to have been busy
since the time of happening of the alarm event.
A further object of this invention is to provide circuits of the
sort discussed above which can be relatively easily and
inexpensively produced and which are substantially free of
maintenance and repair requirements.
DESCRIPTION OF THE DRAWINGS
These and other features and objects of this invention are
discussed hereafter in association with the accompanying drawings
in which:
FIG. 1 is a circuit diagram of an alarm system including the line
lock-out and override circuits in accordance with this invention;
and
FIG. 2 is a series of curves of voltages occurring at various
places in the circuit of FIG. 1 during the passage of a period of
time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to FIG. 1, there is shown a subscriber station 10 in
an alarm system, and it is assumed that the alarm system has a
plurality of stations similar to that illustrated in FIG. 1, or in
any event of the known type which transmits a pulsed signal to a
line where the characteristics of the pulse are known. The station
10 has an alarm terminal input 12, or a plurality of such terminals
each connected to different sensors to indicate an alarm status at
any one of those sensors. The alarm input or inputs 12 are, in any
event, connected to an alarm signal processor 14. The signal
processor 14 is such as to have an output which can be positively
changed in the event of an alarm input, and at a voltage and having
other signal characteristics compatible with the circuitry of the
station. The output of the signal processor 14 goes to a latch 16
which serves as a memory latch, as discussed in greater detail
hereafter. The output from the memory latch 16 goes to a trigger
circuit 18, and thence to a master control latch 20, as discussed
in greater detail hereafter. The output of the control latch 20
goes to a line lock-out circuit indicated generally at 22, which
includes line monitor circuitry indicated generally at 24, a line
access timer indicated generally at 26, an override time indicated
generally at 28, and parallel gates 30 and 32. The outputs of the
gates 30 and 32 are tied together, and go to a transmitter 34 which
is adapted for transmitting a pulsed signal to a line, via line
terminal 36. A ground terminal 38 is also provided in the
subscriber station 10 of FIG. 1, and it should be noted that in the
normal, quiescent circumstances, a DC voltage exists between ground
and line. Thus, the station 10 and any station like it which is
connected to the line has means which are responsive to an alarm
event connected to the alarm input 12; transmitter means 34 for
transmitting a pulsed signal train to the line via line terminal 36
where the pulsed signal train is indicative of an alarm event at
the station; means 24 which sense the presence on the line of a
pulsed signal from any station which is connected thereto,
including itself; and line lock-out means 22 adapted to preclude
operation of the transmitter 34 when a pulsed signal from another
station connected to the line is sensed. The line lock-out means 22
is adapted to permit operation of the transmitter 34 after a
predetermined period of time following the last sensed pulse from
another station connected to the line, as discussed in greater
detail hereafter; and the line lock-out means 22 further includes
the override timer 28 which is operative only for a predetermined
period of time from the occurrence of an alarm event at the station
and the insertion of an alarm input at terminal 12, so that
operation of the transmitter 34 is precluded in the event that the
line monitor circuit 34 continuously senses a line busy condition
at terminal 36 for the period of operation of the override timer,
and so that upon expiration of that period when the line monitor
circuitry 24 has continuously held off or precluded operation of
the transmitter 34, the transmitter operates anyway, irregardless
of the presence of other signals which may be on the line and are
being sensed at terminal 36.
There are four sets of conditions which may happen with respect to
an alarm event at a station and the condition of the alarm line at
the moment that the alarm event occurs. Those conditions are as
follows:
i. an alarm event occurs, and the line is free.
ii. no alarm event has occurred, and the line is busy.
iii. an alarm event occurs, the line is busy and the line lock-out
circuitry becomes operative.
iv. an alarm event has occurred, the line has been continuously
sensed as being busy, and the override timer operates to permit
transmission of signals from the transmitter to the line.
Each of the circumstances set out above is discussed in greater
detail hereafter, with reference to the figures and a full
explanation of the operation of the circuits according to this
invention.
CONDITION I
When an alarm event occurs, an alarm signal appears at input 12 and
is passed to the signal processor 14. The alarm signal then goes to
the memory latch 16, which may be a bi-stable device having two
specific outputs at different levels for purposes of controlling
the transmitter 34 in manner discussed hereafter so that the signal
transmitter to the line is indicative of the alarm input or a reset
of the alarm condition to its quiescent state. Assuming for the
moment that an alarm input has occurred, the output of the memory
latch 16 goes to trigger 18, which has a single pulse output to set
the master control latch 20 -- which is a bi-stable latch -- in its
state which is indicative of an alarm event having occurred.
Assuming, for the moment, that the line is free and has been free
for more than a predetermined period of time so that immediate
acquisition of the line can be obtained by transmitter 34, the
operation of the transmitter 34 is initiated by an appropriate
output from gate 30 in a manner discussed hereafter. When the
transmitter 34 begins its operation to transmit a pulsed signal
indicative of an alarm event at the station to the line via line
terminal 36, a signal status conditioner 40 and a rounds counter
42, each of which is associated with the transmitter 34, become
operative. Thus, when an alarm event is being signalled by
transmitter 34, the rounds counter determines the condition of the
memory latch 16 by a memory status connection 44; and permits a
specific number of rounds of coded, pulsed signals to be
transmitted from transmitter 34, which are indicative of the alarm
event. Usually the coded the pulse train is transmitted to the line
3 or 4 times, under the control of the rounds counter which has
determined from the memory latch that an alarm event has occurred
and the memory status of the memory latch is such as to indicate
that fact.
[Because the memory latch is bi-stable, and the control latch is
also bi-stable, when an alarm event occurs and the control latch 20
switches to its alarm status, an inhibit signal is sent by an
inhibit connection 46 to the memory latch 16 to preclude it from
switching away from its alarm status. When the alarm condition has
been removed, an alarm reset signal -- or in any event, a
difference in the state of the signal which otherwise indicates an
alarm event -- appears at alarm input 12. In that event, the
processor 14 passes another signal to memory latch 16 -- which may
be way of a pulse going in the opposite direction than a pulse
which set the memory latch to its alarm status, so that the memory
latch 16 is then reset to its reset status or non-alarm status.
Once again, the transmitter is caused to operate, sending a
different signal or in the more usual case a single pulse train
under the control of the round counter which determines the reset
memory status as opposed to an alarm memory status. Following the
reset signal transmission from transmitter 34, the signal status
conditioner resets control latch 20 to its quiescent, non-alarm
status.]
The above discussion has been to provide a description of the
operation of the control circuit when conditions are such that an
alarm event results in immediate acquisition of the line by the
transmitter 34; and also when the alarm condition is reset to its
quiescent or normal operating status.
CONDITION II
The premise of condition II is that no alarm input occurs, but that
the line is sensed as being busy because of the presence on the
line of a pulsed signal from another station, which pulsed signal
is sensed by the line monitor circuitry 24. It should also be noted
that the line monitor circuitry 24 will sense the presence of a
pulsed signal on the line from transmitter 34 in the same station
10, and that condition is discussed hereafter.
It should first be noted that each of gates 30 and 32 is a NAND
gate, having two inputs and a single output. The conditions of
operation of a NAND gate are such that two high inputs to the NAND
make a low output; and any low input to the NAND gate makes a high
output. It should also be noted that operation of the transmitter
34 is initiated only when the output from either gate 30 or 32 goes
low; i.e. when both inputs to either gate are high.
A transistor 48 -- preferably an NPN transistor -- is connected
from its base to the line terminal 36 through the series connection
of resistor 50 and capacitor 52. Between the base and emitter of
transistor 48 is a parallel RC network comprising resistor 54 and
capacitor 56; and the ends of that parallel RC network which are
remote from the base of transistor 48 are connected through series
resistor 58 to the ground terminal 38. The series resistor 58 is
provided as an isolation resistor so that the station 10 does not
have a hard connection to ground; and its value is orders of
magnitude less than the value of resistor 50. Capacitor 52 is a DC
blocking capacitor so that the line monitor circuitry 24 is AC
coupled to the line terminal 36. The values of the series RC
network 50, 52 and the parallel RC network 54, 56 are chosen so
that the base of transistor 48 of the line monitor circuit 24 is
connected by a band-pass filter to the line terminal 36 and ground
terminal 38, and so that the line coupling to the line terminal 36
is frequency and amplitude dependent. The networks pass only
signals to the base of transistor 48 to turn transistor 48 on when
signals are detected at line terminal 36 which have the
characteristic of pulses which are of a predetermined length and
magnitude, thereby having a predetermined energy storage
differential from quiescent state. The only signals which appear at
line terminal 36 and which act to turn the transistor 48 on are
those having the characteristic of an alarm transmission on the
line 36. Thus, square-wave pulses having the wrong magnitude or
duration, and therefore the wrong energy storage characteristic,
would not pass to the base of transistor 48; and superimposed AC
signal such as 60 cycle hum could not pass to the AC coupled line
monitor circuit, and in any event would be integrated to zero
energy storage by the RC network 54, 56.
The collector of transistor 48 is connected to the bottom end of a
resistor 60 and a capacitor 62 which are in parallel, and the top
end of each of resistor 60 and capacitor 62 is connected to an
internal positive voltage terminal 64. The values of resistor 48
turns on for a sufficient length of time, the RC network 60, 62 has
a time constant which is set at a predetermined period of time. In
other words, a known period of time is required for capacitor 62 to
charge to a specific voltage, which may be the voltage at positive
terminal 64 or very slightly below that voltage with reference to
ground. In any event, the RC network 60, 62 functions as a timer,
referred to hereafter as the acquisition timer, and in the usual
case the acquisition timer has a time constant of approximately
five seconds. The acquisition timer is connected through series
resistor 66 to a trigger input 68 of gate 30.
It is important to note the operation of the acquisition timer and
its relationship to gate 30 and the line monitor circuit 24.
Reference is made to FIG. 2, and particularly curves 200, 202 and
204. Those curves are plots of voltage versus time, taken at line
terminal 36, the base of transistor 48 and the collector of
transistor 48, respectively. It will be noted that at the beginning
of each pulse 220, the base voltage of transistor 48 reduces and
may become slightly negative; and that at the termination of each
pulse 220, the base voltage increases and then decays in accordance
with the time constant characteristic of the RC networks connected
to it. At the beginning of the first pulse 220(a), at which time
transistor 48 turns on, the collector voltage reduces from
approximately that of positive terminal 64 to substantially zero,
and then begins to rise with a rising characteristic determined by
the acquisition timer RC circuit 60, 62. The interval between
pulses 220 is less than the predetermined time constant of the
acquisition timer as mentioned above. At the beginning of each new
pulse, the collector voltage again collapses and begins to rise in
accordance with the characteristic of the RC circuit of the
acquisition timer, since the capacitor 62 is discharged every time
the transistor 48 turns on. After a number of pulses, the last
pulse of a pulsed signal train of another station connected to the
line occurs, and that is indicated on curve 200 as pulse 220(n).
Once again, at the beginning of pulse 220(n), the base voltage of
transistor 48 behaves as before, and the collector voltage of the
transistor 48 collapses. However, it will be noted that a period of
time t.sub.1 as indicated at 214 then begins. The significance of
period t.sub.1 is discussed hereafter, but for purposes of the
present discussion, it will be noted that the length of period
t.sub.1 is at least 5 seconds. During that interval, which is the
predetermined period of time which is set as the time constant for
the acquisition timer RC network 60, 62, the collector voltage
continues to rise in accordance with the time constant until it
reaches a voltage indicated in curve 204 as voltage v.sub.t. That
voltage may be equal to or slightly less than the voltage of
positive terminal 64 v.sub.64. No further discussion with respect
to curve 204 follows at this time, because the next circumstance
shown in that curve after the collector voltage reaches v.sub.t is
relevant with respect to Condition III discussed hereafter. In any
event voltage v.sub.t is the threshold or trigger voltage for gate
30 at which one of the two inputs to the gate can be considered to
be high. Thus, it will be seen that after a predetermined period of
time which is equal to the time constant of the acquisition timer
RC circuit 60, 62, the trigger input to gate 30 goes high and gate
30 is thus preconditioned with one high input. That predetermined
period of time follows the last sensed pulse at line terminal 36,
irrespective of the alarm status of the station. Therefore, in
order for the operation of the transmitter to begin, it is merely
necessary for a high input to be imposed on trigger 30 at its
enable input 70 -- it being remembered that operation of the
transmitter 34 will be initiated immediately upon gate 30 having
two high inputs so that its output can go low and the transmitter
34 may begin to operate.
Thus, so long as any pulses are being sensed at line terminal 36 --
except pulses from transmitter 34 at the same station, as discussed
hereafter -- and for a predetermined period of time after the last
sensed pulse, operation of the transmitter 34 is precluded because
gate 30 is not in a condition to pass an alarm signal. With respect
to Condition I therefore, when the line is free and has been free
for a length of time greater than the predetermined period which is
the time constant of the acquisition timer RC circuit 60, 62, the
setting of the master control latch 20 to its alarm status
immediately causes a high input at enable input 70 of gate 30, and
since the trigger input 68 is already high in that circumstance,
the output of gate 30 immediately goes low and operation of
transmitter 34 is initiated.
With reference to the circumstances of Condition II, so long as the
line remains busy and no alarm event has occurred, the line
lock-out circuitry 22 acts to preclude operation of the transmitter
34; and the line lock-out also precludes the possible operation of
transmitter 34 for a predetermined amount of time equal to the time
constant of the acquisition timer RC network 60, 62.
CONDITION III
Assume now that an alarm condition occurs so that an alarm input
signal reaches input 12 and causes memory latch 16 to assume its
alarm status signal level. Assume further that at the instant that
the alarm condition occurs, the line monitor circuits 24 sense a
line busy condition at line terminal 36 so that gate 30 has a low
input at its trigger input 68. These conditions are established in
FIG. 2 where the occurrence of an alarm condition is assumed to
occur after pulse 220(b) at a time t.sub.x as indicated in curve
206. At that time, the control latch 20 switches to its alarm
condition status, and puts a high input at enable input 70 of gate
30 and enable input 74 of gate 32, which are tied together. At the
same time, the override timer 28 is started by beginning a charging
operation of capacitor 76 which, with resistor 78 forms an RC
network having a time constant of approximately two minutes. At the
end of the 2 minute period, the voltage of capacitor 76 will have
reached the trigger level V.sub.t for gate 32, as discussed
hereafter with respect to Condition IV. Diode 80 provides
protection for the capacitor 76 when the control latch 20 goes to a
high output, and provides means whereby the capacitor 76
immediately discharges when the control latch 20 assumes a
non-alarm status and its output goes to its low level.
Thus, the slope of curve 212 after time t.sub.x is characteristic
of the time constant of the override timer 28.
It will be noted that at time t.sub.x the output of gate 30 remains
high, indicative of the condition that there is at least one low
input to that gate, because it will be noted that the trigger input
68 of gate 30 is low -- that is, below the trigger voltage v.sub.t.
Indeed, it will be noted that the voltage at trigger input 68 of
gate 30 follows the voltage at the collector of transistor 48,
because no current flows and there is no voltage drop across
resistor 66. Pulse 220(n) finally occurs and is sensed at line
terminal 36, after which the collector voltage of the transistor 48
continues to rise until it reaches the trigger voltage v.sub.t of
gate 30. It is assumed in curve 210 that the trigger voltage
v.sub.t is slightly below voltage v.sub.64, and it will be noted
that the voltage at the trigger input 68 incureases at time t.sub.z
which is the time at which the collector voltage of transistor 48
reaches trigger voltage v.sub.t. At that precise instant the output
of gate 30 goes low because it is at that instant that gate 30 has
two high inputs. At that moment, operation of the transmitter 34 is
initiated, and pulse 220(z) is transmitted from the transmitter 34
and is sensed at the line terminal 36. Thus, the alarm condition of
the station begins to be transmitted to the line. As noted above,
the control latch 20 remains high, and it is therefore necessary
that the sensing of the pulse 220(z) at line terminal 36 by the
line monitor circuit 24 not be such as to turn transmitter 34 off.
In other words, the line lock-out must be inhibited with respect to
its operation because of sensing pulses from its own associated
transmitter.
This latter condition is accomplished by inverter 82 and diode 84
which are in series from the outputs of gates 30 and 32, and which
are also in series with trigger input 68 of gate 30. [It will also
be noted that diodes 86 and 88 are in the output circuits from
gates 30 and 32, respectively, for protection of the gates when
their output goes low; because it will be noted that the output
terminals of the gates 30 and 32 are connected through resistor 90
to the internal positive voltage terminal 64.] When the output from
gate 30 goes low, the output from inverter 82 to diode 84 to the
trigger input 68 goes high because of the connection through
resistor 90 to the internal positive voltage terminal 64; and the
output of the inverter 82 to trigger input 68 of gate 30 remains
high so long as the output from the gate 30 remains low. Thus,
during the condition when output from gate 30 is low because the
alarm condition still exists and transmission of that fact is being
made by transmitter 34 to the line via line terminal 36, the
trigger input 68 remains high notwithstanding the operation of the
line monitor circuit 24 and the manner in which the voltage of the
collector of transistor 48 is operating. When the rounds counter 42
stops operation of the transmitter 34, the other stations can seize
the line and begin transmission of an alarm signal following
operation of the acquisition timers of those stations to permit
access to the line.
It should be noted that, in the event that two further stations
have an alarm event while a first station is transmitting, and each
of those other two stations is held off from the line by their own
respective line lock-out circuitry, acquisition by one or the other
of those two stations to the exclusion of the other is a function
of the precise instant at which the acquisition timer in one of the
stations reaches voltage v.sub.t of the gate 30 of that station.
There are certain component variations, even between identical
components made seriatim on the same assembly line, so that the
time constant characteristic of the acquisition timers of various
stations are not precisely identical. It will also be noted that
when the gate 30 of any station goes to a low input, operation of
the transmitter 34 of that station is immediately initiated and the
first pulse from that station is generated so as to establish
acquisition of the line by that station.
CONDITION IV
Finally, a condition may exist on the line, as noted above, where
an alarm event occurs at a station, and where the line monitor
continuously senses a line busy condition for a predetermined
period of time. In those circumstances, an override timer operates
to initiate transmission of the alarm status at that station,
regardless of the condition of the line.
This latter condition is achieved by operation of the timer network
including resistor 78 and capacitor 76, as mentioned above. It has
been noted that at time t.sub.x, being the time at which an alarm
event occurs at the station presently being discussed, operation of
the override timer begins as noted in curve 212 of FIG. 2. As noted
above, the enable input 74 of gate 32 is tied to the enable input
70 of gate 30, and each of those enable inputs goes high at the
same time when the control latch 20 assumes its alarm status level.
If the line lock-out remains functional for the entire
predetermined period which is the time constant of the override
timer 28 so that operation of the transmitter 34 is precluded, then
at the end of that predetermined period the trigger input 72 of
gate 32 goes high and the output of gate 32 goes low so as to
initiate operation of transmitter 34. When that happens, the output
of inverter 82 goes high, and once again the trigger input 68 of
gate 30 is driven high so that its output goes low and so that
sensing of signals at line terminal 36 does not operate to preclude
further transmission of signals from transmitter 34.
As noted, when the transmission of a coded, pulsed signal
indicative of an alarm event is finished, the signal status
conditioner 40 resets the control latch 20 to its low level, and
the enable inputs 70 and 74 of gate 30 and 32 respectively go low
so that the outputs of those gates go high and further operation of
the transmitter 34 is precluded.
It should also be noted from FIG. 2 that the length of time period
t.sub.1 indicated at 214 is 5 seconds plus or minus, which is the
time constant of the acquisition timer RC circuit 60, 62, and which
is the predetermined period of time after the sensing of a pulsed
signal on the line from another station connected thereto that the
transmitter of that station is permitted to operate to transmit a
pulsed signal to the line indicative of an alarm condition at that
station. The length of the time period t.sub.2 indicated at 216,
which is the time passage from time t .sub.x when an alarm
condition occurs and an alarm input appears at terminal 12 until
the time t.sub.z when the first alarm pulse from that station is
transmitted, is less than approximately 2 minutes. When the time
period from t.sub.x to t.sub.z goes beyond 2 minutes, override
timer 28 operates to make output of gate 32 low and thereby to
initiate operation of the transmitter 34. The length of time of
time period t .sub.3 indicated at 218 in FIG. 2, which is the
length of time between any two pulses in a coded, pulsed signal
train from any station indicating an alarm condition at that
station, is less than 5 seconds.
Other types of circuit components can be substituted for those
which are shown in FIG. 1, such as clock driven timers, other logic
circuits than NAND gates, transistors and other integrated
circuitry having the same or reverse operating polarities than
those shown, etc. In any event, the operation of the acquisition
timer and of the override timer, and their relationship to the line
monitor and the gates of the line lock-out circuitry according to
this invention have been clearly discussed with relationship to the
circuits and curves which have been exemplary of the type of
operation of such circuits; and any such alarm system or variation
thereof as contemplated above would, in any event fall within the
ambit of the accompanying claims.
* * * * *