Note: Descriptions are shown in the official language in which they were submitted.
108771S
; This invention relates to a four wire satellite
control system for a multi-satellite intrusion alarm.
Intrusion alarms normally operate by transmitting
a wave field (typically ultrasonic or electramagnetic radiation)
into an area under supervision~ receiving a portion of the
reflected field,and comparing the two. If a moving intruder
is present, a doppler shift is detected in the received field.
The doppler frequency is processed and is used to produce an
alarm signal.
Since each transmitter-receiver unit can supervise
only a relatively small area, such as a room or hall, therefore
in large buildings a number of separate units must be used. To
reduce duplication of equipment, it is usual to provide a master
unit and a number of separate satellites connected to the master
unit. A satellite unit is placed in each area to be supervised.
The satellites carry out some signal processing, but it is
the practice to place some of the signal processing circuits in
the master where they provide common processing for several
satellites.
A disadvantage of conventional multi-satellite intru-
sion alarms is that the cables connecting the satellites to the
master usually require numerous separate conductors. These
cables are therefore expensive, bulky, and difficult to
instal. In addition, conventional systems usually reguire
that the connecting cables between the satellites and the master
be shielded (since low level signals are being transmitted to
the master for further analysis), increasing further the
expense and bulk of the cable. The bulky shielded cables are
also difficult to instal unobtrusively.
A still further disadvantage of conventional intrusion
alarm systems relates to the practice, in large buildings where
many satellites are required, of connecting the satellites
to the master in groups~ Each group of satellites
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~08771S
constitutes a "zone". When one satellite in a zone generates
an alarm signal, it is normally difficult to determine which
of the satellites in the zone actually generated the
alarm. In conventional systems, when a security guard
investigates these zones in question, his movement usually
causes other satellites in the zone to be triggered. This
adds to the difficulty of tracing the movements of the intruder
or of analyzing the false alarm which caused the alarm signal.
Accordingly, it is an object of the invention to
provide an intrusion alarm control system in which, in a
preferred embodiment, each satellite is connected to the
master by only four wires, which normally need not be shielded.
One of the wires supplies power to the satellite; a second
is a common return line; the third wire is a drive signal line
which carries a drive signal from the master to the satellite;
and the fourth is an alarm line which carries a high level
alarm signal from the satellite to the master. In a preferred
embodiment of the invention, means are also provided in the
master and in each satellite so that when an alarm signal
is produced by a satellite, the system can be switched to a
condition in which the satellite in question enunciates the
alarm, but none of the other satellites in the system will
produce an alarm signal while an authorized person checks
the supervised premises. This facilitates analysis of the
cause of the alarm signal.
Further objects and advantages of the invention will
appear from the following disclosure, taken together with the
accompanied drawings, in which:
Fig. 1 is a block diagram showing a conventional
connection arrangement of satellites to a master control unit;
Fig. 2 is a block diagram of a satellite according
to the invention;
10~7 715
Fig. 3 is a block diagram showing a portion of
a master control unit of the invention;
Fig. 4 is a schematic of a control circuit of
a satellite;
Fig. 5 shows a drive wave form produced at a satellite;
Fig. 6 shows a portion of the control circuit of
Fig. 4 in supervisory condition with the condition of certain
logic elements indicated thereon;
Fig. 7 shows the Fig. 6 circuit in alarm condition;
Fig. 8 shows the Fig. 6 circuit with the drive
signal off;
Fig. 9 shows the Fig. 6 circuit in walk test
condition;
Fig. 10 is a schematic of a logic circuit of a
master control unit; and
Fig. 11 shows wave forms produced by the logic
circuit of Fig. 10.
General Description
Reference is first made to Fig. 1, which shows a
typical connection system for a master control unit and
satellites. The connection system of Fig. 1 has been used in
most conventional alarm systems and is preferably also used
in the alarm system of the invention. As shown, a master
control unit 22 is connected to four satellite zones 24, 26,
28, 30. Each satellite zone typically consists of five
satellites, which are indicated as satellites 1 to 5, 6 to 10,
11 to 15 and 16 to 20. The satellites of each zone are
connected together and to the master control 22 by cables 32.
In operation, if an intruder is detected by any
satellite in a zone, for example in satellite 1 of zone 24,
a signal (which in conventional systems usually requires further
analysis) is sent to the master control unit 22. The master
lQ8771S
control unit 22 generates an appropriate alarm signal, which
may be sent by a telephone line 34 either to the alarm company
whose duty it is to supervise the premises in question, or
to police headquarters, or as desired.
As indicated previously, the cables 32 connecting
the satellites to the master are usually shielded, and usually
contain numerous conductors. Because of this, installation
of the satellites is usually a difficult and expensive task.
According to the invention, means are provided in
the satellites and in the master control unit 22 so that the
cable 32 need contain only four conductors. These means are
shown in block diagram form in Figs. 2 and 3. Fig. 2 shows
a typical satellite, for example satellite 1 of zone 24.
Satellite 1 includes four terminals 40a, 40b, 40c, 40d
which are connected by conductors 42a, 42b, 42c, 42d to four
corresponding terminals 44a, 44b, 44c, 44d in the master 22
(Fig. 3). As will be explained, conductor 42a is a drive
conductor, conductor 42b is an alarm conductor, and conductors
42c, 42d are power supply conductors.
In the example here illustrated, it is assumed
that the transmitted field is ultrasonic sound at a frequency
of 40 KHz. Accordingly, the master 22 includes a 40 KHz
transmitter 46, which forms part of a logic circuit 48. The
transmitter 46 applies a 40 KHz drive signal to terminal 44a,
and thence through drive conductor 42a to terminal 40a of the
satellite 1. In satellite 1 the 40 KHz signal is squared by a
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~087715
Scnmidt trigger 50, which improves the waveform of the drive
signal and ensures that its peak amplitude is constant. The
40 KHz signal is then sent to transmitter 52 (Fig. 2), which
radiates a 40 KHz ultrasonic sound field.
A portion of the reflected field is received by a
transducer-receiver 54, amplified by amplifier 56, and then
directed to a synchronous detector consisting of transistor Ql.
The base of transistor Ql is driven by the 40 KHz drive signal.
The signal from the transistor Ql collector is passed to a
band pass filter 58, which removes the 40 KHz component and
also removes very low frequencies. The signal from the band
pass filter 58 is then directed to a signal processor 60. The
signal processor 60 processes the signal from filter 58 and
produces an alarm signal if the signal from filter 58 contains
the doppler frequencies which are likely to have been generated
by a moving intruder (40 Hz to 300 Hz for a 40 KHz transmitted
sound field). Various known forms of signal pro~essing circuits
may be used for processor 60. A preferred signal processing
circuit is shown in my co-pending application serial no. 262,685
filed concurrently herewith.
The alarm signal (if any~ from signal processor 60
may be of extremely short duration, so it is directed into
a pulse stretcher 62 (typically a Schmidt trigger) which
produces a pulse of fixed length when it is triggered. The
pulses (if any) from the pulse stretcher 62 are fed to a control
circuit 64, which then sends an appropriate high level signal
back to the master 22 via conductor 66, terminal 40b, alarm
conductor 42b, and terminal 44b. This signal is received in
in the master by a detector 68, which then provides a signal
to operate an alarm signal generator 70. The signal from
generator 70 may be of any desired form, e.g. it may operate
1~)87715
a telephone to alert the alarm company.
As shown in Fig. 3, the logic circuit 48 includes
a three position rocker switch 72 having a rocker element 74.
The three positions of rocker switch 72 are (i) the position
shown in Fig. 3, in which rocker element 74 contacts lower
terminal 76, (ii) a position in which rocker element 74
contacts upper terminal 78, (iii)a central position in which
rocker element 74 contacts neither of terminals 76, 78. The
position shown in Fig. 3 is the normal supervisory position.
In this position, rocker switch 72 controls logic circuit 48
so that the 40 KHz signal from oscillator 46 is applied to
terminal 44a.
If an alarm signal is produced by generator 70,
and after an authorized person arrives at the premises to
investigate, he will place rocker switch 72 in its central
position, in which rocker element 74 does not contact either
of terminals 76, 78. As will be explained, the logic circuit
48 then removes the 40 KHz drive signal from terminal 44a,
so that the satellite 1 (and the other satellites in zone 1)
will no longer generate an alarm. This enables the person
to investigate the premises protected by zone 1 without
creating additional alarms. (He may also switch the
corresponding rocker switches for the other zones to their
central positons, thus also preventing any of the other
satellites from generating an alarm signal. One switch
actuator may be used for the rocker switches of all the zones.)
When the authorized person switches off the 40 KHz
drive signal at the master 22, the control circuit 64 of
Fig. 2 responds to the combination of the terminated 40 KHz
drive signal, and the alarm signal which was previously
received from pulse stretcher 62, and operates a speaker 80
in the satellite. Thus, when the investigating person walks
through the area supervised by the satellite which generated
-` ~087715
the alarm signal, he will hear the speaker 80 and will
know which satellite generated the alarm.
When the investigator moves rocker element 74,
power is removed from terminal 82 of alarm signal generator
70. This, by conventional means, places generator 70 in
a constant alarm condition, so that the alarm company will
know that the system is not in its normal supervisory
condition.
After the investigating person has completed his
investigation, he can then place the system in a "walk test"
condition by moving the rocker switch 72 so that the rocker
element 74 contacts terminal 78. This operates the logic
circuit 48 of Fig. 3 to resume supply of the 40 KHz drive
signal to the satellites, including the satellite 1 of
Fig. 2. The control circuit 64 of satellite 1 reacts to
the resumption of the 40 KHz drive signal at terminal 4Oa
by altering the control of the speaker 80, so that the
speaker 80 will now be operated whenever an alarm pulse
from pulse stretcher 62 is produced. Therefore, as the
investigator walks through the area supervised by satel-
lite 1, he can test and determine the extent of coverage
of satellite 1 and whether it is operating properly or
generating false alarms. The same applies to the other
satellites in zone 1 (and in any other zones where rocker
switches have been moved to the "walk test" position).
After the walk test has been completed, the
investigator moves the rocker switch 72 back to its
original position, in which rocker element 74 contacts
terminal 76. This causes the logic circuit 48 to send a
timed reset signal along drive conductor 42a, as will be
described, to actuate the control circuit 64 of each
satellite to resume its original supervisory mode of
operation. Power is also reapplied via terminal 82 to
the alarm signal generator 70.
As will also be described, during the time when the
1~87 715
satellite is not generating an alarm signal, it sends a
supervisory signal over alarm conductor 42b to the master
22. If the supervisory signal ceases for example because
the cable 32 is cut or short circuited, this operates the
detector 68 which causes the alarm signal generator 70 to
operate. Similarly, if for some reason the 40 KHz drive
signal from the master 22 to the satellite 1 ceases, the
control circuit 64 of the satellite reacts by ceasing to
supply the supervisory signal to terminal 40b, again causing
detector 68 to operate.
The remaining two conductors 42c, 42d of Figs.
2, 3 supply +6 volts and a common return respectively to
the various components shown in Fig. 2. These two con-
ductors are shown as connected directly to a six volt
power supply 90 in the master 22, and are indicated as
being connected to the components of Fig. 3 by the dia-
gramatic showing of these components as being connected
to +6 volts and ground
Detailed Description
A - Circuit Description
Reference is next made to Fig. 4, which shows
in detail the satellite control circuit 64. As shown in
Fig. 4, the 40 KHz drive signal from terminal 40a is
fed through resistor Rl to the Schmidt trigger 50, which
produces a constant peak amplitude square wave train 98
(Fig. 5) from the drive signal. The wave train 98 is fed
to the transmitter 52 and the base of transistor Ql, as
described, and is also fed through diode Dl to the input
of a second Schmidt trigger 100. The positive side of
diode Dl is connected through resistor R2 to the +6 volt
supply and is also connected through capacitor Cl to ground.
The output of Schmidt trigger 100 is fed to an
inverter 102 and also through capacitor C2 to the reset
terminal 104 of a memory latch 106. The output of the
g _
1087'715
inventer 102 is fed to one input 107 of a NAND gate 108,
and also the set terminal 109 of a second memory latch 110.
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~)877~5
The output of latch 110 is directed to one terminal 112 of
a NAND gate 114. The output of the NAND gate 114 is fed
to one input 116 of NOR gate 118. The output of NOR gate 18
is directed to the input of an oscillator 120 (typically 5KHz)
consisting of NAND gate 112, inverter 124, resistors R3 and R4,
and capacitor C3. The output of the oscillator 120 is fed
through amplifier 128 to the speaker 80.
The sguare wave train from the input trigger 50
is also fed through a second diode D2 to the input 130 of
another Schmidt trigger 132. The input 130 of the Schmidt
trigger 132 is connected to ground, through the parallel
combination of resistor R5 and capacitor C4. The output of
Schmidt trigger 132 is connected to the reset terminal 134
of the memory latch 110.
The output of Schmidt trigger 62 (the pulse
stretcher) is connected to the set terminal 138 of memory
latch 106 and also to an input terminal 142 of NAND
gate 114. The output of latch 106 is connected
to an input terminal 140 of NAND gate 108. The output of
NAND gate 108 is connected to input 148 of NOR gate 118.
The output of Schmidt trigger 62 is also connected
through resistor R6 to the base of transistor Q2, the
collector-emitter circuit of which is connected between
ground and terminal 42b.
Finally, the Fig. 4 circuit includes a 10 Hz oscillator
150, consisting of NAND gates 152, 154, timing resistors R7
R8, and timing capacitor C5. Oscillator 150 applies a
10 Hz signal to terminal 40b so long as the 40 KHz signal
is present at terminal 40a, as will be explained.
B - Operation - Supervisory Condition
The detailed operation of the Fig. 4 circuit is
as follows. So long as the 40 KHz driving signal is present
at terminal 40a, Schmidt trigger 50 produces the square wave
1~87715
signal 98 shown in Fig. 5, varying between +6 volts (when
the driving signal is low), and ground twhen the driving
signal is high). Signal 98 maintains capacitor Cl discharged
so long as the 40 KHz driving signal is present at terminal
40a. This is because diode Dl is reversed by the "on" half
cycles of signal 98 permitting capacitor Cl to charge
slowly th~ough resistor R2 during "on" half cycles, but
during "off n half cycles diode Dl is forward biased, dis-
charging capacitor Cl through diode Dl and through a low
resistance connection to ground (not shown) which is made in
the Schmidt trigger 50.
The opposite situation prevails with regard to
capacitor C4. This capacitor is normally charged, since
during "on".half cycles of signal 98, diode D2 is forward
biased, permitting rapid charging of capacitor C4, while
during "off" half cycles, diode D2 is reverse biased,
causing capacitor C4 to discharge slowly through resistor R5.
So long as capacitor Cl remains discharged, the
output from Schmidt trigger 100 is high (i.e. +6 volts),
since it is an inverting trigger, and the output from
inverter 102 is low (i.e. ground), so the memory latch 110
is not set. Latch 110 therefore applies a low to input 112
of NAND gate 114. Inver~er 102 also applies a low to input
107 of NAND gate 108. The output of NAND gate 108 is now
high, applying a high to the second input 148 of NOR gate 118.
So long as both inputs of NOR gate 118 are high. the output
of gate 118 is low, inhibiting oscillator 120 (gate 118 is a
negative logic NOR gate, as indicated by the two open circles
at its inputs, and therefore functions as if it were a positive
logic NAND gate). The speaker 80 therefore remains silent.
This situation is shown in Fig. 6, in whicy highs are indicated
by + signs and lows are indicated by - signs.
In addition, so long as the 40 KHz driving signal
is present, a high is applied from Schmidt trigger 100 to
,..
NAND gate 152 of 10 Hz oscillator 150, and a second high is
~)8771S
applied from the input of trigger 132 to NAND gate 154 of
oscillator 150. Oscillator 150 operates in conventional
manner to apply a 10 Hz square wave train of about 6 volts
amplitude to terminal 40b. The 10 Hz wave train is transmitted
to the master 22 (Fig. 3) and received-by the detector 68.
So long as the detector 68 receives the 10 Hz signal, it will
not operate the alarm signal generator 70.
C - Supervisory Condition - Alarm
If an intrusion occurs, causing a high pulse (+ 6
volts) from Schmidt trigger 62, this pulse turns on transistor
Q2 for the duration of the pulse. Transistor Q2 grounds
terminal 40b, stopping transmission of the 10 Hz signal from
oscillator 150 to the detector 68. The absence of the 10 Hz
signal triggers the detector 68, causing it to operate the
alarm signal generator 70.
In addition, the high from Schmidt trigger 62 is
applied to one input 142 of NAND gate 114 (See Fig. 7).
However, since the other input 112 to gate 114 remains low
(since latch 110 has not been set), the output from gate 114
remains high. There is, therefore, no change in the output
of NAND gate 11-4 that would cause NOR gate 118 to remove
the inhibit signal (a low) from oscillator 120.
The high from Schmidt trigger 62 also acts to set
latch 106 (see Fig. 7) placing a high on input 140 of
NAND gate 108. However, input 107 of N~D gate 108 remains
low (due to inverter 102) and the output of gate 108 remains
high, and again there is no change in the condition of
NQR gate 118. The speaker 80 thus remains silent, so as
not to alert the intruder, although an alarm has been
transmitted to the master and hence to the alarm company.
D - Drive Signal OFF
When an authorized person responds to the alarm
1087715
and arrives at the supervised premises to investigate, he
will move the rocker switch 72 (Fig. 3) to its intermediate
position to shut off the 40 KHz drive signal. This shuts
off the transmitters of all of the satellites and prevents
them from responding to further movement. In addition, when
the 40 KHz drive signal is shut off, the output from trigger 50
stays high; diode Dl remains reverse biased, and capacitor Cl
charges through resistor R2, producing a low at the output
of trigger 100.
The low at the output of trigger 100 produces a
high at the output of inverter 102 (see Fig. 8) setting
latch 110 and also applying a high to input 107 of NAND gate 108.
Since latch 106 was set by the previous alarm pulse from
trigger ~0, and applies a second high to input 140 of NAND
gate 108 (see Fig. 8) gate 108 now has two high inputs.
Its output therefore goes low, applying a low to input 148
of NOR gate 118. The output of NOR gate 118 now goes high,
enabling oscillator 120. The output of the oscillator 120
is amplified by amplifier 128 and is fed to speaker 80.
Thus, when the investigator reaches the area which the
satellite 1 supervises, he will hear its speaker and will
know that the intruder was in that area or that it generated
a false alarm. If the speakers of any other satellites are
sounding, he will also know that these satellites generated
alarm signals. No other alarm signals will be generated,
because the 40 KHz driving signal has been turned off. It
will be seen that speaker 80 sounded when two conditions
occurred, namely (1) an intrusion was previously detected,
and (2) the 40 KHz drive signal was turned off.
E - Walk Test
After the investigation has been completed, it will
normally be desired to walk test the system, to ensure that
it is operating properly. At this time, the rocker switch 72
1087715
(Fig. 3) is moved so that its rocker element 74 contacts
terminal 78. This turns on the 40 KHz drive signal again,
again discharging capacitor Cl.
When capacitor Cl is discharged, trigger 100 goes
high (see Fig. 9), resetting memory latch 106 through
capacitor C2. Now, with the 40 KHz drive signal available
to the satellites, when the authorized person moves in the
area supervised by the satellite 1, a high is produced by
trigger 62 and is fed directly to input 142 of NAND gate 114.
The other input 112 to NAND gate 114 is also high, since
latch 110 was set when the 40 KHz drive was turned off
previously. The two high inputs to NAND gate 114 produce
a low at its output. This low is applied to input 116 of
NOR gate 118, which then removes the inhibit from the
oscillator 120. The result is that the speaker 80 sounds
during the time when the authorized person is actually
moving in the area under supervision. This enables
testing of the satellite in question and also facilitates
setting of the levels at which it will generate an alarm
signal.
F - Return to Supervisory Condition
After the walk testing has been completed, and
the system is to be placed back into its supervisory condition,
the rocker switch 72 is returned to its original condition
shown in ~ig. 3. By means to be described, this produces a
timed .4 second low signal on drive conductor 42, followed
by a timed .4 second high signal, followed by the normal
40 KHz drive signal. The timed .4 second high signal is
sufficient for capacitor Cl to discharge to its normally
discharged condition and is also sufficient time for capacitor
C4 to discharge through resistor R5, causing the output of
the second trigger-132 to go high. - This places a high signal
1087 715
on the reset terminal 134 of latch 110, resetting this
latch and thereby disa~ling any further enunciation of the
speaker 80. When the 40 KHz drive signal resumes, after
the timed signals, the system is back in supervisory
condition.
G - Description of Master Logic Circuit
The logic circuit 48 of the master 22, and the
wave forms produced thereby, are shown in detail in
Figs. 10 and 11. When the rocker switch 72 is in the position
shown the 40 KHz oscillator 46 operates and its signal is fed
to input 200 of OR gate 202 to operate driver amplifier 204.
Amplifier 204 then feeds the amplified 40 KHz drive signal
to the drive terminal 44a. There is no input to the second
input 206 of OR gate 202 at this time, because input 206
is fed by AND gate 208, one input 210 of which is a single
shot multivibrator 212 which is not operative at this time.
When the rocker switch 72 is operated so that its
rocker element 74 is in its intermediate position, in which
element 74 does not contact either terminal 76, 78, then +6
volts is removed from the second input 214 of NAND gate 216.
Gate 216 is a negative logic NAND gate, as indicated by the
open circles at its inputs, and produces a high at its output
only when both inputs are low, i.e. it functions like a positive
logic NDR gate. Both the inputs of NAND gate 216 are now low,
thereby producing a high at the input 218 of OR gate 220, which
in turn produces a high at the input 222 of AND gate 224. The
second input 226 of AND gate 220 is also high at this time,
because of inverter 228, the input o~ which is grounded througn
resistor R10. The outpllt of AND gate 224 therefore goes high,
inhibiting the 40 KHz oscillator 46, which ceases operation.
~hen oscillator 46 turns off, the drive terminal 44a is
grounded by means not shown in the driver amplifier 204.
The wave forms thus produced at drive terminal 44a
are shown in Fig. 11. The 40 KHz drive signal is shown at 230,
, . ,. . _
1087715
and the ground signal produced when the 40 KHZ oscillator 46
is inhibited is shown at 232. As described, when the 40 KHz
oscillator 46 is inhibited, an investigator can walk into
the supervised area without causing a further alarm.
When the rocker switch 72 is switched to its
walk test condition, in which rocker element 74 contacts
terminal 78, this supplies a high to the input of inverter 228,
causing its output to go low, so that input 226 of AND gate
224 goes low. AND gate 224 therefore removes
the inhibit or high signal from oscillator 46, and the drive
terminal 44a now receives the 40 KHz drive signal again. As
previously described, the satellites will now detect motion
and the speakers 80 will sound at the time when the motion
occurs, so that the system can be walk test. In addition,
input 234 of NAND gate 216 goes high and input 214 of this
gate goes low, ca~sing the outPut of gate 216 to go low.
To return the system back to supervisory position,
the rocker switch 72 is returned to its position as shown
in Fig. 10. As the rocker element 74 moves, both inputs
214, 234 to NAND gate 216 arelow for a brief interval. The
output of gate 216 therefore goes high for a brief interval
and triggers a single shot multivibrator 236 which produces
a .4 second high output pulse. This high pulse at input 238
of OR gate 220 produces a .4 second high at input 222 of
AND gate 224. AND gate 224 now has two high inputs (input
222 from OR gate 220 and input 226 from inverter 228), so
the 40 KHz oscillator 46 is inhibited for .4 seconds. The
.4 second off pulse in the drive signal is indicated at
250 in Fig. 11.
When the single shot multivibrator 236 times out, and
since by this time switch 72 will have reached the position
drawn, the output of OR gate 220 goes low again, since it will
have lows at both its inputs. The low output of OR gate 220
triggers the second single shot multivibrator 212. Multivibra-
tor 212 produces a .4 second high pulse at its output. AND gate 208
1087~715
i
now has two high inputs, namely input 210 from multivibrator
212, and the other input 240 supplied directly from terminal
76 and the +6 volts supply. AND gate 208 therefore produces
a high output for .4 second (the timing duration of multi-
vibrator 212) and this applied to input 206 of OR gate 202,
produces a high at its output. The high output of OR gate
202, fed to the driver amplifier 204, produces a high pulse 252
(Fig. 11) at drive terminal 44a for the timing duration of
multivibrator 212 (.4 seconds).
As soon as multivibrator 212 times out, the
high input to input 206 of OR gate 202 is removed, and the
normal 40 KHz drive signal 230 is reapplied to the drive
terminal 44a. The system is now back in normal supervisory
operation.
In the system described, it will be seen that
the alarm signal transmitted by the satellites to the master is
a high level signal, i.e. it is the removal and subsequent absence
of the high level signal produced by oscillator 150. A "high
level" alarm signal as here used means a signal which differs
by a reasonably substantial amount from the previously
prevailing signal, so that even if the signal conductor is
unshielded, it will not normally pick up stray signals that
would be interpreted as an alarm signal. For example, the
difference will usually be at least one volt and preferably
higher in a cable of length not exceeding 500 feet. For longer
cables, a higher difference will usually be employed. Here,
+6 volts has been used for a system in which the cable length
is typically up to 1000 feet.
It will also be appreciated that certain features
of the invention may be used in systems which transmit low
level signals over shielded cables containing more than
_ 1 7 --
10877~5
four conductors. For example, the feature of inhibiting the
speaker of a satellite which has detected a disturbance, until
the drive signal is turned off, the termination of the drive
signal causing that speaker (or other alarm indicator) then to
enunciate, may be used in other systems, as may the walk
test feature.
