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FIELD OF THE INVENTION
The present invention is concerned with improvements in or relating to
radio broadcast communication systems, and in particular to a new low
power system providing broadcast communication between a number of
individual display modules and a central base station transmitting
information to the modules and also receiving information therefrom.
REVIEW OF THE PRIOR ART
There have been a number of prior proposals to automate in some way the
provision of item price information in a retail grocery store. Such a
system is attractive to store operators because of the economic benefits
that result, for example, from reduction or elimination of the labour
costs associated with maintaining the shelf labels and signs up-to-date;
reducing or eliminating the need to provide price tags on the individual
items; reducing or eliminating loss on stock due to price change lags and
the difficulty of quickly repricing a large number of individually priced
items; and to permit optimization of price distribution in the store with
the possibility of rapid and economical provision of time limited
specials. To this end there have been a number of proposals for such
systems.
Several important technical problems have prevented the cost effective
development of such systems. For example, the shelves that are now used in
most retail industries are constantly being rearranged. Any direct wiring
therefore becomes an expensive impracticality. Moreover, cost
considerations make it important that individual display modules be priced
low and expensive anti-fretting gold connectors used to connect the
modules to the wiring would overprice the units. Nevertheless, much effort
has been focused on the creation of clever connectors, and wiring schemes
as the solution. Wireless systems including infrared, acoustic and radio
broadcast have been proposed, but most have assumed that such a system
would simply be too unreliable for transmitting important pricing and
merchandising information.
U.S. Pat. No. 4,002,886, issued to Sundelin, discloses an "electronic
shelf" consisting of modules that are attached to the front edge of the
shelf and supplied by wire connections with the data required for display.
It teaches that as an alternative to wiring each of 10,000 or more modules
directly to the master computer, a simple address decoding system could be
used where a unique address is first transmitted followed by the data.
Each module in turn has its own unique address and, if the transmitted
address corresponds to the module address, then the data is accepted by
the module.
U.S. Pat. No. 4,028,537, issued in 1977 to N.C.R., proposes that a serial
addressing scheme be used. Each module is serially connected to the next
module similar to a Christmas tree string of lights, and they propose that
address decoding be accomplished by subtracting 1 from the current number
before sending it on to the next module. The module that receives a zero
accepts the data as being its own.
U.S. Pat. No. 4,500,880 issued in February 1985 to Motorola and proposes
that the UPC code be used as the address, in place of an arbitrary number.
DEFINITION OF THE INVENTION
In accordance with the present invention there is provided a radio
broadcast system comprising a broadcast transmitter and at least one
broadcast receiver comprising:
means for generating at the transmitter a first carrier of a first
reference frequency N and for broadcasting that carrier;
means for generating at the transmitter a second carrier of second
frequency N/n derived from the first reference carrier and for modulating
the second carrier in accordance with information to be transmitted
thereby;
means at the receiver for receiving the first carrier and for dividing it
by the divisor n to produce a corresponding demodulating signal of
frequency N/n; and
a detector at the receiver receiving the second modulated carrier and
demodulating it with the said demodulating signal to generate a resulting
information signal.
Also in accordance with the invention there is provided a system for the
operation of radio receiving shelf-mounted modules by signals from a
broadcast transmitter comprising:
at least one metal shelf unit comprising a plurality of horizontal metal
shelves each having an outer longitudinal edge;
a plurality of said radio receiving modules each mounted on a respective
shelf outer longitudinal edge;
a broadcast radio transmitter and antenna transmitting radio signals to be
received by the said modules; and
said antenna comprising an antenna segment for each shelf unit, the segment
lying upon a surface of the respective shelf unit parallel to the said
shelf longitudinal edges of the unit for electromagnetic coupling with the
unit and the production of a corresponding increased field signal strength
at the shelf longitudinal edges to be received by the modules mounted
thereon.
Further in accordance with the invention there is provided a radio
broadcast system comprising a base broadcast transmitter/receiver and at
least one module broadcast receiver/transmitter comprising:
means for generating at the base transmitter/receiver a reference carrier
in the form of sequential discrete envelopes thereof of predetermined
duration;
means for generating within the envelope at the base transmitter/receiver a
base data word and for transmitting the base data word to the module
receiver/transmitter;
means in the module receiver/transmitter for receiving the base data word
and in response thereto generating a timing period interposed between the
received base word and a module word to be transmitted
means in the module transmitter/receiver for transmitting the module data
word upon termination of the said timing period; and
the lengths of the base and module words and the timing period being such
that the transmitted module word terminates with the termination of the
corresponding envelope.
Further in accordance with the invention there is provided a radio
broadcast system receive module for receiving a reference signal of a
first frequency and a second data modulated signal of frequency which is a
multiple of the reference frequency comprising:
a module body;
a first loop antenna coil mounted in the module body in a respective first
plane; and
a second loop antenna coil mounted in the module body in a respective
second plane orthogonal to the said first plane.
Further in accordance with the invention there is provided a radio
broadcast system comprising a base transmitter/receiver and a plurality of
shelf mounted module receivers/transmitters wherein each module comprises:
a microprocessor;
a visible button accessing a respective visible register of the
microprocessor;
at least one concealed button accessing a respective concealed register of
the microprocessor; and
the microprocessor being addressable to enable the concealed button,
whereby data can be entered into the microprocessor by operation of the
concealed button.
Further in accordance with the invention there is provided a radio
broadcast system comprising a base transmitter/receiver and a plurality of
shelf mounted receiver/transmitter modules each receiving data broadcast
from the base and each capable of transmission to the base, each of said
modules being designated for a specific product item, the system also
comprising at least one mode module designated for a group of product
items and addressable for entry of data generic to the said group.
Further in accordance with the invention there is provided a radio
broadcast system comprising a base transmitter and a plurality of module
receivers, wherein each module includes as a power source a capacitor, and
a rectifier charging circuit for the capacitor, the power for the
rectifier charging circuit being obtained from the module broadcast
receiving antenna.
Thus, a wireless display module for an "electronic shelf" has four major
requirements:
1. Two Way Communication;
2. Long Battery Life (3-5 years+);
3. Minimal Error Rates; and
4. Low Cost.
To simultaneously achieve all four requires several compromises. First to
achieve low error rates and two way communication a phase modulation
system is used. This previously has required a very complex circuit both
to encode and decode the analog signal consisting of a phase locked loop
or square law device, several amplifiers and encoding and decoding
circuitry. A second major area of concern is that while with some
difficulty it is possible to create a one way link of base station to
module, the return signal from module to base station represents a major
challenge. Power consumption in any CMOS device is due largely to
capactive discharge; thus, as the driving frequency for reception
increases so does the power consumption. However, as the transmission
frequency decreases, the efficiency for fixed transmission becomes very
poor.
These problems are reduced with this invention by a unique phase encoding
system employing a special reference carrier. This reference carrier is,
in a preferred embodiment, nominally 132 kHz and initially is activated to
frame the transmission from the base station in an envelope of
predetermined length. The module takes the 132 kHz carrier and divides it
by 2 using a conventional flip-flop to create a 66 kHz internal reference.
The base station can then transmit digital data by phase shifting a second
66 kHz carrier also derived from the reference. The module makes a direct
comparison with the 132 kHz divided by 2 signal to obtain a modulated
digital output. When the module transmits back it again uses the 132 kHz
signal as a reference and creates a 66 kHz carrier. This 66 kHz carrier is
phase modulated to encode the digital data. The module transmitted signal
is transmitted within the reference envelope a predetermined period after
the data is received from the base station. The base station has the
advantage that it therefore knows with a great deal of precision the
frequency and timing of the return signal. This makes it possible to
extract acceptable digital data with low signal-to-noise ratios with a
high degree of reliability.
DESCRIPTION OF THE DRAWINGS
Particular preferred embodiments of the invention will now be described, by
way of example, with reference to the accompanying diagrammatic drawings,
wherein:
FIG. 1 is a perspective view illustrating a typical layout of part of a
store in which the apparatus of the invention is employed;
FIG. 2 is transverse cross section through a shelf unit of FIG. 1 to
illustrate the enhanced broadcast field that is obtained;
FIG. 3 is a front elevation of a shelf module of the invention, some of the
internally mounted components thereof being shown in broken lines;
FIG. 4 is a schematic diagram of the operating circuit of one of the
modules;
FIGS. 5a through 5e illustrate the broadcast signals received by the
modules and the digital signals produced therefrom for operation of the
module;
FIG. 6 is a schematic, illustration of the format of the operating
binary/word that is transmitted to the module;
FIGS. 7a through 7e illustrate transmission of base station data to a
module and vice versa within a reference signal framing envelope;
FIG. 8 illustrates apparatus for investigating the best phase relationship
for transmitting and receiving for each module;
FIG. 9 is a plot of a typical table of the different transmit/receive phase
relationships in the modulator and detector at the base station;
FIG. 10 is a more detailed schematic circuit diagram of the pipper circuit
of FIG. 4;
FIG. 11 is a more detailed schematic circuit diagram of the decoder circuit
of FIG. 4;
FIG. 12 is a more detailed schematic circuit diagram of the encoder circuit
of FIG. 4;
FIG. 13 is a more detailed schematic circuit diagram of the sync logic
circuit of FIG. 4;
FIG. 14 is a more detailed schematic circuit diagram of the phase
detector/modulator circuit of FIG. 4; and
FIG. 15 is a circuit diagram of a chargeable circuit for replacement of the
battery of the circuit of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be specifically described in its application to a
self-service retail food store, particularly one of the supermarket type,
in which typically there may be from about 5,000 to about 10,000 different
items to be sold, each requiring its price to be clearly and positively
identified, and each requiring that price notice to be readily changeable,
often at very short notice, to take account of seasonal changes, etc. in
wholesale prices, and to implement the marketing strategy of the store. It
will be evident, however, that the invention is also applicable to other
types of stores, such as clothing and general department stores, and to
completely different types of installation, such as industrial plants,
warehouses and distribution centres, exhibition and convention centres,
and the tool and supply cribs of manufacturing establishments.
FIG. 1 illustrates part of a typical retail store consisting of a plurality
of spaced parallel multiple shelf units 10, each having a plurality of
shelves 12, on the upwardly inclined front edge of each of which is
mounted a plurality of longitudinally spaced shelf unit modules 14, one
for each item whose price is to be displayed. The store also includes a
plurality of check-out stations 16, each of which includes a point-of-sale
terminal having a scanner able to read the bar code that is now almost
universally an integral part of the item labels, and to display and record
the corresponding price in the cash register. The stations 16 typically
are controlled from an in-store main computer 18 to which information may
be supplied as required via a telephone link 20 from a central office, or
by direct keyboard, EPROM, tape, or floppy disc input, as will be apparent
to those skilled in the art. This information is also supplied as required
from the main computer 18 to a system computer 22 (which may also have its
own similar input 23), which in turn is connected to a base station
transmitter/receiver 24. The computers and the base station between them
store the information required by the store in connection with the items
sold, such as:
(a) the identifying bar code;
(b) the item price that day;
(c) information as to previous price history;
(d) details of a temporary sale price to be offered that day at
predetermined times;
(e) the corresponding unit prices;
(f) the aisle, shelf and shelf position location;
(g) the number of facings on the shelf;
(h) the size of a standard unit for re-ordering;
(i) the list of words that each module can reproduce upon command; and
(j) the program that will result in announcements to be displayed on the
module, such as "ON SALE", "15% OFF", etc., and the times at which it is
to be displayed.
In this embodiment the base station 24 is a phase modulated radio frequency
transmitter, the output of which is fed via switches 26 controlled from
the station 24 via a separate control line 27 to the parallel segments 28
of the in-store broadcast antenna, which is disposed so that the parallel
loop planes of the segments are horizontal. Each immediately adjacent pair
of switches controls the antenna segment between them. Each of these
segments has the two horizontal power carrying leads of the respective
horizontal loop lying along the respective top surfaces of the two
associated row of metal shelf units 10 so that each is electromagnetically
coupled to its respective unit. With such an arrangement and at the
frequencies employed the transmission is principally near field inductive
and the practical range of each antenna segment does not extend much more
than its own dimension beyond the shelf unit. The switches 26 permit the
selection of the antenna segment or segments that are required to be
energized at any time, so as to avoid energization of modules 14 that are
not to be addressed, avoiding unnecessary operation thereof and power
consumption, as will become evident from the description below. In this
embodiment the connections to the antenna segments are taken through the
utilities space above the store suspended ceiling requiring downwardly
extending portions 30, but they could also be led through the floor and up
the ends of the shelf units.
The shelf units 10 of such a store are almost universally of thin sheet
steel because of load bearing requirements and it is found unexpectedly
that, at the frequencies at which it is preferred to operate the system,
which will be described in more detail below, placing part of the antenna
segment 28 in sufficiently close contact with the metal structure so as to
be electromagnetically coupled thereto results in greatly increased local
radiation fields at the outer longitudinal edges of the shelves on which
the modules 14 are located, as is indicated by the broken-line outlines 32
in FIG. 2. Thus, in a test installation voltages measured at the module
locations were expected to be in the range of 0.5-3 volts, but instead
were found to be in the range 1-9 volts, and moreover the voltages at the
lower shelves further from the antenna were higher than at the higher
shelves.
Referring now to FIG. 3, a shelf mounting module of the invention comprises
a plastic molded case 34 that is generally rectangular as seen in plan and
elevation, the front face of which has a rectangular aperture 36 behind
which is mounted an LCD display 38 that is capable of displaying the
required information upon suitable energization of the component segments
thereof. A label is applied to the front face, the upper part of which
contains item identification, while the lower part carries the
corresponding bar code and instructions for operation of a visible unit
price pushbutton 40. The manner of operation of the unit price button 40
is more specifically described in our U.S. Pat. No. 4,603,495, the
disclosure of which is incorporated herein by this references.
The module also has mounted therein behind the label two "concealed"
pushbuttons 42 and 44 disposed respectively above and below the visible
button 40, which during normal shopping hours are usually disabled to
prevent their accidental operation by, for example, a child touching the
module. The functions and operation of these two concealed buttons when
they are enabled will be described below. The case 34 also mounts a low
impedance, low Q, air-cored receiving/transmitting loop antenna coil 46
disposed with the plane of the loop parallel to the casing front face and
with their longer sides parallel. The case further mounts a higher Q,
higher impedance ferrite-cored receiving loop antenna coil 48 disposed
with its loop plane at a right angle to the casing front face and thus at
a right angle to that of the coil 46; in this embodiment its longer loop
side is also parallel to the case longer edge. The loop is positioned as
centrally as possible and, with the relative orthogonal placement,
minimizes the coupling between them. It will be noted from FIG. 2 that the
modules are mounted on the shelf edges inclined at an angle to the
vertical, so that the loop planes of the two antennae 46 and 48 are not
orthogonal to that of the loop antennae segments 28, but are inclined at
that angle, which is necessary for other than minimal coupling between
them. The above mentioned electromagnetic coupling is also found
unexpectedly to effectively increase this angle, as though the field is
being bent, so that the transmission efficiency from both of the coils to
the store antennae is increased with minimum coupling between the coils
themselves. Each module also contains a circuit board which is not
illustrated in FIG. 3 but is shown schematically in FIG. 4.
The power for each module is provided by a power source 50, which in this
embodiment is a lithium battery of 0.2 amp hour capacity having a
potential life for operation with the circuit of the invention of about
3-5 years. In view of the fact that a typical retail store will contain at
least 5,000 modules this is the extent of the life that is preferred by
the industry, since battery replacement of so many modules is a
time-consuming and costly operation. The manner in which the circuit of
the invention is able to obtain such an extended shelf life with such a
battery will be described below.
The base station transmitter transmits a first reference carrier signal of
frequency N, which in this embodiment is 132 kHz, the frequency being
determined by division down from a crystal controlled oscillator to obtain
the desired stability. Provided the module is powered to receive a signal,
as will be described below, this is received by the smaller ferrite-cored
antenna 48, amplified by amplifier 52 and divided by an integer n, which
in this embodiment is 2, by divider 54 to produce a demodulating or
heterodyning signal of 66 kHz frequency (N/n) that is fed to a circuit 56,
to be described in more detail below, which is operative alternatively as
a bi-phase detector or a modulator. The divider output is also used as a
clock signal and for that purpose is fed to a pipper 58, a divider 60 and
a decoder 62. The transmitter also transmits an information containing
signal, to be described in more detail below, consisting of a second
carrier at 66 kHz, also derived from the same crystal standard, phase
modulated by a coded digital signal, this second modulated carrier being
received in the module by the larger air-cored antenna 46 and fed to the
circuit 56 configured as a phase detector. The output of the bi-phase
detector is an information-containing encoded, digital pulse signal with
pulses that are positive-going or negative-going with respect to ground
resulting from demodulation of the second modulated carrier signal from
antenna 46 employing as a demodulating reference the divided signal from
divider 54. This digital output signal is fed to a narrow band filter and
amplifier circuit 66, in which it is shaped as required and unwanted
frequency components (such as the 132 kHz harmonic) are removed. In this
embodiment a pass band filter of 3 kHz is employed.
A high Q, ferrite-core coil 48 is preferred for the reference frequency
antenna since it is relatively immune from the effects of ambient noise,
which is relatively high in the particular environment of a food store
with extensive lighting, refrigeration and air conditioning installations,
particularly to the effects of "spikes" which might otherwise cause
unwanted frequency and phase changes. On the other hand, a low Q air cored
coil is preferred for the receive/transmit antenna 46, particularly when
it is required to transmit, since more power can be radiated as compared
to a ferrite-cored antenna, and the receiver bandwidth can be greater to
permit higher BAUD rates to be used.
FIGS. 5a-5e show the sequence of signals beginning with that received by
the antennae and subsequently that obtained from the phase detector 56.
Thus, FIG. 5a shows a typical 132 kHz first carrier signal received by
antenna 48, and FIG. 5b shows the resultant divided demodulating reference
signal from divider 54. FIG. 5c shows a typical phase modulated signal
obtained from antenna 46 having two phase transitions at X and Y. The
signal at 5d is the output of the phase detector resulting from detection
using the reference frequency signal 5b, and that at 5e is the resultant
signal after smoothing and filtering, consisting of either positive- or
negative-going pulses about the zero volt line 0V. Since all of the
subsequent circuits are of binary digital type, the high pulse value is
"1", while the low pulse value is "0".
The amount of information required to be transmitted to the module is
relatively limited and it is found adequate to operate with a 32-bit
binary operating word, as illustrated by FIG. 6, subdivided into eight
4-bit "nibbles" N4-N11. The word is preceded by three password nibbles
N1-N3 and ends with two sync nibbles N12 and N13, whose function will be
described below. The first data nibble N4 of the word is a module
instruction start, while the second nibble N5 is an instruction
modification to the instruction start, the two instructions combining to
instruct the module as to the action that is to be taken with the data
nibbles N6-N9. The last two nibbles N10 and N11 are both complement check
sum coded for the data nibbles, this relatively large check sum coding
being employed to ensure accuracy for the data under the difficult
conditions in which the modules operate. The complement is employed to
ensure that a positive response is always obtained, so that a "1" is
always detected avoiding the ambiguity caused if no response were
obtained, which might be due to module failure. Another level of security
is provided by encoding the digital signals at the transmitter and
decoding in the module, and vice-versa when the module is transmitting,
using, for example, bi-phase mark or space coding. Since the system is not
in any way time-critical, a conservative coding system can be employed
despite the fact that it results in half speed transmission. Such coding
of digital data is described, for example, in Pages 384-395; 535-536 of
"Introduction to Communication Systems" by F. G. Stremier, published 1982
by Addison Wesley, Redding Mass, which is incorporated herein by this
reference. At the base station the coding and encoding will be included in
the software of the controlling microprocessor. The coding system employed
in this embodiment is such that upon encoding both "0" and "1" will
produce pulses with transitions at the ends of the respective bit periods,
while a "1" will additionally result in a transition at the middle of a
bit period; and vice-versa upon decoding. A conservative coding of this
type has the advantages that it gives a zero average voltage, which is
more certain than one which gives an average positive or negative voltage
that can vary and perhaps cause loss of data, and that it constitutes a
built in clock making it easier to synchronize the coded and uncoded bits
of the original data. It is found in practice important that the coding
system used is polarity insensitive, so that initiating conditions of the
circuits employed will not affect the validity of the data.
Referring again to FIG. 4, the overall control of the system is maintained
by a microprocessor chip 68, which can be a standard chip as employed in a
digital watch or clock, such a chip already including, besides its
microprocessor and internal clock, the registers for the control of the
LCD display 38 which corresponds to the usual LCD watch or clock display;
the control being exercised through connection 70 with the requisite data
stored in the many storage registers provided in the chip. For example,
the data for item price, which usually must be displayed continuously,
will be stored in the register that is normally continuously accessed,
while the corresponding data for unit price display is stored in another
of the registers which is accessed to replace the item price information
on the display upon the shopper pressing the visible button 40, the button
assembly being connected to the clock chip through a respective register
72. The chip also may contain program registers, as many as three, which
can be programmed to cause the chip to cycle through the display registers
in a preset sequence, so that individual words in those registers can be
made to display in sequence, thus providing a special announcement upon
addressing the particular program register, each program giving rise to a
respective message selected from the words available in the registers.
Such a chip may, for example, have as many as fourteen display registers,
the smallest of which are of 16-bits capacity with the lead 4 bits
available for display instructions, while the remaining 12 bits are
available for display data; the chip may also contain as many as four
maintenance registers of smaller capacity, e.g. 8 or 4-bits, which can be
used for other functions such as are described below.
Another important function performed by the microprocessor chip 68 is to
provide a much reduced duty cycle for the radio frequency parts of the
module, such as the amplifiers 52 and 66, which are of relatively high
power consumption. The chip will include a programmable on and off
register and in the chip employed this is of 16-bit capacity with the
first 4 bits used to set the length of turn on time and the remaining 12
used to set the length of turn off time. Thus, typically the chip
continuously turns on the RF circuits for a period of 0.5 seconds and, if
no first reference carrier signal is received during that period, it turns
them off again for a much longer period, typically 10 seconds, to give a
duty cycle of 20. As long as the first carrier signal is detected, as
described below, the chip remains turned on until the reference ceases for
the "RF ON" time which in this embodiment is 0.5 second duration. Such a
cycle will usually increase the battery life by a factor of about 2 times,
since each module is operative for only a very small fraction of the total
time, but of course the microprocessor chip itself and other parts of the
circuit must remain powered at all times.
The base station will usually remain powered, but quiescent, until it is
instructed to transmit to one or more of the modules, whereupon the first
reference carrier is transmitted for at least 101/2 seconds, to ensure
that transmission occurs during the "on" portion of all the module duty
cycles; at the end of this transmission all of the modules will therefore
be "on". The first carrier is then switched off for a period of about 50
milliseconds, which is too short for the modules to switch off, and both
the first reference and the second modulated carriers are now transmitted
simultaneously. This permits the first carrier to be used to "frame" the
transmitted data and the data received from the module, as will be
described below.
Referring again to FIG. 4, with the RF portions of the module powered by
the clock chip signal from "RF POWER ON" and the reference carrier and the
modulated carrier received during that period, the output of the amplifier
66 is fed to pipper 58 from "REC DATA" terminal to "REC DATA" terminal,
the clock signal from the divider 54 being fed to "66 kHz CLOCK". The
pipper produces an output pulse or pip each time there is a state change
in the received data and these are fed from its "PIPS" terminal to the
"PIPS" terminal of the decoder 62 which decodes the bi-phase coded data
back to normal binary code data. Thus, the decoder, which also receives
the 66 kHz clock signal, determines whether a pip occurs in the middle of
a time period and, if so, generates a "1" and, if not, generates a "0".
The decoded binary signal is fed from terminal "DEC DATA OUT" to terminal
"INPUT" of a 4-bit shift register 74 in which the signal shifts while the
data in the register is fed from terminal "D OUT" to terminal "D IN" of
sync logic circuit 76. When synchronism is detected by sync logic circuit
76 between the first password nibbles N1-N3, and after a one nibble delay,
a "LATCH DATA" signal is sent from that terminal of circuit 76 to the
"LATCH" terminal of a 4-bit latch 78, and the subsequent data nibbles
N4-N11 are subsequently latched into the latch from terminal "D OUT" to
terminal "D IN", and fed through tristate buffers 80 to the "4-BITS DATA"
terminal of the microprocessor chip for utilization therein. Tristate
buffers are required since the data moves in both directions to and from
the microprocessor. The password N1-N3 will be the same for all modules
and is employed to ensure that the module does not attempt to respond to
spurious data; three nibbles are employed for added security; typically,
the word will be a unique three digit number, the first of which will
usually be zero. The first transmission or transmissions supplied to the
chip 68 have in the instruction and data nibbles N4-N11 an identifying
instruction or instructions for the module to be addressed; upon the chip
68 detecting that it is being addressed, it is enabled to receive further
data and write it into its registers, while if it does not detect an
identifier, it remains quiescent and ignores the further data received
from the buffers.
As will become evident, it is essential for correct operation that the
reference carrier is present; it is detected by divider 60 which transmits
a one-sixteenth divided clock signal (4125 Hz) from terminal Q.sub.3 to
the corresponding terminal Q.sub.3 of the sync logic 76; effectively the
sync logic circuit counts the number of cycles received in a time period
set by this clock signal and, if sufficient are counted for this to be the
required carrier, it sends a "carrier detect" signal to the respective
microprocessor chip terminal, whereupon the clock chip returns a "receive
enable"signal to the sync logic. The carrier detect signal is also used as
the reset signal for the shift register 74 and the latch 78. The sync
logic also, upon detection of the required password N1-N3 flags the chip
68 through the "DATA READY" connection every four bits synchronous with
latching the data into the latch, so that it is ready to receive the data
to be used. Upon conclusion of the receipt of each four bits, the "data
ready" signal is cleared by the microprocessor chip by pulsing the "DATA
ACCEPTED" connection.
Upon the reference carrier ceasing, the carrier detect signal to chip 68
also ceases and a time out time period starts to operate that will usually
be of the same length as the turn on time and produced by the same
register, the RF circuits being switched off after this time until a new
time out period of 10 seconds elapses. This means that instructions to the
module must be transmitted at a rate faster than this off time period.
A system as already described with individual battery operated modules,
each of which can be individually addressed by a broadcast transmitter, so
that no hard wiring is required, is already of great value in the type of
installation to which it is directed. Some way usually is needed to
confirm that data has been safely received, and systems for this will be
described below. However, the value of the system is even greater by
providing that the modules can transmit appropriate information back to
the base station and the store computer. For example, it is then possible
for re-stocking personnel to walk along the aisle and immediately upon
visual inspection of an item transmit back the identity of the item, its
current shelf location and the quantity required for re-stocking. All of
this is to be accomplished, if possible, without decreasing the battery
life more than is absolutely necessary, in order to achieve the desired
target of 3-5 years life or longer. The manner in which this is
accomplished in this embodiment will be described after further
description of the protocol employed to transmit data to the module.
As has been indicated above, the operation of a radio frequency broadcast
system of the invention involves two different difficult problems, namely
the extremely noisy environment in which the inherently low power system
must operate, and the need for extreme battery life which implies
extremely low power consumption. The power of the base station can of
course be made as high as is necessary with relatively low additional
cost. A phase modulated system is selected because of its inherent high
noise tolerance, and digital coding of the transmitted data is employed
again because of the low power digital circuit elements than can be
employed to manipulate such data. Encoding of the transmitted digital
signals in both directions provides yet another level of security for the
subsequent accurate detection of the data. The conventional procedure in
demodulating phase modulated signals is to employ a phase locked loop in
the detector, but in the very noisy environments encountered there is the
danger that the loop would lock onto adjacent interference instead of the
signal, or take so long to lock onto the signal among the ambient noise
that data transmission becomes impossibly slow, even though speedy
transmission is not usually required. A phase locked loop therefore would
need to be kept in constant operation and could not duty cycle as
described above, and would in addition require | | |
