U.S. patent number 3,879,723 [Application Number 05/317,486] was granted by the patent office on 1975-04-22 for destination sign system using liquid crystal display devices.
This patent grant is currently assigned to Transign, Inc.. Invention is credited to Thomas F. Hornung.
United States Patent |
3,879,723 |
Hornung |
April 22, 1975 |
Destination sign system using liquid crystal display devices
Abstract
A destination sign is disclosed for use in buses and trains with
a remote control which permits the operator to selectively cause
display of any one of a large number of names or words. The sign is
comprised of liquid crystal type display devices and the
information to be displayed is selected from a memory device where
it is stored in binary word form. A clock controlled scanner causes
sequential reading of plural memory devices a word at a time which
are fed through a first parallel/serial register to the remote sign
in serial form, bit-by-bit. At the remote sign the serial data,
under synchronized control means from the clock, is fed through a
serial/parallel register to a set of latch registers under the
control of a clock controlled scanner. The latch registers hold a
data word corresponding to each character of the liquid crystal
display and provide continuous actuation thereof. The synchronizing
signal from the clock not only synchronizes the data feed through
the serial/parallel register, but also by means of a tone burst
generator and detector means provides a reset signal for the
serial/parallel register after all data words have been
transmitted. Thus the latch registers are updated at the end of
each sequence of data transmission. In addition to the continuous
energization and updating of the liquid crystal display devices of
the sign, the sign includes back-lighting means, louver means, and
partial reflector means to enhance the readability of the sign.
Inventors: |
Hornung; Thomas F. (Berkley,
MI) |
Assignee: |
Transign, Inc. (Pontiac,
MI)
|
Family
ID: |
23233872 |
Appl.
No.: |
05/317,486 |
Filed: |
December 21, 1972 |
Current U.S.
Class: |
345/51;
345/204 |
Current CPC
Class: |
G02F
1/133524 (20130101); G09G 3/18 (20130101) |
Current International
Class: |
G09G
3/18 (20060101); G02F 1/13 (20060101); G02F
1/1335 (20060101); G09f 009/32 () |
Field of
Search: |
;350/16LC
;340/336,324R,324M,334 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Curtis; Marshall M.
Attorney, Agent or Firm: Reising, Ethington & Perry
Claims
The embodiments of the invention in which an exclusive property or
privelege is claimed are defined as follows;
1. A programmable display system for information comprising: a
plurality of field-responsive liquid crystal character display
devices disposed in lineal juxtaposition to form a sign, each of
said character display devices comprising a plurality of separately
actuable character-forming segments whereby each device may
represent any of a plurality of individually recognizable
characters, input means for producing any of a plurality of digital
input programs, data storage means connected to said input means
and responsive to said programs for producing a plurality of
multiple bit data words each representing segments of a character
in said sign to be actuated to represent the programmed
information; and transmitting means for distributing said data
words to said segments of said liquid crystal character display
devices, said transmitting means comprising first register means
connected with said storage means for serializing the bits of said
data words, clock means, first scanning means controlled by said
clock means and connected with said storage means for causing said
plurality of data words to be read one at a time into said register
means, said input means, storage means, clock and transmitting
means being located remotely from said sign, receiving means
located proximate said sign and including a latch register means
for each character of said sign; second register means for
receiving said plurality of data words with the bits thereof in
serial form and for converting the bits of each word into parallel
form, a first transmission line connecting the first and second
register means for transmission of the bits of the data words in
serial form, each of said latch register means being connected
between said second register means and a respective character
display device, power supply means for said liquid crystal display
devices, a plurality of electronic switch devices connected
respectively to said segments and to said power supply and having
an actuating input for selective actuation thereof, means connected
between said latch registers and said switch inputs for selectively
actuating the switch devices in accordance with said data words,
second scanner means connected by a second transmission line to
said clock and controlled thereby and connected to said second
register means for causing transfer of one word at a time from the
second register means to successive latch register means, whereby
each latch register means maintains continuous actuation of its
respective display device during the subsequent actuation of the
remaining display devices in said sign, generator means for
generating a synchronizing signal having a frequency which is less
than the frequency of said clock means, said generator means being
connected with said second register for synchronizing the actuation
thereof with said data words, detector means connected with said
second register for detecting the occurrence of said synchronizing
signal at the end of transmission of all of said plurality of data
words, and reset means connected between said detector means and
said second register for resetting said second register after
receipt of said plurality of data words whereby the succeeding
transmission of said data words will be effective to provide data
to said latch registers in accordance with such succeeding
transmission.
2. A display system as defined in claim 1 wherein said sign
comprises in addition to said plurality of liquid crystal character
display devices, a separately actuable back-lighting means for
illuminating said sign under low ambient light conditions.
3. A display system as defined in claim 2 wherein said sign
includes in addition, a set of louvers between the liquid crystal
character display devices and the back-lighting means for
concealing the back-lighting means from certain predetermined
viewing angles.
4. A display system as defined in claim 3 including a partially
mirrored surface area between the liquid crystal character display
devices and the louvers for partially reflecting ambient light
incident on the sign.
Description
INTRODUCTION
This invention relates to display devices of the type used to
present alpha-numeric information and particularly to a
programmable destination sign system for use in mass transit
vehicles and the like wherein liquid crystal display devices are
employed.
BACKGROUND OF THE INVENTION
It is well known that most mass transit vehicles such as buses and
trains carry destination signs either externally, internally or
both for the purpose of conveying certain information to passengers
and respective passengers. One well known prior art destination
sign employs a curtain or scroll of flexible material such as
fabric or plastic having a serially arranged sequence of
destination statements or the like which may be selected by
manually rotating the curtain or scroll until the proper message
appears. A later improvement on such destination signs employs
programmable input means and a curtainmoving servo system such that
an operator might select a given message automatically and from a
remote station simply by providing certain address information by
way of switches, punch keys or the like.
Both of the prior art devices described above have the disadvantage
of requiring a relatively large curtain or scroll having all of the
possible destinations or other messages prefabricated therein.
Accordingly, such destination signs lack adaptability with respect
to changing destination indicators or with respect to the use of
such systems for displaying information other than that which is
principally related to destination.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention a programmable display
system for information is provided having the attributes of
mechanical simplicity, ease of message programming and extremely
high adaptability to varying information conditions such that a
large number of messages may be available for selection without the
requirement for a bulky prefabricated message curtain or scroll. In
general this is accomplished by means of a display system
comprising a plurality of display devices of the so-called liquid
crystal type. The use of liquid crystal display devices for various
purposes, including advertising and message communication, has been
documented in the technical literature as well as in the patent
literature. Liquid crystal display devices are described, for
example, in an article entitled "LIQUID CRYSTAL DISPLAYS" by R. W.
Gurtler and Craig Maze appearing in the November 1972 issue of the
IEEE SPECTRUM, pages 25 through 29. The use of such crystal display
devices in a destination sign system or the like completely
eliminates the need for a prefabricated mechanical curtain or
scroll and facilitates the use of an electronic system for
generating a pattern of data signals which may be applied in the
proper fashion to an array of liquid crystal message segments to
instantly compose any of a large number of various information
displays.
In accordance with the second feature of the invention a
destination sign system or the like is provided wherein the input
and message programming portions are interconnected with one or
more remote liquid crystal sign portions by way of a minimum number
of electrical conductors thereby reducing wiring costs and
complexities in destination sign systems and the like wherein the
sign portions are often necessarily located substantial distances
away from the input or control devices. In general this is
accomplished by means of a digital input portion wherein the
message to be displayed is programmed by means of a simple digital
code and wherein the data signals representing the number and
sequence of liquid crystal character segments to be actuated is
generated in parallel form, transmitted to the sign in serial form
along with a synchronizing clock signal, reconverted to at least
partial parallel form at the sign, and applied to the liquid
devices in parallel or partially parallel form.
In accordance with a third, more detailed, feature of the
invention, a synchronizing signal is provided whereby the message
displayed in the liquid crystal sign is repeatedly refreshed in a
periodic fashion and renewed in response to the entering of a new
input code only after a complete display cycle has been realized.
In general this is accomplished by means of a synchronizing and
storage arrangement comprising a sync signal generator operating in
conjunction with the system clock to establish a periodicity to the
message data transfer from programmer to sign and a latch
arrangement which operates to present continuous data oriented
actuation signals to the liquid crystal segment switches during the
presentation of any given message.
In accordance with a still further feature of the invention liquid
crystal lifetime is increased by the use of a power supply which
provides for the generation of an effective alternating current
energizing wave form while at the same time generating D.C.
energization voltages for the data transfer system logic. This
preferred power supply is energizable by an A.C. input signal and
includes a rectifying bridge and capacitor arrangement to generate
a stable D.C. reference, and further includes chopper means
decoupled from the D.C. signal takeoff point for generating an
effective alternating current signal for application to the
combination of a liquid crystal sign segment and a switch
therefor.
A still further feature of the invention involves the provision of
an improved destination sign for vehicular use wherein the message
to be displayed exhibits enhanced readability under a broad range
of lighting conditions from bright sunlight to total darkness. In
general, this is accomplished by means of an arrangement of liquid
crystal display devices in combination with a partially reflective
mirror surface, a thin and selectively oriented louver film and
artificial back-lighting means such as flourescent tubes, the whole
arrangement being oriented and constructed so as to provide maximum
readability from the typical viewing angle while at the same time
preventing the observation of the flourescent tubes.
In summary the present invention has for a particular objective the
provision of a substantially improved destination sign system for
vehicular use so as to permit the operator to quickly and easily
generate any one or more of a large number of messages to be
displayed and to replace the usual scroll type curtain which has
been present in the destination sign field for many years. It is,
however to be understood that the present invention is not limited
in application to vehicular destination signs but may find
advantageous application in other areas including, for example,
airline and other transportation terminal displays for time tables,
gate numbers, arrivals, departures, etc., rapid transit vehicle
destination and run number displays including monorails, convention
rails, etc., advertising media, entertainment, news presentations
and so forth.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a transit sign system embodying the
present invention;
FIG. 2 is a plan view of a liquid crystal display element for
representing alpha-numeric characters in the system of FIG. 1;
FIG. 3 is a side view of the liquid crystal display character of
FIG. 2;
FIG. 4 is a schematic circuit diagram of a power supply for liquid
crystal display device energization;
FIG. 5 is a detailed block diagram of an illustrative embodiment of
the invention; and
FIG. 6 is an exploded side view of a destination sign for buses and
like transit vehicles in combination with the system of FIG. 1.
DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT
Referring to FIG. 1, the block diagram of the illustrative
embodiment of the invention is shown to comprise a digital signal
generating input section 10 for use by an operator in specifying,
according to a predetermined code, any of a large number of
messages to be graphically displayed by means of a multi-character
crystal display sign 12. The input section 10 includes means,
hereinafter described in greater detail, for generating serialized
digital data signals which are transmitted to the remotely located
liquid crystal sign 12 by means of the so called "twisted pair"
transmission line 14. In addition, the input section 10 comprises
means for generating clock signals for synchronizing purposes,
these signals being transmitted from the input section 10 to the
sign 12 by means of a second twisted pair transmission line 16. As
indicated in FIG. 1 the twisted pair transmission lines 14 and 16
represent the only electrically conductive connections between the
input section 10 and the sign 12. Accordingly the sign 12 may be
remotely located from the control area, the wiring requirements
between the two being minimized by the relatively small number of
conductive paths required. As is also shown in FIG. 1, additional
signs may be conducted in parallel with sign 12.
In FIG. 1 the sign 12 is shown to comprise a lineal array of
juxtaposed liquid crystal display devices 18. In FIG. 1 the display
devices are labeled from left to right C-l through C-8, it being
understood that each device 18 is capable of representing each of
the alphabetic and numeric characters in accordance with a
particular number and arrangement of input data signals which are
received by that device. Although only eight liquid crystal display
devices 18 are shown in FIG. 1 it is to be understood that the sign
12 may comprise a larger or a smaller number of such devices and a
sixteencharacter sign is more typical. Each of the liquid crystal
display devices 18 is energized by means of a digit driver 20
comprising a decoding arrangement and switch set hereinafter
described in greater detail. The digit drivers 20 are actuated in
groups of eight display bits or display device segments at one time
by means of an eight-bit serial-to-parallel converting register 22.
The register 22 is connected to receive the serialized data signals
from the input section 10 by way of the twisted pair transmission
line 14. In addition, the digit drivers 20 are caused to operate in
a synchronized sequence by means of a clock driven scanner 24 which
is connected to receive the clock signals by way of the twisted
pair transmission line 16. The clock scanner 24 is connected to
each of the digit drivers 20 for distributing the data signals to
the devices 18 in groups of eight bits each until all devices
receive such signals as are required to compose a given message.
Finally, a power supply 26 is connected to the switch portions of
each of the digit drivers 20 to present an eighty volt alternating
current, square waveform for energization of the liquid crystal
display segments in accordance with the data signal pattern.
Input section 10 of the destination sign system shown in FIG. 1
comprises a three-digit, manually-operated input device in the form
of a set of three thumbwheel switches. Each of the thumbwheel
switches has ten positions and accordingly the device 28 is capable
of selecting any one of as many as nine hundred ninety nine message
codes, each code representing a different message to be programmed
into and displayed by the sign 12. Again the thumbwheel switch
device 28 as well as the number of selectible messages is specified
for purposes of example only and, as will be apparent to those
skilled in the art, the subject invention is not limited to
thumbwheel switch input devices or to manual input devices of any
kind. For example the input or control means of the system shown in
FIG. 1 may comprise a data processing machine such as a computer, a
radio receiving network, a flow transducer, etc.
The thumbwheel switch input device 28 is connected to a code
converter 30 which operates to convert the binary coded decimal
signals from the thumbwheel switches 28 to pure binary form and to
enter the message code in parallel form into a programmable read
only memory 32. The read only memory is a large digital storage
device capable of converting a relatively small number of input
signal combinations in binary code into a larger number of output
signals having a predetermined relationship to the input signals.
Accordingly it will be apparent to those skilled in the art that
the read only memory 32 is simply a device for converting
information from a highly compressed code form into a more or less
compressed form (in this case a less compressed code form) thereby
to generate an equivalent signal combination comprising the number
of actual signals which are required to actuate any given output
device. The read only memory 32 may take any of various forms but
is preferably in the form of a diode matrix or a magnetic core
array of a well known prior art type.
The signal combinations which are generated by the read only memory
32 are presented in parallel form on output lines 34. These output
lines are connected to a parallel-to-serial converter 36 which
operates to present data signals in serial form and at a clocked
rate. These serialized data signals are presented to a line driver
38 such as a suitable amplifier device which is connected directly
to the twisted pair transmission line 14. Accordingly it is to be
understood that for each message code a plurality of data signals
are generated by means of the read only memory 32. These data
signals are then serialized by means of the parallel-to-serial
converter 36 and transmitted to the sign 12 by way of the twisted
pair transmission line 14.
The transmission of serialized signals in binary code form requires
the synchronization of the sign 12 and the input section 10 so that
the various digits or bits in the binary wave train transmitted
over the data transmission line 14 can be separated from one
another and properly distributed among the liquid display devices
18 of the sign 12. For this purpose a clock oscillator 40 is
provided. The output of the clock oscillator 40 is connected to a
second line driver 42 to transmit the clock signal over the twisted
pair transmission line 16 to the sign 12. In addition, the output
of the clock oscillator 40 is connected through a frequency divider
to the read only memory 32 and the converter 36 so that the signals
are read from the read only memory 32 and presented to the data
line 14 at the clock rate.
Looking now to FIGS. 2 and 3 one of the liquid crystal display
devices 18 will be described in greater detail. Although the liquid
crystal device 18 is essentially a prior art device, some detailed
description is believed to be in order thereby to facilitate a
better understanding of the need for and use of various data
signals which are generated in the system described in FIGS. 1, 4
and 5.
Each liquid crystal display device 18 comprises a rectangular thin
film 46 of liquid crystal material, the particular material being
preferably of the nematic type as described in various prior art
publications including U.S. Pat. No. 3,622,224 issued to Joseph J.
Wysocki and Robert W. Madrid on Nov. 23, 1971. The thickness of the
liquid crystal film 46 is on the order of 0.006 inches and
accordingly the dimensions and proportions shown in FIGS. 2 and 3
are not to be construed as a representative of an actual device but
rather have been exaggerated for purposes of illustration. Film 46
of liquid crystal material is hermetically sealed between two flat
plates 48 and 50 of transparent and nonconducting material such as
plastic or glass. The inner surface 52 of plate 48 is coated with a
transparent silver oxide electrode which extends continuously over
the face thereof and is electrically connected to one of the
externally exposed and vertically depending conductive terminals 54
shown on the lower portion of plate 48 in FIG. 2. The inner surface
56 of plate 50 is selectively and discretely plated with
transparent silver oxide segments 58 which are arranged in the
fourteen segment pattern shown in FIG. 2 so as to be selectively
energizable for the purpose of representing each of the alphabetic
and numeric characters. Each of the segments 58 is electrically
connected to one of the terminals 54 thus to permit selective
application of voltages to the segments for the purpose of creating
a high intensity field between selected segments 58 and the
electrode facing on the inner surface 52 of plate 48. As is well
documented in the prior art with respect to liquid crystal devices,
this application on a highly localized field produces an optical
mutation of the liquid crystal material within the field so as to
permit the excited area of the liquid crystal display device 18 to
be optically distinguishable from the unexcited areas. Accordingly,
each of the liquid crystal display devices 18 in the sign 12
requires some combination of 14 individually selectable data bits
plus proper connection of the common electrode on the inner surface
52 of plate 48. It will be readily appreciated that a 16 character
sign having 14 segments per character requires 224 separate data
input signal paths.
Referring now to FIG. 4 a schematic representation of the power
supply 26 from the circuit of FIG. 1 is shown. Power supply 26
comprises input terminals 60 which are adapted to receive 120 volt
A.C. energization from a typical transit vehicle converter. Input
terminals 60 are connected across the primary coil of a step down
transformer 62, the secondary coil of which is connected across the
input terminals 64 of a rectifying diode bridge 66. The output
terminals 68 of the diode bridge are connected across a capacitor
70 which receives an eighty volt D.C. charge so as to provide a
constant eighty volt D.C. signal at the circuit point 72 for use in
connection with various circuitry logic requiring D.C.
energization. The upper plate of capacitor 70 is connected through
a resistor 74 to the collector electrode of a transistor switch 76
which acts as a chopper to chop the eighty volt D.C. signal
appearing at the output terminal 68 of bridge 66 and to apply an
effective eighty volt peak-to-peak square wave signal to the liquid
crystal segment 18' through a decoupling capacitor 78. Transistor
76 is turned on and off by a 25 Hz oscillator 80 having a 50
percent duty cycle. The output of the oscillator 80 is connected
through a bias resistor 81 to the base of the transistor 76. In
addition the application of the chopped eighty volt signal to the
liquid crystal segment 18'which represents a portion of any given
liquid crystal display device is controlled by means of an SCR
switch 82. As shown in FIG. 4 the gate electrode 84 of the SCR
switch is connected to receive a data signal.
Summarizing the operating of the liquid crystal display device of
FIG. 2 when used in the circuit of FIG. 1 and energized by the
circuit of FIG. 4 it can be seen that the receipt of a suitable
data signal at the register 22 in the system of FIG. 1 along with
the suitable clock signal results in the actuation of at least one
of the digit drivers 20 to select at least a segment 58 of a liquid
crystal display device 18 for excitation. Whenever this occurs a
data signal is applied to the gate electrode of SCR switch 82 to
cause the eighty volt peak-to-peak A.C. signal to be applied
through the decoupling capacitor 78 to the liquid crystal 18' which
is represented by the segment 58 selected for high field
excitation. The A.C. signal being applied to that representative
portion of the liquid crystal display device 18 causes an optical
transmutation of that portion of the liquid crystal display device,
thus establishing that portion or segment as part of the message to
be displayed.
Referring now to FIG. 5 there is shown a more detailed block
diagram of a destination sign system employing liquid crystal
characters 18. The block diagram of FIG. 5 follows the general
organization illustrated in FIG. 1 and like components are
identified with like reference characters.
In FIG. 5 the three digit thumbwheel input device 28 is shown to
include three output lines connected to the binary coded
decimal-to-binary code converter 30. The converter 30 is provided
with eight output lines 90 which are connected in parallel to each
of twelve read only memories of which memory 32A, 32B and 32C are
illustrated in FIG. 5 as being representative. It will be
appreciated that the individual read only memories illustrated in
FIG. 5 make up the memory 32 shown in FIG. 1. The read only
memories are each provided with a set of eight output lines
connected in parallel to the input of the parallel-to-serial
converting register 36. The memories 32A, 32B and 32C together with
the remaining nine memories which are not shown are read into the
register 36 in sequence under the control of the system clock 40
which is connected through the frequency divider 44 to a word
memory scanner 92 having twelve sequentially actuated output lines.
Output line 94 is connected to read only memory 32A to enable data
transfer from that memory to the register 36. Output line 96 from
scanner 92 is connected to read only memory 32B to output an
eight-bit word from that memory to the register 36 at the next
clock time. Finally output line 98 enables the transfer of an
eight-bit word from read only memory 32C to the register 36 at the
twelfth clock time. The clock 40 preferably operates at 6 KHz and
the divider 44 has the effect of dividing this frequency by a
factor of eight. The result is a sequence of 12 "read enable"
signals on line 94, 96 and 98, the thirteenth time being reserved
for a tone burst sync signal to be explained hereinafter. The need
for a sequence of 13 read times results in the dividing factor of
frequency dividers 102 and 118 being 104 or thirteen times eight.
As explained with reference to FIG. 1 the output of the
parallel-to-serial register 36 is clocked out to the line driver
amplifier 38 under the control of the clock 40 to present a serial
data wave train in binary code to the transmission line 14.
In FIG. 5 the clock 40 is also shown to be connected to an
inverting OR gate 100 which operates to apply the clock signal to
the line driver amplifier 42, the output of which is connected to
the twisted pair transmission line 16. OR gate 100 selectively
passes to the line driver amplifier 42 one of two signals
comprising the basic clock signal or a synchronizing signal in the
form of a short duration tone burst. To generate the tone burst,
the output of clock 40 is connected through a frequency divider 102
to one input of OR gate 104. In addition a constantly running tone
burst generator has the output connected to the other input of gate
104. The output of gate 104 is connected to a second input of gate
100. The frequency division factors of dividers 102 and 44 are
related by a factor of thirteen, thus giving rise to a periodicity
of tone burst or sync signal generation which is the same as the
periodicity of the data cycle produced by the word memory scanner
92. In other words, the entire data train is repeated and
transmitted in the first 12 clock times of divider 44 and the sync
signal occurs at the thirteenth clock time.
The twisted pair transmission line 14 which carries the data signal
train is connected to a receiver amplifier 110 and thence to the
serial-to-parallel register 22 which is disposed at the destination
sign location. Similarly twisted pair transmission line 16 carrying
the clock signal is connected to the input of the receiver
amplifier 112, the output of which is also connected to the
serial-to-parallel register 22 for synchronizing purposes. As will
be apparent to those skilled in the data processing art the
function of the clock signal as applied to the register 22 is the
establishment of bit times or strobe times which are synchronized
with the data cells which exist in the serialized signal train on
twisted pair transmission line 14. Each clock or strobe signal
operates to open a time window to permit the data waveform state
which exists during that window to enter the register 22 as a bit
of intelligence, the value of the bit depending on the voltage of
the input wave form, either high to represent a binary "one" or low
to represent a binary "zero". The output of amplifier 112 is also
connected to a frequency discriminating tone burst detector 114
which operates to single out the high frequency tone burst from the
standard clock signal and to apply a reset pulse to the register 22
upon the occurrence of the tone burst. The tone burst detector
output signal is gated by OR gate 116 having one input connected to
the output of the tone burst detector and the second input
connected to receive the clock signal after having been reduced in
frequency by a factor of 104 by the frequency divider 118.
Accordingly, the contents of serial-to-parallel register 22 are
"refreshed" at the end of each data train.
The output of the serial-to-parallel register 22 comprises eight
output lines 120 which are energized simultaneously and in
parallel. The lines 120 are connected to a set of 16 "data latches"
corresponding to the sixteen liquid crystal character display
devices 18 which are used in a particular destination sign. Each
data latch comprises the combination of two four-bit latch devices;
for example in FIG. 5 the left hand-most latch comprises four-bit
latch 122A and four-bit latch 122B connected to receive the eight
bits from the register 22 in groups of four each. The next latch
comprises four-bit latch device 124A and four-bit latch device
124B, it being understood that sixteen such combinations of two
four-bit latches are provided.
The latch devices are simply holding registers implemented in
T.sup.2 L logic to maintain the data content of one message from
register 22 during the change to a new message signal. A suitable
device is available from National Semiconductor Inc. of Pasadena,
Calif. In other words, the latches operate as a buffer. The
eight-bit data signals are distributed between the four-bit latch
pairs by means of a scanner 126 which receives the clock signal
after having passed through a frequency divider 128 having a
division factor of eight. Scanner 126 has sixteen output lines of
which line 130 is connected to enable the latch pair comprising
devices 122A and 122B. A second output line 122 from scanner 126 is
connected to enable the latch pair comprising devices 124A and
124B. A third output line 124 from scanner 126 is connected to a
still further pair of latch devices. Accordingly, the eight-bit
data read from the register 22 are applied to the sixteen pairs of
latch devices, one pair at a time, to apply data to the liquid
crystal characters 18 one at a time and typically from left to
right in the sign 12 of FIG. 1. It is to be understood, however,
that during the initial application of data to the liquid crystal
characters 18 of the sign 12, the distribution of data on the
character by character basis occurs so rapidly as to present the
visual impression of an instantaneous and simultaneous actuation of
all characters in the particular message being displayed.
The function of the latch devices 122, 124, etc., is to
continuously present a combination of data signals to a plurality
of sixteen read only memory devices 136 which convert the eight-bit
input code thereto into a fourteen-bit output code appearing on
lines 138. As shown in FIG. 5 the output lines 138 carry D.C.
voltages when energized and are electrically connected to the gate
electrodes of the SCR switches 82 in circuit with the various
segments of the liquid crystal character 18 which might be selected
by any particular output code. As shown in FIG. 5, SCR 82a is
controlled by the first of fourteen lines in the output lines at
138 whereas SCR switch 82b is controlled by the twelfth output line
and SCR 82c is controlled by the thirteenth output line. Fourteen
such SCR switches exist for each liquid crystal character 18 and 16
of the read only memory devices 136 exist for a 16 character
sign.
Looking now to FIG. 6 an exploded view of a preferred configuration
for sign 12 is illustrated. The sign 12 shown in FIG. 6 is
particularly well suited for use as an external destination sign on
transit vehicles wherein the destination message is preferably
readable under varying lighting conditions including both day and
night conditions. Sign 12 comprises a plurality of liquid crystal
display devices 18 of the type described with reference to FIGS. 2
and 3. Each of the liquid crystal devices 18 is backed up by a 50
percent reflective mirrored surface 140 and a plastic louvered film
142 having individually spaced louvers 144 disposed on an upward
angle approximately 30.degree.. A suitable louvered film is
available from the 3M Company and is referred to by the
manufacturer as a "display film." The laminated combination of
liquid crystal device 18, mirror 140, and louvered film 142 is
disposed within an opaque housing 146 having a plurality of
selectively energizable fluorescent lighting tubes 148.
As shown in FIG. 6 the sign 12 is typically viewed from somewhat
beneath the normal elevation of the sign such that the viewing
angle is intercepted by the 30.degree. louvers to prevent the
viewer from observing the flourescent lighting tubes 48 under
normal conditions. At the same time, the louvers permit excessive
back-lighting to be exhausted upwardly where it is not visable to
the observer. As is also shown in FIG. 6, sunlight is typically
reflected off of the 50 percent mirrored surface 140 to the viewer
enhancing the contrast of the liquid crystal display when properly
energized.
To summarize the operation of the circuit of FIG. 5, the operator
selects a message to be displayed and positions the thumbwheels of
switch device 28 to represent that message. From the thumbwheel
positions, code conversions and expansions are carried out until a
data signal comprising a relative large number of bits is
generated. This data signal is then serialized and transmitted
along with a clock signal and a reset signal to the sign 12. At the
sign, the serialized data train is synchronously detected and fed
into the register 22 for conversion into a sequence of eight-bit
parallel words. These words are distributed among the buffer
latches 122, 124, etc. which retain the current data code until a
new message is selected. The latches feed the memories 136 which
output signal combinations in such number as to energize all of the
selected segments of respective display devices 18. The message is
continuously displayed until a new message code is set via the
thumbwheels.
It is to be understood that the specific embodiments of the
invention which have been described herein are illustrative only
and are not to be construed as limiting the invention.
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