U.S. patent number 3,898,644 [Application Number 05/396,797] was granted by the patent office on 1975-08-05 for tv display system.
This patent grant is currently assigned to QSI Systems, Inc.. Invention is credited to Larry K. Baxter.
United States Patent |
3,898,644 |
Baxter |
August 5, 1975 |
TV display system
Abstract
A character display system having a raster type cathode ray tube
display including a continuously updated display segment for such
information as date and time. Data defining the updated portion of
the display is stored in a circulating shift register having a
counter stage and an output stage connected to a character
generator. The data is shifted in synchronism with the raster scan
so that the output stage of the register synchronously provides
information to the character generator to generate the updated
portion of the display. The counter stage is appropriately
incremented when containing data to be modified, thereby updating
the display information being circulated.
Inventors: |
Baxter; Larry K. (Lexington,
MA) |
Assignee: |
QSI Systems, Inc. (West Newton,
MA)
|
Family
ID: |
23568639 |
Appl.
No.: |
05/396,797 |
Filed: |
September 13, 1973 |
Current U.S.
Class: |
368/28; 368/34;
377/20; 345/467; 345/26; 348/525; 368/10; 368/223; 377/129;
968/935 |
Current CPC
Class: |
G04G
9/0058 (20130101); G09G 1/16 (20130101) |
Current International
Class: |
G09G
1/16 (20060101); G04G 9/00 (20060101); G08B
005/36 () |
Field of
Search: |
;340/324AD ;58/152R
;235/92EA,92N |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trafton; David L.
Attorney, Agent or Firm: Cesari and McKenna
Claims
I claim:
1. A character display system for displaying characters in raster
form, said system comprising
A. a raster encoder that converts the combination of
1. vertical timing signals,
2 horizontal timing signals and
3 character code signals representing a character to be displayed
into signals corresponding to a raster presentation of the
character,
B. means for generating the horizontal timing signals,
C. means for generating the vertical timing signals,
D. a circulating character shift register that
1. shifts in response to shift signals,
2. contains, in the form of said character code, the characters to
be displayed, and
3. includes
i. output conductors that provide to said encoder the character
code input therefor, said characters being arranged in said shift
register so as to be shifted to said output conductors in the order
in which they are to be displayed,
ii. a counter as one stage thereof,
E. means for incrementing said counter to update the displayed
information,
F. means for applying shift signals to said shift register in
synchronism with the tracing of the portions of the raster lines
associated with the respective displayed characters.
2. The system defined in claim 1 including
A. means for generating segment signals corresponding to the
presence of predetermined parts of the displayed information in a
predetermined stage of said character register, and
B. means for changing the contents of said predetermined register
stage in synchronism with said segment signals, thereby to update
the information displayed by said system.
3. The system defined in claim 2 including
A. a carry shift register having a stage corresponding to each
stage in said character register and shifting in response to said
shift signals,
B. means for inserting a bit into a first stage of said carry
register when said counter overflows, and said first stage
corresponding to the character register stage containing a first
digit in a number whose next less significant digit is contained in
said counter,
C. means for incrementing said counter whenever a second stage in
said carry register, corresponding to said counter, contains said
bit, thereby to advance said first digit when there is an overflow
of said next less significant digit.
4. A system as defined in claim 3 having the capability of
displaying different items of countable information having
different moduli, said system comprising
A. a first decoder connected to decode the contents of one or more
character register stages to provide output signals corresponding
to predetermined contents of said one or more stages,
coincidence means for gating each of said decoder output signals
with a segment signal, thereby to provide an overflow signal for
each item when that item
1. is contained in said one or more stages and
2. exceeds the modulus for that item, and
C. means for clearing each item in response to the overflow signal
corresponding thereto.
5. A system as defined in claim 2 having the capability of
displaying different items of countable information having
different moduli, said system comprising
A. a first decoder connected to decode the contents of one or more
character register stages to provide output signals corresponding
to predetermined contents of said one or more stages,
B. coincidence means for gating each of said decoder output signals
with a segment signal, thereby to provide an overflow signal for
each item when that item
1. is contained in said one or more stages and
2. exceeds the modulus for that item, and
C. means for clearing each item in response to the overflow signal
corresponding thereto.
6. A system as defined in claim 5 including means for updating a
first item of information in response to the overflow of a second
item of information, and
A. in which said coincidence means further provides a carry signal
upon the coincidence of an output signal with a segment signal,
and
B. including means for applying the carry signal resulting from
overflow of said second item to the stage of said carry register
then corresponding to the character register stage containing the
least significant digit of said first item, thereby to insert a
carry bit into that carry register stage and consequently advance
said least significant digit when that digit is next contained in
said counter.
7. The system defined in claim 6 in which the items of information
are time divisions and including
A. a clock and
B. means for advancing one of said time divisions in response to
the output of said clock.
8. A system as defined in claim 7
A. in which said time divisions include the year, the month and the
day of the month,
B. including a second decoder connected to said character register
to provide month length signals indicating the lengths of months
contained in the character register.
C. in which said coincidence means further responds to said month
length signals in providing carry and overflow signals at the ends
of months.
Description
BACKGROUND OF THE INVENTION
This invention relates to the display of alpha numeric characters
on a television screen. In particular, it relates to the display of
continually updated information in alpha numeric form on a
television screen that simultaneously displays a conventional
picture.
The invention is primarily directed to those applications where a
picture of an event or series of events is recorded on video tape
for later display on a television screen and certain current
auxilliary information relating to each event must be concurrently
recorded for simultaneous display with the picture to which it
relates.
For example, many banks position television cameras to view teller
transactions. The pictures are recorded on video tape and later
displayed if they are to be visually reviewed. Then if a bad check
is passed or there is a holdup, the authorities can review the
recorded pictures to identify the wrongdoers. To make the evidence
more binding in court, it is desirable that the system record,
along with the viewed event, the date and time thereof so that each
person whose picture is recorded can be positively connected with a
specific date and time.
Conventionally the date and time are entered into the system
electronically. The various elements of this information, i.e.
seconds, minutes, hours, days, months and years, are stored in
counter-registers which are connected to operate as a running
electronic calendar-clock. The digital contents of these registers
are applied to a decoder which converts the signals to
corresponding television raster presentation signals. The latter
signals are then combined with the output of the television camera
as the latter scans the area to be viewed. A suitable synchronizing
and switching arrangement applies the contents of the respective
registers in turn to the decoder so that they are fed into the
video system in the proper order. Then, when the recorded, combined
video signal is later displayed, numerals representing the date and
time of the recorded pictures appear in a string across the bottom
of the screen.
While this system effectively records the required information, it
requires relatively involved and expensive switching circuitry to
apply the contents of the various data-containing registers to the
decoder. The principle object of the present invention is to
simplify and thereby reduce the cost of the circuits.
SUMMARY OF THE INVENTION
In my system the auxilliary information to be recorded and/or
displayed is stored in a circulating character register, each
character of the information occupying one stage of this register.
One stage of the shift register is connected to a video raster
decoder so that as the contents of the shift register are rotated,
each character in turn is applied to the decoder for combination
with the video signal. In cases where the recorded information is
numerical information to be updated by a counting operation, such
as the keeping of time, this is preferably accomplished by
including a counter as one stage of the shift register. Then
whenever a quantity stored in the register is to be increased, the
system adds to it as it passes through the counter.
The counting process requires a carry function in several different
situations. For this I prefer to use a separate carry shift
register having a stage corresponding to each stage in the
character register and arranged for insertion of carry bits into
certain stages. Whenever a carry bit reaches the stage
corresponding to the counter, it increments the counter and thereby
increases the value of the digit then contained in the counter.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of the raster-encoding portions of the
circuit;
FIG. 2 is a diagram of the character storage and selection portions
of the circuit; and
FIG. 3 is a diagram of a display provided by the circuits of FIGS.
1 and 2.
In the following description, actuation of a circuit element, e.g.
setting or clearing of a flip-flop, takes place on the trailing
edge of the actuating signal unless otherwise stated or indicated
by the circuit configuration. Thus, when actuation on the leading
edge of a signal is desired, the signal is first passed through an
inverter so that the leading edge of the inverted signal has the
same effect as the trailing edge of the noninverted signal.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1, a television camera 10 set up for a
surveillance operation provides a video output signal that is
applied to a recorder 12 by way of a summing circuit 14. If
desired, signal may be simultaneously displayed on a conventional
remote television monitor 15. The other input of the summing
circuit comes from a character generator, generally indicated at
16, that supplies signals corresponding to alpha numeric
characters. These characters are displayed along with the scene
viewed by the camera 10. The characters represent any desired
information that is to be updated from time to time. In the system
described herein they represent the date and time.
At this point it will be helpful to review the arrangement of the
character display on the cathode ray screen. As shown in FIG. 3,
the system displays in order a series of date and time segments:
the month, day of the month, year, hour, minute and second. Thus,
the indicated time and date are 14 hours, 23 minutes and 36 seconds
on July 23, 1973.
The characters are formed in a conventional 5 .times. 7 matrix,
i.e. 5 elements wide and 7 elements high. Each of the vertical
elements corresponds to one line of the television raster.
Specifically, the video display first sweeps across line 0 to form
the top portion of each of the characters and then in succession it
sweeps across lines 1-6 to complete the character display. The
times during which the electron beam traces the respective lines
are correspondingly designated V0-V6.
Similarly, the system has time divisions corresponding to the
horizontal movement of the electron beam along each of the lines.
Thus, the first character occupies a zone corresponding to
horizontal times H0-H6. The character itself occupies the five
elements H0-H4, H5 and H6 being used for intercharacter
spacing.
In the display shown in FIG. 3, the characters are divided into
time-segment blocks of two characters each, with punctuation marks
separating the time segments as indicated. This requires additional
spacing between the second character of each segment and the first
character of the next segment and for this purpose I assign ten
horizontal intervals H0-H9 to the second character in each block.
Again the character itself occupies intervals H0-H4.
Each element in the field of each character of the FIG. 3 display
is uniquely identifiable with a pair of vertical and horizontal
time intervals. In order to generate the character, i.e. develop
the electrical signals which will be used to display it, a digital
code for the character is applied to a decoder, as described below,
along with signals representing the respective vertical and
horizontal time signals. The decoder provides an output for each
time signal pair, indicating whether the display should be dark or
light during that time interval, i.e. in the corresponding position
of the display. The combination of these dark and light intervals
provides the visual presentation of the character.
For example, when the character 7 is to be displayed, as shown in
FIG. 3, the decoder provides a "dark" output signal during the
vertical time V0, from horizontal time H0 through time H4. The
decoder also provides a dark signal when horizontal time H4 occurs
during vertical time V1. On the other hand, there is a "light"
signal during the coincidences of horizontal times H0-H3 with
vertical time V1.
In FIG. 3, I have designated the intervals occupied by the
respective time segments as t.sub.MO, t.sub.D, t.sub.y, t.sub.H,
t.sub.Mi, and t.sub.s, respectively. The system develops and uses
correspondingly designated signals as described below.
With further reference to FIG. 1, the output of the camera 10 is
also applied to a sync separator 18 which extracts horizontal and
vertical sync pulses. These pulses are applied to variable
horizontal and vertical delay units 20 and 22.
An oscillator 24 which, in the present example, has a frequency of
2.1 MHz applies a series of pulses through a gate 26 to a
horizontal counter 28. The gate 26 is enabled by the output of a
flip-flop 30 when the latter is set by the delayed sync pulse from
the horizontal delay unit 20. The counter 28 nominally divides the
frequency of the oscillator 24 by a factor of ten, except when its
counting cycle is shortened, as described below, in which case it
divides by a factor of seven. The counter 28 is provided with
suitable gating so that in each of various counting states the
counter 28 provides an output signal on a different one of its
output lines. The signals from the counter 28 appear during the
intervals bearing the corresponding designations in FIG. 3.
Accordingly, since characters are generated during the horizontal
time intervals H0-H4, the output signals H0-H4 from the counter 28
are applied to a raster encoder 32.
A second timing input for the encoder 32 is supplied by a vertical
counter 34, which is similar to the counter 28. The counter 34
counts the t.sub.s signals, which are applied by way of an AND gate
36. Specifically, the counter 34 increases its count at the end of
each displayed line of characters (FIG. 3).
The counter 34 has eight states and eight corresponding output
signals, V0-V6, which correspond in turn to the intervals V0-V6 in
FIG. 3, and also a state V7. These signals are applied to the
raster encoder 32 as shown. The counter is cleared to its V0 state
by the output of the vertical delay unit 22.
A third input for the encoder 32 is the content of a character
register 38 which contains the codes for the various characters to
be displayed. In the present example, where the characters are all
numerals, the codes in the register 38 will merely be the binary
representations of the numerals to be displayed.
The output of the encoder 32 is applied to the summing circuit 14
by way of a gate 40 and an OR circuit 42. The gate 40 is enabled by
the coincidence of two signals. The first is a horizontal enabling
signal from a flip-flop 44. The flip-flop 44 is set, through an
inverter 45 by the leading edge of the H0 signal and reset by the
trailing edge of the H4 signal. It is thus set during the intervals
H0-H4, i.e. the horizontal time intervals during which each of the
characters is displayed.
The second enabling signal for the gate 40 is a vertical enabling
signal provided by the output of an inverter 46 whose input is the
V7 signal from the vertical counter 34. Thus the vertical enabling
signal is applied to the gate 40 when the counter 34 is in any of
its states V0-V6, i.e. the vertical intervals during which the
characters are generated (FIG. 2). When the counter 34 reaches its
V7 state, the character generator 16 has completed the generation
of the displayed characters. At this time the vertical enabling
signal from the inverter 46 ceases, thereby disabling the gates 40
and 36.
This cuts off the output of the encoder 32 from the video channel.
It also prevents the counting of further t.sub.s signals until the
next delayed vertical sync signal clears the counter 34. By varying
the delay in the vertical delay unit 22 one may thus adjust the
vertical position of the characters in the video display. Similarly
one may adjust the horizontal position of the characters by
variation of the delay in the horizontal delay unit 20.
As shown in FIG. 2 the character register 38 is a circulating shift
register having a stage for each of the characters to be displayed.
Thus in the present example the register 38 contains 12 stages.
Eleven of these are conventional shift register stages 38a-38k. The
twelfth stage is a counter 48.
More specifically, each stage of the register 38 has sufficient
capacity for the code of a single character. When only numerals are
to be displayed, four bits are required for a binary representation
of the decimal numerals 0-9. Accordingly, each register stage
contains four binary storage elements. The contents of these
elements are shifted in parallel from stage to stage in response to
the H7 pulses.
The counter 48 may be a conventional binary counter arranged
internally for decimal operation. Thus, it advances its count by
one in response to a CI pulse and, whenever its content is a nine,
the receipt of a further CI pulse clears it to 0, with the
corresponding emission of a carry pulse (CO). The counter 48 has
four character output lines, 48a-48d, that provide the input
signals for the raster encoder 32 (FIGS. 1 and 3).
The circuit of FIG. 2 also includes a carry shift register 50 in
which each stage has a capacity of one bit. The carry register 50
contains any carry bits that have to be stored temporarily. A
principle reason to use a carry register is the order in which a
string of characters is displayed on a television screen. Normally,
the electron beam traces each line from left to right. Thus the
higher order, tens, digits are fed to the raster encoder 38 before
the lower order, ones, digits.
Illustratively, if the minute count is 29 and 2 will pass through
the counter 48 before the 9 and when the 9 is in the counter 48,
the 2 will be in the register stage 38A. Then, when the minute
count is to be advanced, a CI pulse will clear the counter 48 to
zero and cause the counter to emit a CO (carry) pulse. However, the
CO pulse cannot add to the 2 in the tens position of the minute
count until the 2 has circulated back to the counter 48. Therefore,
the CO pulse passes to the carry register 50 where it sets a stage
50a correspnding to the stage 38a in the character register 38. The
resulting carry bit in the stage 50a is then shifted through the
register 50 by the H7 pulses in step with the shifting of the 2
through the register 38. As the 2 finally reaches the counter 48,
the carry bit reaches the stage 50L. The stage 50L is connected to
the counter so as to apply the carry bit to the counter as the CI
signal. In the example under discussion, this increases the content
of the counter 48 to 3. The minute count has thus completed its
advance from 29 to 30.
The shift register 50 also handles carry signals generated for
other purposes as described below. First, however, it will be
helpful to review the preferred timing arrangement for the
information updating operations.
As further shown in FIG. 2 a flip-flop 52 operates as a
divide-by-two counter of the H7 signals emitted by the horizontal
counter 28 (FIG. 1) after the right hand end of each character in
the display (FIG. 3). The output of the flip-flop thus cycles once
for each pair of characters in the display, i.e. once for each time
segment. To insure the correct sequence of signals from the
flip-flop 52, it is initially cleared by the delayed sync pulse
from the horizontal delay unit 20, i.e. at the beginning of each
line of a character display. Thus the flip-flop is in its cleared
state during the display of the left hand character in each time
segment and is in its set state during the display of the right
hand character in each time segment.
As pointed out above, the first digit in each time segment requires
seven horizontal time intervals, whereas the second digit requires
ten intervals to accommodate the following punctuation mark. The
flip-flop 52 controls the length of the counting sequence of the
horizontal counter 28 to accomplish this difference in horizontal
timing.
Specifically the SCND signal provided by the flip-flop 52 when it
is in its set state passes through an inverter 58 whose output is
therefore an FRST signal corresponding to the first digit in each
time segment. The FRST signal enables the gate 60 (FIG. 1) to feed
back the H7 signal from the counter 28 to a reset input of the
counter by way of an inverter 61. Thus, at the beginning of the H7
time interval during the tracing of the first character in each
time segment, the counter 28 is reset to a 0 count to limit it to
seven horizontal time intervals for the first digit. On the other
hand, during the tracing of the second digit of each time segment,
the FRST signal is absent, the gate 60 is thereby disabled and the
horizontal counter 28 counts through a full ten intervals,
H0-H9.
The circuit of FIG. 2 also includes a divide-by-six counter 54
connected to count the H9 signals. The output of the counter 54 is
decoded by a decoder 56. Since the H9 signal occurs only at the end
of each time segment, the counter 54 counts the time segments and
the decoder 56 provides an output on one of six output terminals
during each time segment. These signals are designated in
accordance with the display illustrated in FIG. 3. The counter 56
is also cleared by the delayed sync signal from the horizontal
delay unit 20 and, when it is cleared, the decoder 56 emits the
t.sub.MO signal.
The decoder 56 is enabled by the output of a gate 62 upon the
coincidence of the SCND signal from the flip-flop 52 and the H6
signal from the horizontal counter 28. Thus the decoder 56 provides
outputs only during the H6 time intervals immediately following the
right hand digits of the respective time segments.
Since the system displays time, it includes a clock 64 synchronized
to a stable frequency source such as the power line. Assuming a
power frequency of 60Hz and a corresponding frequency of the clock
64, a frequency divider 66 divides the output of the clock to
provide the basic one-second time intervals for the display.
Preferably the divider 66 provides two output signals, one at 1Hz
and the other at 2Hz, the latter output being used to set the
correct time into the system initially.
More specifically, to set the time, the operator applies the 2Hz
output of the divider 66 to the set input of a flip-flop 68 by way
of a push button switch 70 and selector switch 71. The resulting
output of the flip-flop 68 provides one enabling input for a gate
72. Also, by means of a selector switch 73, the operator selects,
for example, the t.sub.MO output of the decoder 56 as a second
enabling signal for the gate 72. Then, when the t.sub.MO signal and
the set output of the flip-flop 68 coincide, the gate passes the
next pulse from the oscillator 24 to the stage 50j of the carry
register 50.
During the t.sub.MO signal, the second digit of the month is
displayed, i.e. the second digit of the month is in the counter 48
in the character register 38. Therefore, the first digit of the day
of the month is in the register stage 38k and the second digit of
the day is in the register stage 38j. Accordingly, the output of
the gate 72 inserts a carry bit into the carry register stage
corresponding to the second digit of the day of the month.
Following the next H7 (shift) pulse this carry bit is in the carry
register stage 50k. The content of the stage thus enables a gate 74
to pass the next pulse from the oscillator 24 to the flip-flop 68
and thereby reset the flip-flop to disable the gate 72.
The next H7 pulse moves the carry bit to the stage 50L and the
content of this stage thereby enables a counter input gate 76. The
gate 76 then passes the following H1 signal to the counter 48 as a
CI signal to advance by one the content of the counter, i.e.
advance the day of the month to the next date.
One-half second later the output of the counter 66 again sets the
flip-flop 68 and the foregoing sequence is repeated to update the
day of the month once again. This continues until the correct day
of the month appears in the display, at which time the operator
releases the switch 70 to prevent further advance of the date.
If the second, i.e. the least significant, digit of the date must
advance past 9 to reach the correct date, the circuit performs a
carry operation to advance the first digit in the manner described
above.
In similar fashion the operator adjusts the other time segments
contained in the character register 38 by setting the selector
switch 73 to appropriate positions and actuating the switch 70
until the correct numbers appear in the display. The switch 73 is
preferably provided with an indicator showing which time segment is
adjusted for each position of the switch.
Ordinarily the last time segment to be adjusted in this manner will
be the seconds segment. When the correct seconds count appears on
the display, the operator moves the switch 71 to its other position
to apply the 1Hz signal to the flip-flop 68. The seconds count in
the character register 38 thus automatically advances in the manner
described above, but at a rate of one per second, thereby to
provide a continuous display and record of the seconds segment of
the time.
When the seconds count reaches 60, it should reset to zero and the
minute count should advance by one. The system accomplishes this by
means of a decoder 78 that decodes the contents of the counter 48
and register stage 38a. When the combined content of these two
stages reaches 60, the decoder 78 provides an output on a line
78a.
The outputs of the decoder 78 are applied to a coincidence logic
unit 80 along with the time segment outputs of the decoder 56. If a
signal on line 78a coincides with the t.sub.S signal, as it will
when the minute count reaches 60, the logic unit 80 will emit an OF
(overflow) signal. This signal clears the counter 48 and register
stage 38a, thereby resetting the contents of both stages to
zero.
At the same time, the logic unit 80 inserts a seconds-minutes carry
bit (S-M) into the carry register stage 50b by way of an output
conductor 80b. At this time, the corresponding stage 38b in the
character register contains the right hand digit of the minute
segment. Consequently, when this digit next reaches the counter 48,
the carry bit in the stage 50b will reach the stage 50L. The H1
signal at the time will thus pass through the gate 76 as a CI
signal for the counter 48, thereby increasing the minute count by
one.
Even though the second count reaches 60, it remains at that figure
only momentarily and consequently the viewer never perceives this
number in the video display.
In similar fashion, when the minute count reaches 60, a decoder
output on line 78a coincides with the t.sub.M signal to provide an
OF signal from the logic unit 80 along with a minutes-hours carry
signal (M-H) on line 80b. This clears the minute count to zero and
advances the hour count.
When the hour count reaches 24, the decoder 78 provides an output
signal on line 78b when the hour time segment is contained in the
register stage 38a and counter 48. This will coincide with the
t.sub.H signal and the logic unit 80 will again emit an OF signal,
this time clearing the hour count to zero. The logic unit 80 will
also provide an hours-days carry signal (H-D), but this time on a
line 80d connected to the carry register stage 50d. Reference to
FIGS. 2 and 3 will show that, at this time, the character register
stage 38d contains the right hand digit of the day of the month.
Accordingly, when that digit next reaches the counter 48, the carry
bit inserted into the register stage 50d will reach the carry stage
50L. The resulting CI signal will thus advance the date by one
day.
Since the number of days in a month varies according to the month,
I have provided the logic unit 80 with a further input indicating
the month, in order to control the date reading which will reset
the day of the month and advance the month. Specifically, the
decoder 78 has output lines 78d, 78e and 78f that provide signals
when the date reaches 29 (February, non-leap year), 31 and 32,
respectively. A month decoder 82 provides outputs when the contents
of character register stages 38b and 38c correspond to months
having 28, 30 and 31 days respectively. For example, if these two
register stages contain the number 07, corresponding to July, the
decoder 82 provides an output on line 82a, as it does for all other
months having thirty-one days. The stages 38b and 38c contain the
month whenever the stage 38b and counter 48 contain the day of the
month.
Thus, whenever the combined content of the stage 38a and counter 48
is the day of the month, as indicated by the t.sub.D signal, and
that content overflows, i.e. exceeds the number of days in the
month indicated by the output of the month decoder 82, the logic
unit 80 emits an OF signal to clear the stage 38a and counter 48.
For example, with a month decoder output on line 82a during July,
the logic unit 80 will emit its output signals when the
day-of-the-month count reaches 32. The logic unit also transmits a
days-months carry signal (D-M) over line 80b to the carry register
stage 50b to advance the month count in the same manner that the
minute and hour counts are advanced, as described above.
On the other hand, unlike the hour, minute and second time
segments, the lowest day-of-the-month count is not zero but one.
Accordingly, when the day-of-the-month count exceeds its maximum
value, it must be reset to 01. This can be accomplished in several
ways. For example, the logic unit 80 can be arranged to apply a
signal to the counter 48, directly resetting counter to the count
of one at the same time that the register stage 38a is cleared.
Alternatively, the counter can be cleared as described above, along
with the stage 38a, and then advanced by one count. The latter step
can be accomplished by means of a CI signal developed after the
counter 48 has been cleared and before the next shift (H7) pulse is
applied to the register 38. However, I prefer to use the shift
register 50 to develop a delayed count-advance signal.
Specifically, the carry register 50 includes a stage 50x that
precedes the stage 50a. In this respect the stage 50x corresponds
to the counter 48 in the shifting sequence. Thus when the
day-of-the-month count overflows, the coincidence logic unit 80
applies a carry signal SC to the stage 50x by way of an output line
80x. This carry signal shifts into the stage 50a at the same time
that the content of the counter 48 shifts into the stage 38a. Thus
the carry signal reaches the carry register stage 501 the next time
that the right-hand digit of the day of the month reaches the
counter 48. The resulting CI signal then sets a 1 into the counter
48 to provide the correct day of the month.
The circuit of FIG. 2 may also include a switch (not shown) that
changes the connections to the decoder 78 to provide a signal when
the day of the month reaches 30 instead of 29, in order to take
into account an increase in the number of days in February from 28
to 29 during leap years.
Whenever the combined content of the register state 38a and counter
48 is 13, the decoder 78 provides an output on line 78g. When this
output signal coincides with the t.sub.MO signal, it indicates an
end-of-the-year overflow of the month count. The logic unit 80 will
therefore emit an OF signal to clear the register stage 38a and
counter 48. The logic unit will also supply a months-years carry
signal (M-Y) to carry register stage 50h over a line 80h. At this
time the character register stage 38h contains the right hand digit
of the year count. Therefore, when the latter digit reaches the
counter 48, the carry signal will arrive at the stage 501 to
advance the year count by means of the CI signal.
Moreover, since the month reading must be reset to 01 at the end of
the year, the logic unit 80 applies an additional carry signal to
the register stage 50x to reset the month count in the same manner
that the day-of-the-month count is reset.
The punctuation marks shown in FIG. 3 are formed by a circuit shown
in FIG. 1. With reference to the latter figure. the vertically
centered dots following the month and day segments are generated by
means of a gate 84 upon the coincidence of the V3 signal with
either the t.sub.MO or t.sub.D signal as applied to an OR circuit
86. The output of the gate 84 is applied to the summing circuit 14
by way of the OR circuit 42.
Similarly, the colons following the hour and minute segments of the
display are generated by a gate 88 upon the coincidence of outputs
from OR circuits 90 and 92. The inputs for the OR circuit 90 are
the t.sub.H and t.sub.MI signals and the inputs for the OR circuit
92 and the V0 and V6 signals.
* * * * *