:.
I reconmend this thesis (project) for acceptance by the
Honors Program of Ball State University as partial fulfillment for the Honors curriculum.
I
Thesis Advisor
Associate Professor of Physics
and Astronomy
Ball State University
-
DESIGN PRINCIPLES AND PRELIMINARY
CONSTRUCTION OF A VIDEO-KEYBOARD INTERFACE
By
Kenneth L. Bowers
Senior Honors Project
ID 499
Dr. Ralph Place, Advisor
May
,1974
'? lit.:.
C. 11
•
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TABLE OF CONTENTS
Page
LIST OF FIGURES . . . . . . . . . . . . . . . . . iii
LIST OF TABLES . . .
I.
II.
III.
IV.
. . . . . . . . . . . . . . .
iv
IiJr:2RODUC'rION. .
1
ORGANIZATION.
2
A.
Keyboard. .
4
B.
Ilfemory. .
11
C.
'riming. .
D.
Cursor..
E.
Television.
. . . . . . . . . . . ..
6
G
. . . . . . . . . . . .
7
OPERATION OF CIRCUITS . .
9
A.
Power Supply. .
9
B.
rriming. .
13
C.
f1emory. .
20
D.
Cursor. .
. . . . . . . . . . . . .
22
CONSTRUCTION. .
28
A.
Printed Circuit Boards.
28
B.
Construction Yet To Be Completed. .
29
i
TABLE OF CONTENTS
v.
(Continued)
Page
30
SCHEMATICS. . .
A.
Mainframe.
30
B.
Timing.
.
30
C.
Cursor.
.
30
REFERENCES. . . . . . . . . . . . . . . . . . .
-
ii
31
LIST OF FIGURES
Figure
1
Block diagram of Keyboard and interface
organization. . . •
. .•
3
2
Mainframe schematic •
10
3
Main timing chain schematic
16
4
Derived timing schematic . . .
18
5
Page "A" or page "B" memory schematic
21
6
Page "A" character schematic (needed only
on "A" page circuit board). • • . . . . .
23
Cursor input conditioning and sequencer
schematic . . . . . . . .
....
24
Cursor character position and counter
schematic . . . . . . • .
•...
26
7
-
Page
8
iii
LIST OF TABLES
TABLE
1
2
Page
ASCII character code. . . . .
.
.
. .
Key clock waveforms at various points of
. . . . . . ..
TV typewriter circuit
.iv
5
14
I.
INTRODUCTION
During the past few years a tremendous interest
has been generated in the field of digital electronics.
Everywhere one turns there are digital clocks, digital
meters, digital thermometers and now a digital television.
Calculators the size that fit in your hand suddenly can do
remarkable calculations.
Computers are finding an even
wider spread application, from data processing to scientific
research.
Having studied moderate amounts of electronics and
applications in digital electronics the author's interest
was spurred by the possibility of a video-keyboard interface.
The possible adaptation to the existent terminal
system at Ball State seemed conceivable.
Upon
reading
"TV Typewriter", Radio-Electronics, Sept. 1973, it was
decided that this would be an educational as well as
practical experience.
What follows is an indication of
design principles, preliminary construction, and what still
remains to be done .
•
2
II.
ORGANIZATION
The basic video interface is designed to take the
output from an ASCII coded keyboard and generate the
corresponding characters on a television screen.
This
device is self-powered and contains a TV transmitter to
output directly to a television via the antenna input.
The
input is through a typewriter keyboard that delivers the
proper ASCII code to the interface input lines.
The input
may also be generated by a combination of six switches and
a pushbutton.
resulting unit.
Complete editing is a capability of the
The output can be directed so as to be
displayed anywhere on the screen.
Character input rate is
asynchronous, as the input is serial.
The character rate
is reduced because of this feature but circuit simplicity
is gained l and the input rate is still up to thirty characters
per second.
As seen in Figure 1, the organization of the "TV
Typewriter" is quite simple.
to input the data.
One necessity is the keyboard
This input must be in ASCII code.
The
input goes directly to the memory board which is generally
in the recirculate mode, meaning the input characters are
continually shifted through the memory and then back again.
The characters are recirculated again and again in the
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CURSOR
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4
memory and output continually to the television until new
characters are input.
The position on the screen of the
output characters is determined by the cursor.
Sequential
timing is governed by the timing board which determines
when characters are output, when blanks are output, and
when the memory board is clocked.
The timing sequence
insures that at the proper time and proper place, the
character is output to the TV via the antenna lead-in and
is seen on the screen.
A.
Keyboard
The keyboard generates ASCII code.
lines, each of which can be in a ground,
With six input
HOH
state, or at
+5V, rll" state, there are 2 6 possible input combinations
ranging from 000000, 000001, 000010, to 111110 and 111111.
These 64 different conditions represent 64 different letters,
numbers, and punctuation.
ASCII
C~merican
Standard Code
for Information Interchange), as illustrated in Table 1,
assigns a different one to each of these conditions.
Thus
the state of the six input lines CAl-6) determines the
character and when the seventh input line (A7) is grounded
these inputs are passed along to the memory of the interface.
B.
Memory
The memory board stores 512 words of six bits each
in a recirculating shift register.
-
This continually re-
circulates the stored material thus creating a memory.
board also contains a single line memory which will
The
5
TABLE 1
ASCII character code
Chc~r
AG A5 A4 A3 A2 Al
Char
A6 A5 A4 A3 A2 Al
@
0
0
0
0
0
0
0
0
1
1
0
&
0
0
0
0
0
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
1
0
1
0
1
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
blank
E
F
G
0
0
0
0
0
0
0
0
H
0
I
0
J
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
1
0
1
+
0
0
comma
0
1
1
1
0
1
.
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
1
0
1
0
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
8
9
A
I3
C
D
K
L
~Il
lJ
0
p
Q
R
S
r;"1
.L
U
V
W
X
y
Z
[
/
]
1\
undo
&
-
0
0
0
0
0
0
0
0
0
0
0
1
1
!!
#
$
%
(
)
*
/
2
3
4
5
6
7
<
=
>
?
0
0
0
0
0
0
0
1
1
1
1
1
1
0
0
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
1
1
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
0
1
1
1
0
0
1
1
1
0
1
0
1
0
6
recirculate each line of memory (32 words) over and over
again twelve times for the scanner to output.
For simplicity
and easy add-on any input device may be connected to the
single line memory and character generator.
For any output
the ENABLE line goes high, thus as long as only one such
device is ENABLEd, this device will
provide the output.
This is known as bus organization.
C.
Timing
The timing board contains a timing crystal and
TTL countdown chain to produce all necessary timing signals.
The main timing signals are obtained directly from the
crystal and the derived timing is obtained through proper
combinations of the main timing signals.
These signals
provide the necessary timing to sequence the entire operation
of the unit.
D.
Cursor
The cursor determines where a character is to go
on the TV screen, controls entry of characters and conditions the input signal.
The conditioning of the keypressed
signal eliminates noise associated with it.
This is filtered
out and a timing delay is provided to allow the keyboard to
input the entire character serially before any action is
initiated.
The cursor also determines if the input is to
begin a new line, a new page or follow the previous input.
7
-
E.
Television
The TV displays the characters.
to any unused low channel (2-5).
It may be tuned
The picture tube of the
television contains an electrode structure capable of
producing a narrow beam of electrons.
This beam is
directed toward the front of the tube where it strikes
a luminescent screen.
Where the beam strikes the screen
it causes a small spot of light to appear.
This electron
beam's direction is controlled by two pairs of deflecting
plates, one of which causes the beam to move horizontally,
the other makes it traverse vertically.
The current in these
plates controls the motion of the beam.
The plates cause
the electron beam to move across the screen (scan) in a
series of alternate horizontal lines.
Thus the rapid
successive illumination of the screen creates the effect
of a uniform and simultaneous illumination to the eye and
it "sees" a picture.
The potential applied to the control electrode
determines the brilliance of the output.
Thus characters
on the screen are generated by the sweeping scan of an
electr~n
beam, which crosses the screen in 62
takes 33 msec to get to the bottom.
~sec
and
The brightness of
the dot is changed by changing the picture tube's cathode
current.
The lower the signal the whiter the dot.
maximum signal produces a black screen.
negative transmission. 2
Thus
This is known as
8
The television screen is scanned into a total of
525 lines.
The entire scanning process must be accomplished
in 1/30 sec.
by 1/30 sec
This implies a frequency of 525 lines divided
=
15,750 lines/sec. 3
The TV typewriter scans
at 15,840 lines/sec, very close to the normal television
rate.
9
III.
A.
OPERATION OF CIRCUITS
Power Supply
The TV typewriter is operable on a standard 110V,
60-cycle, ac line.
seen in Figure 2.
The schematic for the mainframe is
The dual secondary transformer yields
a +5 volt supply from the +6v taps and regulated by ICI.
The -12V and +12V supplies are from the -12V and +12V
transformer taps.
The -5V supply is derived from the
-12V by a series combination of 6.8v and 5.1V Zener diodes.
The +5V should be at connector pins 58 and 59, the -5V at
connector pin 57, and the -12V at connector pin 56.
+12V will be at the optional keyboard power point.
The
This
+5V supply will deliver one amp or more.
The rocker switches control various operations.
OFF-ON (Sl) controls the power.
the transformer.
OFF.
When ON, 110V ac reaches
There is no power when the switch is
LINE-FULL (S2) determines whether a single line, a
group of lines, or a normal full scan will be displayed.
FULL scan is normal operation.
In LINE scan connector pin
36 is connected to a timing clock that resets IC7 on the
cursor.
This moves one character per frame.
A-B (S3)
decides whether page A memory or page B memory is to be
displayed and which page will enter characters.
Normal
operation will have the same page displayed and loaded.
10
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-
FIgure 2.
r~inframe schematic.
(Reprinted from
Radio-Electronics, Sept. 1973)
11
KEEP-CHANGE (s4) is the memory protection switch.
is output if +5V is on the ENABLE lines.
Memory
This sends pin 3
of ICl-6 on the memory board positive and connects the
output to the bus lines Bl through B6.
if the PROTECT lines are grounded.
There is no output
This sends pin 3 of
ICl-6 to ground and prevents any output to the bus lines
Bl-6.
KEEP will override A-B (S3) and ground both PROTECT
lines, thus no output.
CHANGE will allow normal operation,
with output from either page A memory or page B memory
depending upon A-B (S3).
When keypressed data is entered, a
on connector pin 22.
is put
~round
REPEAT (S5 - momentary) will apply
the blinker (U clock) to REPEAT the characters.
This will
put down four characters per second as the blinker (U clock)
is a 4 Hz signal.
This blinker is at connector pin 23.
HOME (S6 - momentary) resets the cursor to the upper lefthand corner of the screen.
First power is removed from the
keyboard forcing inputs Al through A6 to ground.
applied to input A6 via memory diode D4.
pin 24.
This
+5V is
This is at connector
IC3 on the cursor is held until HOME (S6) is released.
g~ound
is at connector pin 25.
In KEEP position, HOME
simply resets the cursor and the memory maintains the output.
In CHANGE position, HOME replaces the output with the new
Al-6 output.
This is a 100000 ASCII code or a blank.
the entire screen is erased as the cursor is reset.
Thus
CURSOR
OFF-ON (S7) determines whether the winking cursor appears
on the screen.
OFF grounds connector pin 28, preventing
12
-
the winking cursor from appearing.
ON allows connector
pin 22 to go positive and the winking cursor is visible.
ADD-SUBTRACT (S8) controls cursor and character entry
direction.
ADD moves forward or down a line, since
connector pines 27 and 30 are shorted to provide a large
capacitance.
The pulse from this is so long, it overrides
two ncrmal pulses and the character counter (¢l clock) goes
forward one character.
Connector pin 29 is also connected
to the down counter (P clock) which is at connector pin
26.
This moves the cursor down one line.
SUBTRACT moves
backward or up a line since connector pins 27 and 30 are
connected by a small capacitance and a brief pulse is added
to the character counter (¢l clock), which then goes back-
-
wards one character.
Connector pin 29 is grounded and
only a brief pulse appears so the cursor moves up a line.
The rf circuitry uses Ql as an oscillator and DIO
as a modulator.
The modulator controls the amplitude of
the signal out.
The video output at connector pin 20 is
put across R7.
The current through R7 determines the high
frequency rf resistance of DIO and thus how much of the
carrier will be amplitude modulated and sent to R8.
Thus
DIO modulates the amplitude of the carrier depending upon
the video signal.
The more current the darker the screen.
Maximum video output yields a perfectly blank screen.
signal across R8 is the output.
-
drive a television.
The
It is too strong to directly
An extra eight inches of twin lead
overlapped at the end by two inches of twin lead cuts the
13
signal by capacitive coupling.
The adjustable capacitor,
trimmer capacitor c6, is used for tuning to a particular
channel.
The tuning range is from 55 to 80 MHz.
standards the frequency bands are:
By FCC
Channel 2 is 54-60
MHz, Channel 3 is 60-66 MHz, Channel 4 is 66-72 MHz and
Channel 5 is 76-82 MHz.
Thus this may be tuned to Channel
2, 3, 4, or 5.
B.
Timing
To sequence all the events properly a complex
timing system is necessary.
The timing board consists of
the main timing, the clock being a crystal oscillator, and
the derived timing, which consists of combinations of main
timing signals.
in Table 2.
The various clock waveforms may be seen
The main timing (Fig. 3) is a 4561.920 KHz
crystal oscillator and a string of divide-by-two and
divide-by-six IC's.
ICI is a dual astable oscillator.
a
4 Hz blinker (U clock).
Half of it is
This is applied at connector
pin 23 to REPEAT characters being output, at four characters
per second.
second.
This also winks the cursor four times per
The cursor may be blanked so as not to be visible
by putting the CURSOR OFF-ON switch in the OFF position.
The other half of ICI is a 4561.920 KHz oscillator (A clock).
This is the reference frequency for the system and the rate
at which the output register, ICIO, on memory board A is
clocked.
This can be seen at connector pin 47.
ICIO yields
14
TABLE 2
Key clock waveforms at various points of TV
typewriter circuit.
Frequency
Waveform
Clock
1.3 1 llsecl
1st
Character
Start oJf
Scan Line
Oth
Character
A
2nd
Character
4.561 HHz
'
2.28096
B
r.mz
760.32 kHz
C
D
~~________~
~220
LJ
u
<PI
760.32 kHz
u
<P2
Start of
Sca Line
msec
LJ
u
63 llsec
L
Stops after 32nd
character until
new line
Stops after 32nd
character until
new line
End of
Scan Line
E
380.16 kHz
F
190.08 kHz
G
95.04 kHz
H
47.52 kHz
I
15.84 kHz
J
15.84 kHz
---Active line scan-32 characters
~Blank &_ _-1
Retrace
15
TABLE 2
Clock
(Continued)
l;vaveform
Frequency
Scan line
K
7920 Hz
L
3960 Hz
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2640 Hz
26 l [ 0 Hz
rI
Line 1
rrransfer
9-12
Blank
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__________~I
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horizontal-i4 scans~2 characters output~
scans
12 horizontal scans
r---------------- 60
Hz
o
1320 Hz
p
600 Hz
Q
360 Hz
R
120 Hz
S
60 Hz
16
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U
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CLOCK.
4.56 MHz
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-
F1gure 3.
(Reprinted
timing chain schematic.
from TIadio-E1ectronic's, Sept. 1973)
~'Jlain
17
serial character output.
Characters are always output
unless ICIO is inhibited by connector pin 21 to blank the
last four scans of each line.
This frequency is the dot
rate for our video output.
IC2 divides-by-six the basic rate to 760.32 KHz
(D clock).
This is the rate at which characters are loaded
into the output register ICIO on memory board A.
part of IC2 and Ic4 provide a divide-by-48.
IC3 and
This yields
an output that will be the horizontal rate of 15,840 Hz
(J clock).
This is the rate at which the horizontal scans
of the raster will be made at and this can be found at
connector pin 53.
A divide-by-twelve from Ic4 and IC5 counts
the scan lines (0 clock).
The scan line counter (0 clock) is a frequency of
1320 Hz.
A divide-by-22, in IC5 and Ic6 with feedback of
the T clock from ICIO, determines the twenty-two possible
character rows on the screen.
rate of 60 Hz (S clock).
60 Hz or in 16.7 msec.
This yields the vertical
Thus each line is generated at
The scanner must scan each line
twelve times to put down the entire output of that line.
Thus the scanner operates at 1.31 msec per scan.
The derived timing (Fig. 4) yields the control of
the typewriter and proper sequencing.
raster-scan dot-matrix characters.
actual composite scanned picture.
the scanner compose the raster.
The typewriter uses
The raster is the
Thus all the scans of
These scans cover a five-
dot wide by seven-dot high array for each character.
There
18
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4.
l"rrRNAI HST PC/INT
Derived timing schematic.
(Reprinted
from Radio-Electronics, Sept. 1973)
19
is a one-dot wide by seven-dot high array between characters
for spacing.
Seven passes of the scanner are necessary for
each line of characters, since the scanning is done horizontally on the screen.
The first scan of the raster will
output blanks as the memory is loaded with the new output.
The next seven scans will output the characters as are in
the line register.
The last four scans for each character
line output blanks for spacing between character rows.
The line register is then loaded with new output from the
memory board and ·the next character line may be generated.
Bursts of timing are necessary to bring in new characters,
blank the first scan line, scan seven lines, and then blank
-
four
~ore
scan lines.
Only on scan lines 1, 13, 25, 37, . .
. . is the memory connected to the line register to bring
in new characters to be output.
Half of lC7 AND's (negative logic) the Nand S
clocks to blank the last four scan lines of each character
row.
Thus scan lines 9-12, 21-24, 33-36, 45-48, . . . are
blanked.
The result can be seen at connector pin 21.
This
clock goes high during the aforementioned scan lines to
prevent the output register lClD on memory board A from
outputting video.
The other part of lC7 also AND's (negative
logic) clocks K, L, M, and N to give a low output on scan
lines 1, 13, 25, 37, . . . .
This connects the line register
to the memory only during scan lines 1, 13, 25, . . . .
Normally the line register simply recirculates the output.
Thus new output is brought in to start each new character
20
line.
This line register switch from recirculate to update
is visible at connector pin 17.
Ic8 generates the line clock ANDing (negative logic)
the J and D clocks.
This line clock circulates characters
throueh the line register at the rate of thirty-two characters
per line with a delay before output.
The other sixteen
character positions are blanked on each line to allow retrace.
This delay is provided by IC9.
This clock is at connector
pin 18.
The main memory clocks ¢l and ¢2 are derived from
ICIO.
¢l is the combination of the line 1 transfer, the
thirty-two pulses per line, and clocks Band C.
is at connector pin 16.
¢2 is the combination of the line
1 transfer, and clocks B, C, and D.
pin 15.
This clock
This clock is at connector
The result is a pair of 32 pulse per line clocks
only on lines 1,13, 25, 37, . . . which run the clock driver
on the memory board.
C.
Memory
One memory is all that is necessary.
Additional
memory boards serve only to increase the memory capacity.
One memory board will store 512 characters or 32 characters
per line by 16 lines.
six bit ASCII code.
Characters are stored in the form of
The basic memory (Fig. 5) consists of
six 5l2-bit recirculating shift registers, ICI through Ic6.
These are driven by IC7 and Ql, Q2.
Ql, Q2 translate TTL
(!rans1stor Transistor f:0gic) clock pulses into MOS (!:1etallic
21
4
3
2
...""
•3
IC2
2524
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~
MEMORY
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5THiT-MEMORY
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2524
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......
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1
5"11'8"
MEMORY
R1J
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R25
22K
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52
CTRl
7
~+5V
..LC6
0.1
c~...c
100
RI.
'!>OP.
F:~gure
5.
Page "A" or page "D" memory schematic
(Reprinted from Radio-Electronics,
Sept. 1973)
22
Oxide Semiconductor) levels.
IC7 simply increases the
power level driving the memory.
Grounding pin 5 of ICl-6 recirculates the memory.
+5V on pin 5 of ICl-6 enters new data.
Thus an update
command on connector pin 51 which is generated by the
cursor would enter new data since this would raise pin
5 on ICl-6 positive.
This occurs as long as no control
commar..d is being received at connector pin 52 and the memory
is not being PROTECTed at connector pin 31.
Output from
the memory is controlled by conditions at pin 3 of ICl-6.
If this pin is grounded there is no output.
If pin 3 of
ICl-6 is positive the memory output is connected to the
bus lines Bl-6 and output is generated.
-
The character generator (Fig. 6) receives the
ASCII code from IC7.
This along with the line address
commands; Ll, seen at connector pin 50, L2, seen at
connector pin 49, and LII, seen at connector pin 48, are
sent to IcB which does the actual character generation.
D.
Cursor
The cursor actually determines when and where a
character is to be entered.
eliminates contact bounce.
The input conditioning (Fig. 7)
This problem, caused by "bouncing"
on contact, produces a noisy signal which would trigger the
circuit incorrectly.
A common problem to all solid metal
contacts, it is eliminated by conditioning and delaying the
-
input pulse.
ground.
A keypressed signal sends connector pin 22 to
Ql then drives a Schmitt Trigger whose output
23
CUR~O~
I~HI81;'
~~
GP.OU~lD
~NQ
W>1.;OR
ViDEO
CLt)CK
\1
OUTPUT
LOAD
ct cr
1),,;2
BLA:r<
~Y
I
100
>
a:
o
::.
w
~
~
oa:
...
!
+5V
-
LINE 1
TRANSFER
Flgure 6.
-
I.t...
~
LINE
. CLOCK
Page "An character schematic (needed only
on "A" page circuit board).
(Reprinted
from Radio-Electronics, Sept. 1973)
24
IC7
7474
~'!/.!LI!I
ut'OATE
FRA~E
O'U·AN[)-ONLY-ON~
TO PQS'TION
COUNTER
FIGuRE 11
,--------------...
()
VOUT
+5V
RI8
0
2.2'
"20
1500
: ; [J
1
G
-=
Flgure 7.
CI& POSITlON~ ""ADO"
COMMANO WITH RESP[Cr TOOl
Cursor Input conditioning and sequencer
schematic.
(Reprinted from Radio-Electronics,
Sept. 1973)
25
trips a monostable IC9 giving roughly a 10 msec delay.
The output of this delay monostable is converted to a
pulse by C12.
The output from Ic8 drives IC7.
IC7 is a
set-reset flip-flop driving a synchronous D flip-flop and
the output (C clock) is one that lasts for one-and-onlyone vertical interval.
This output (C clock) goes directly
to the update control of ICI of the character position
counter.
It also goes to Ic6, which determines if a
line feed, carriage return or control command is being
received.
If a control command is being received no new
data would be entered in the memory.
This is at connector
pin 52.
l-lhile most cursors use a large comparator to
determine character position, here a much simpler phase
shift counter is employed (Fig. 8).
A divide-by-512
counter is driven by the 512 memory timing pulses (¢l clock).
The counter runs continuously, though in bursts.
Once each
frame, the output drops, indicating that this is the place
for a new character.
To back this counter up an extra pulse is added
causing the output to drop one count earlier, thus backing
up one character.
Hold back one pulse and the counter goes
ahead one character.
Actually to go ahead, one very long
pulse is added to override two system clock pulses from ¢l.
For carriage control the divide-by-512 is used
as a divide-by-32, for characters, and a divide-by-16, for
the lines.
To return the carriage the character counter is
26
( CHAR,
!'OS.
!NPUT F
Ct.OCK
C!
.0012
-
";\DO
.OIRECTION
~~~"""""""'~-1+
+sv
59
T I- I- -lii~~·F
-t'
_
-
CS-C9
0.1
l
':::ACT,;,\
1
ro
-AOO lINE- POS,
.... UP
, CLOCK
':
C'
0
jC''SL'OC' _
..
C5 [-';NTEM"
gJ),)l I
UPDHE
LINE. GOES
~~GRHA~~R
FOR UPOATE
JUPCATE
LINE: GOES
~r::;::
fOA UPDATE
'-----,________ •_______ --------
'" DOW~
...
UPOAT(
l'r.E GOES
~~:~:
Q.NLY i:' Cl?L
fROM CURSOR SEOUEWCUIfIGURE 161
-
F:Lgure 8.
Cursor character position and counter
schematic.
(Reprinted from Radio-Electronics,
Sept. 1973)
27
-
reset to its highest count and a count pulse is added or
held tack from the line counter.
To HOME or return to
the upper left-hand corner both counters, line and
character, are reset to their highest count.
IC2 and
half of IC3 form the character counter (E clock), while
Ic4 counts character lines.
At the end of an update, one
of the AND gates in ICI is pulsed by the C clock.
If
connector pins 27 and 30 are open, this pulse is so short
it is added to the ¢l clock and an extra count is created.
If connector pins 27 and 30 are shorted, the pulse is so
long it starts before the first normal clock pulse of ¢l
and lasts until after the second normal clock pulse of
¢l dies away.
Thus one pulse is added but two are wiped
out.
The flip-flop in IC5 controls line feed in a
similar manner.
The clock here is the B clock.
The flip-
flop in IC3 is set on a clear command and released at the
beginning of the next field.
pin 25.
This is seen at connector
This holds everything until the new frame begins.
28
IV.
A.
CONSTRUCTION
Printed Circuit Boards
The first necessity is the construction of the
printed circuit boards.
Once having the foil patterns,
transparent negatives may be produced on a suitable 3M
Thermo-Fax or a 3M copier.
An alternative procedure is
to place film underneath a glass plate with the foil
pattern on it, connections covered by black tape, and
expose the film.
"t,vhichever approach is used the final
print must not allow light to pass through the lines formed
by
con~ections.
The print for these circuit boards was
made on a 3M copier.
To make the circuit board, the print was placed on
top of the actual board, covered with a glass plate and
exposed to an EBV-No. 2 bulb at a distance of ten inches
for six minutes.
The was done in near total darkness.
The
boards were submerged in trichlorethylene for two minutes
and agitated gently.
This stops the development process.
As the boards were removed from the trichlorethylene, they
were allowed to carefully dry by being suspended vertically.
These boards must now harden for a span of hours.
To etch the boards they were submerged in ferric
chloride and again gently agitated.
This removes the copper
29
from all but the connections.
water.
Then they were washed in
After drying they were washed in acetone to clean
the boards.
The holes were then drilled and the boards
were ready for parts placement.
B.
Construction yet to be completed
The construction of the TV typewriter progresses
in an orderly fashion from the mainframe to the timing
board to the cursor and finally the memory.
The power
supply has been carried to the stage where a lack of
acceptable parts has forestalled its conclusion.
The
same is true of the other major segments.
Final construction can be accomplished when the
requisite electronics parts are delivered .
•
30
v.
SCHEMATICS
Several errors are apparent in the schematics as
printed.
To avoid complications in the understanding of
these circuits the errors are here indicated.
A.
Mainframe
Diodes D3 and D4, the negative supply diodes,
are shown backwards in Figure 2.
Connection pin 25 should
go to keyboard input B and the diodes DIO-14.
There is no
connection between keyboard input C and the diodes DIO-14.
Rll and R12 should be deleted.
CURSOR OFF-ON is S7 not
S5.
B.
Timing
The left end of C5 should go to R3 in Figure 4.
The right end of c6 should go to R2.
wards.
¢l and ¢2 are back-
It should be ¢2 at the top is connector pin 15
while ¢l at the bottom is connector pin 16.
C.
Cursor
An additional .05
~F
disc capacitor is necessary
across the top of ICI on the cursor, Figure S from pin 7
to 14.
This allows slight shifts in pulse width and
position.
An inverter formed from pins 11 and 12 of ICS
•
must be placed between Ic6 pin 1 and the A clock on
Figure 7.
The dot to the left of C14 should be no connection.
31
REFERENCES
,-
•
1.
OCalmstadt, H. V. and Enke, C. G. Digital Electronics
for Scient ist s . ITew York, New York: lIT. A.
Benjamin, Inc., 1969.
2.
Everitt, W. L.
Fundamentals of Radio and Electronics.
Second edition.
Englewood Cliffs, Hew Jersey:
Prentice-Hall, Inc., 1958.
3.
Fink, Donald O.
Principles of Television Engineering.
First edition.
New York:
McGraw-Hill Book Company,
Inc., 1940.