:.

-

I reconmend this thesis (project) for acceptance by the

Honors Program of Ball State University as partial fulfill-

ment 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

,'" iJ • I 1 -: <' t r' ,,: i

.1~:rC9

TABLE OF CONTENTS

Page LIST OF FIGURES . . . . . . . . . . . . . . . . . iii

LIST OF TABLES ... . . . . . . . . . . . . . . .

I. IiJr:2RODUC'rION. .

II. ORGANIZATION.

A. Keyboard. .

B. Ilfemory. .

C. 'riming. . . . . . . . . . . . . . . D. Cursor..

E. Television. . . . . . . . . . . . . III. OPERATION OF CIRCUITS . .

A. Power Supply. .

B. rriming. .

C. f1emory. .

D. Cursor. . . . . . . . . . . . . . . IV. CONSTRUCTION. .

A. Printed Circuit Boards.

iv

1

2

4

11

6

G

7

9

9

13

20

22

28

28

B. Construction Yet To Be Completed. . 29

i

-

TABLE OF CONTENTS (Continued)

v. SCHEMATICS. . .

A. Mainframe.

B. Timing. .

C. Cursor. .

Page 30

30

30

30

REFERENCES. . . . . . . . . . . . . . . . . .. 31

ii

-

Figure

1

2

3

4

LIST OF FIGURES

Block diagram of Keyboard and interface organization. . . • . .•

Mainframe schematic •

Main timing chain schematic

Derived timing schematic ...

Page

3

10

16

18

5 Page "A" or page "B" memory schematic 21

6 Page "A" character schematic (needed only on "A" page circuit board). • • . . . .. 23

7 Cursor input conditioning and sequencer schematic . . . . . . .. .... 24

8 Cursor character position and counter schematic . . . . . . •. •... 26

iii

LIST OF TABLES

TABLE Page

1 ASCII character code. . . . . . . . . 5

2 Key clock waveforms at various points of TV typewriter circuit . . . . . . .. 14

.-iv

•

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 inter­

face. 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. Complete editing is a capability of the

resulting unit. 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 gainedl 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. One necessity is the keyboard

to input the data. 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

':rj 1-"

()q s:: ~ (j)

I-'

o l>J ~I-' ()qO P> Cl ;::J ~ 1-'­No.. P> 1-'­c-tP> 1-" ()q o ~ ;::J P> • ;3

o I--'l

~ (i)

« 0' o P> ~ 0..

P> ;::J 0..

1-'-;::J c-t (j)

~ I--'l P1 Cl (j)

) ,>

KEYBOARD

.... , ...,

.... ,I

Inputs(Al-7)

L...--

Memory Clock

Character Update

) )

~![Er·l0RY

V~deo T J" Input~(Al-7)

Video

]\IAINFRArm Inputs(A6-7)

l' Carriage

~ TELEVISION

Ret1urn

TITUNG

60 H! For Synchro~ization

CURSOR " I'

w

-

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. With six input

lines, each of which can be in a ground, HOH state, or at

+5V, rll" state, there are 26 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. The

board also contains a single line memory which will

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 blank 1 0 0 0 0 0 A 0 0 0 0 0 1 1 0 0 0 0 1 I3 0 0 0 0 1 0 !! 1 0 0 0 1 0 C 0 0 0 0 1 1 # 1 0 0 0 1 1 D 0 0 0 1 0 0 $ 1 0 0 1 0 0 E 0 0 0 1 0 1 % 1 0 0 1 0 1 F 0 0 0 1 1 0 & 1 0 0 1 1 0 G 0 0 0 1 1 1 1 0 0 1 1 1

H 0 0 1 0 0 0 ( 1 0 1 0 0 0 I 0 0 1 0 0 1 ) 1 0 1 0 0 1 J 0 0 1 0 1 0 * 1 0 1 0 1 0 K 0 0 1 0 1 1 + 1 0 1 0 1 1 L 0 0 1 1 0 0 comma 1 0 1 1 0 0 ~Il 0 0 1 1 0 1 1 0 1 1 0 1 lJ 0 0 1 1 1 0 . 1 0 1 1 1 0 0 0 0 1 1 1 1 / 1 0 1 1 1 1

p 0 1 0 0 0 0 0 1 1 0 0 0 0 Q 0 1 0 0 0 1 1 1 1 0 0 0 1 R 0 1 0 0 1 0 2 1 1 0 0 1 0 S 0 1 0 0 1 1 3 1 1 0 0 1 1 r;"1 0 1 0 1 0 0 4 1 1 0 1 0 0 .L

U 0 1 0 1 0 1 5 1 1 0 1 0 1 V 0 1 0 1 1 0 6 1 1 0 1 1 0 W 0 1 0 1 1 1 7 1 1 0 1 1 1

X 0 1 1 0 0 0 8 1 1 1 0 0 0 y 0 1 1 0 0 1 9 1 1 1 0 0 1 Z 0 1 1 0 1 0 1 1 1 0 1 0 [ 0 1 1 0 1 1 1 1 1 0 1 1 / 0 1 1 1 0 0 < 1 1 1 1 0 0 ] 0 1 1 1 0 1 = 1 1 1 1 0 1 1\ 0 1 1 1 1 0 > 1 1 1 1 1 0 undo 0 1 1 1 1 1 ? 1 1 1 1 1 1

&

-

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 condi­

tions 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. It may be tuned

to any unused low channel (2-5). 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 ~sec and

takes 33 msec to get to the bottom. The brightness of

the dot is changed by changing the picture tube's cathode

current. The lower the signal the whiter the dot. Thus

maximum signal produces a black screen. This is known as

negative transmission. 2

8

The television screen is scanned into a total of

525 lines. The entire scanning process must be accomplished

in 1/30 sec. This implies a frequency of 525 lines divided

by 1/30 sec = 15,750 lines/sec. 3 The TV typewriter scans

at 15,840 lines/sec, very close to the normal television

rate.

----------------------------------

9

III. OPERATION OF CIRCUITS

A. Power Supply

The TV typewriter is operable on a standard 110V,

60-cycle, ac line. The schematic for the mainframe is

seen in Figure 2. 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. The

+12V will be at the optional keyboard power point. This

+5V supply will deliver one amp or more.

The rocker switches control various operations.

OFF-ON (Sl) controls the power. When ON, 110V ac reaches

the transformer. There is no power when the switch is

OFF. 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.

-

n @-@)

'-se IF- lEST"

FIgure 2.

Rl7 22t(

CAP @ --- 3(J RIGHT9 ~ . LEfl v---o"! ftl'GHT

lONe '_"H uP- I D~N :...,...-----0 ~~ ~CVJN

ONe ('U:'CK. sa 'f'

"ADO-SU8" " "

.!~~ • SWITCJi POS. FOR~S~0

CLR

NCtR O STACk: ,

PIN~.o j"

O Kfyr.t.)ARD PIN 1 .. 0

r~inframe schematic. (Reprinted from Radio-Electronics, Sept. 1973)

10

11

KEEP-CHANGE (s4) is the memory protection switch. Memory

is output if +5V is on the ENABLE lines. This sends pin 3

of ICl-6 on the memory board positive and connects the

output to the bus lines Bl through B6. There is no output

if the PROTECT lines are grounded. 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 ~round is put

on connector pin 22. 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 left­

hand corner of the screen. First power is removed from the

keyboard forcing inputs Al through A6 to ground. +5V is

applied to input A6 via memory diode D4. This is at connector

pin 24. IC3 on the cursor is held until HOME (S6) is released.

This 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. Thus

the entire screen is erased as the cursor is reset. 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. The

signal across R8 is the output. It is too strong to directly

drive a television. 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. By FCC

standards the frequency bands are: 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. The various clock waveforms may be seen

in Table 2. 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. Half of it is

a 4 Hz blinker (U clock). This is applied at connector

pin 23 to REPEAT characters being output, at four characters

per second. This also winks the cursor four times per

second. 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

TABLE 2

Key clock waveforms at various points of TV typewriter circuit.

Clock

A

B

C

Start oJf Scan Line Oth

Character

Waveform

'

1.31 llsecl 1st

Character

D ~~ ________ ~

<PI

<P2

E

F

G

H

I

J

Start of Sca Line

~220 u LJ

u u

63 llsec

2nd Character

msec LJ

L

End Scan

---Active line scan--~Blank & __ -1

32 characters Retrace

14

Frequency

4.561 HHz

2.28096 r.mz

760.32 kHz

760.32 kHz

Stops after 32nd character until new line Stops after 32nd character until new line

of Line

380.16 kHz

190.08 kHz

95.04 kHz

47.52 kHz

15.84 kHz

15.84 kHz

Clock

K

L

:n

Line 1 rrransfer

9-12 Blank

o

p

Q

R

S

TABLE 2 (Continued)

l;vaveform

Scan line

.---, r-l r-, I"l

--, Il II rI

~. __________ ~I I I horizontal-i4 scans~2 characters output~

scans 12 horizontal scans

r---------------- 60 Hz

15

Frequency

7920 Hz

3960 Hz

2640 Hz

26 l [ 0 Hz

1320 Hz

600 Hz

360 Hz

120 Hz

60 Hz

-

F1gure 3. -

BUNKE" U 4Hl

--_ ..... >----@ lei

4024

VIDEO CLOCK.

.. 4.56 MHz

~--------·~··--------GD

1 l o c

IC~

8288 DOT COuNTER

lQf.Y!ftf

• IC3

8288 CHARACTE Ft COUNT£: R

TOPVIEW

Ie_ 7473

CH,~R . .II;cr£H CO",NHR !9PVI~~

®. -U" LINE COUNTER 1920 H1

~'Jlain

from

.. ICS

8286 liNE COUNT ER . !QPVI~~

N

FROM DERIVED TI~ING

SHORTEN<; COUNT TO - 11

e'NrERNAl TEST POINT

IC6 8288

VERTICAL COUNTER !.Q! VIEW

timing chain schematic. TIadio-E1ectronic's, Sept.

a

,. INH"lACF

RST 10P'1I0NAl)

(Reprinted 1973)

16

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. IC3 and

part of IC2 and Ic4 provide a divide-by-48. 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. This yields the vertical

rate of 60 Hz (S clock). Thus each line is generated at

60 Hz or in 16.7 msec. 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. The typewriter uses

raster-scan dot-matrix characters. The raster is the

actual composite scanned picture. Thus all the scans of

the scanner compose the raster. These scans cover a five­

dot wide by seven-dot high array for each character. There

F:lgure 4.

!C1 74:>2

1(;10

7410 TOf"VIEW

IC8 7'432 !.~

,ell 7410 ~

;C'9 7402 !~

IC11 7402

- Tor v~!.~

IJ C

• l"rrRNAI HST PC/INT

Derived timing schematic. (Reprinted from Radio-Electronics, Sept. 1973)

18

-

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 hori­

zontally 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. This clock

is at connector pin 16. ¢2 is the combination of the line

1 transfer, and clocks B, C, and D. This clock is at connector

pin 15. 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. Characters are stored in the form of

six bit ASCII code. 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

R3 11K

w

" ':: <> ~ <r

+SV ~ i!

~C9_S 2~?~ '"

tpCll

u 0.1 S

u .5

• ·SV

!', -}

RI2 220

PROTECT 8 ';UMPEA R23

®o TO 'OK Rle; R1J

SUIT lOOP. 4.1K PAGE

PROHC' R25 A 22K

52

CTRl

4

3 2

• 3

• 3

• 3

Rll 10011 ~

.1 TEST

RI. '!>OP.

"" IC2 ... 2524 :5 TOP VIEW z ~ w'" MEMORY lOa:

~

~ erO ...... ... !!! u"'

CJ IC3 <w ~ 2524 0:0:

" <z 1> a: TOP VIEW r~

S12~ °0 MEMORY <0 .......

b4 u .... ,,:> .... '" 0'" u ... 0

u'

... ICS i? 2524 ~ TOP VIEW

5THiT--MEMORY

---0~ IC6

RIO 2524

122n lOP VIEW 5"11'8" MEMORY

.2 TEST

7 ~+5V c~...c ..LC6

100 0.1

F:~gure 5. Page "A" or page "D" memory schematic (Reprinted from Radio-Electronics, Sept. 1973)

21

-

-

Oxide Semiconductor) levels. IC7 simply increases the

power level driving the memory.

22

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. The input conditioning (Fig. 7)

eliminates contact bounce. 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. A keypressed signal sends connector pin 22 to

ground. Ql then drives a Schmitt Trigger whose output

-

-

> a: o ::. w ~ ~ o a: ...

! +5V I.t... ~

LINE 1 LINE TRANSFER . CLOCK

CUR~O~ I~HI81;'

~~ GP.OU~lD ~NQ W>1.;OR

ViDEO OUTPUT CLt)CK LOAD

\1 ct cr 100

1),,;2 BLA:r<

~Y I

23

Flgure 6. Page "An character schematic on "A" page circuit board). from Radio-Electronics, Sept.

(needed only (Reprinted 1973)

Flgure 7.

+5V 0

-=

TO PQS'TION COUNTER FIGuRE 11 ,--------------...

()

VOUT : ; [J

RI8 2.2'

1 "20 1500

G

IC7 7474

~'!/.!LI!I ut'OATE FRA~E

O'U·AN[)-ONLY-ON~

CI& POSITlON~ ""ADO" COMMANO WITH RESP[Cr TOOl

24

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-only­

one 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

-

F:Lgure 8. -

.OIRECTION

!NPUT F Ct.OCK

";\DO

C! .0012

( CHAR, !'OS.

, CLOCK l':::ACT,;,\ -AOO lINE- POS, .... UP

': C5 [-';NTEM" '" DOW~ ~~~"""""""'~-1+ 59 0 C' gJ),)l I ...

+svT I I lii~~·F jC''SL'OC' _ UPDHE UPCATE UPOAT( -t' - - - ... LINE. GOES J LINE: GOES l'r.E GOES

_ CS-C9 1 ~~GRHA~~R ~r::;:: ~~:~: - 0.1 FOR UPOATE fOA UPDATE Q.NLY i:' Cl?L

ro '-----, ________ • _______ --------fROM CURSOR SEOUEWCUIfIGURE 161

26

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. This is seen at connector

pin 25. This holds everything until the new frame begins.

IV. CONSTRUCTION

A. 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

28

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. Then they were washed in

water. 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. ¢l and ¢2 are back-

wards. 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.