bdtvtype.pdf

It's a super sales promoter, either lo

cally or on a store-wide basis. It's easily

converted to a title machine for a video

recorder. It's a message generator or "an

swer back" unit for advanced two-way

cable TV systems. Tied to a cassette

recorder, it's an electronic notebook and

study aid, or a custom catalog. It's an an

nunciator for plant, schools, and hospitals

that tells not only that someone is needed,

but why and where.

And, if all that isn't enough, it's easy

to convert into a 12- or 16-place electronic

calculator. You can also make a clock out

of it, and, with e.tensive modiication, you

can even make a 32-register, l6-place serial

digital computer out of the beast!

Cost of the project'? Around S120 for

the basic unit. This is slightly under two

month's nonal rentl of commercial units

that don't do nearly as much, and less than

1/10 the cost of anything commercial you

could buy to do the same job. And we feel

that this cost is nally low enough that a

lot of new uses are now not only poSSible,

but reaonable as well.

The low cost comes about by using the

latest available semiconductors, leaving the

keyboard and case as te.ible options, and

working in kit fonn.

Printed-circuit boards and complete

kits are readily available as are any special

or hard-to-get-normally parts. A limited

quantity of high-quality keyboards are also

available from the same source. This is not

the sort of thing you'd want to try as a irst

elecronic project, but if you are wiUing to

slowly and methodically work things out

and carefully reason out any debuging

problems, you shouldn't have an unreason

able amount of rouble getting the thing to

work. Once you're past a certain stage early

in the construction, the V set iself be

comes a self-testing display that reatly sim

pliies debuging.

To make things easier, you can get a

complete copy of the entire stoy that in

cludes additional desin information, how it

works, PC pattens, construction details, etc.

DO NOT AITEMPT CONSTRUCTION

WITHOUT THIS ADDITIONAL INFOR

MATION!

Construction is done in stages. Once

each stage is tested, it is safe to go into the

next, progressively working up to a com

plete unit. The basic machine we'll show

you works from a keyboard or a set of six

switches and a pushbutton. Thanks to the

plug-in construction, low-cost add-on circuit

boards can let you k to a computer or a

cassette recorder, or adapt the unit for 12-

place calculation. These add-ons will be

picked up later f enough readers seem in

terested. They're not needed for most of

the possible applications of this V Type

writer.

Specs of the unit

Complete specs appear in Table l. The

basic device generates and stores 512 char

acters, arranged as sixteen lines of 32 char

acters each. A second page of characters is

easily added intenally to bring the total up

to 1024 characters. For more storage, a

C0 cassette can store well over a hundred

pages, so the total capability is quite large.

The characters available are standard

ASCII ones that include the capital letters,

numerals, and most punctuation.

The TV Typewriter is self-powered and

contains its own miniature V transmitter

which simply clips onto the antenna teni

nals of an unmodiied V tuned to an

unused channel. Several TV's may be

driven simultaneously, and a direct video

output is also available for industrial and

experimental uses. "hile any V can be

"borrowed" and used with the typewriter,

small, high.quality portables give the nicest

presentation, and Slight sze and position

HOW TYPEWRITER ELECTRONICS IS ASSEMBLED within the cue. The Umlng, cUrior and

memoy oard. plug Into the mainframe and each oher, corwd fa.hlon.

2 RADIO-ELECTRONICS

adjustmenl" can further help the appear

ance, although they are nOI neeJed.

The characters are added one at a time

and normally go on the screen JU�I like you

were typing. This is done b)' pr�widing the

proper ASCii character code on seven in

put lines �nd tripping an :ighth "key

pressed" line to enter the character. A

winking cursor tells you whel: the next

character s to go, bUI you can [un this otf

if you want to. Should the screen get illed,

the machine starts over again on the top.

rewriing over the old message.

Besides the normal openitloll you have

a complete editing capability. You can

move the cursor either direction anywhere

you want and then change only the charac

ters or words you wish to, thu. edIting

something you already have on the screen.

This nicely handles mistakes without having

to stan over again. A REPEAT key is avail

able for putting down a group of identicaJ

characters or getting to a iven posiion in

a hurry. There's a KEEP-CHANGE switch to

protect what you have written while you

are moving around, and you can home the

cursor to the upper let either by itself or

erasing the whole picture on the way. Other

switches control the direction the cursor

goes, which page is being displayed, and

optionally whether the mode will be a full

screen one for typewriter use or a line scan

one for calculator use.

Computer people would call this a par

allel input system with off-line editing. A

Single machine command is available; this

is the UNE FEED. Thus, any CTRL key moves

you down a line. Other remote commands

are easily added, but were left off to hold

the cost down. The contents of the memory

can be retransmitted with slmple circuit

modiications, and the whole system is bus

oriented to allow all sorts of add-ons with

out major circuit rework.

Character input rate is asynchronous

and up to 30 characters per second, thus

making the beast three limes faster and

compatible with the industry standard

ASR·33 teletype. Hard copy is via cassette

recorder or Poloroidr photos.

A SURPLUS KEYBOARD Is used

In this version of the V typ.

writer. Unl! on cover has a home

made keyboard baled on the a,

Ucle In the February Issue.

TABLE I

COMPLETE SPECIFICATIONS-TV TYPEWRITER

1024 Characters arranged as 2 pages of 16 lines of 32 charac

ters each.

STORAGE:

Rf Output tuneable from channel 2 through 5; clips directly to

the antenna terminals of one or more unmodified television sets.

Optional positive-white video output.

OUTPUTS:

Parallel, TTL compatible, ASCII character code (Table II) is in

put with positive logic on six lines; a seventh keypressed line is

suddenly brought to ground to input character, Internal de

ouncing. The full 8-bit ASCII code may also be used as an in

put. If done, any CTRL input will be interpreted as a combined

CAAAIAGE RETURN and LINE FEED, CTRL output available for code

extension.

INPUTS:

FORMAT:

Begins in upper left HOME position and proceeds as in normal

typing. Carriage return and linefeed automatic at end of line. At

bottom of screen, jumps to upper left HOME position and re

writes over old text.

EDITING:

Winking cursor indicates next character position. Cursor may be

blanked and may e independently moved in any direction with

or without changing text. One or more letters may be easily

changed at any time.

TIMEBASE:

Internal, crystal controlled TTL divider. Basic video clocking

rate = 4.562 MHz. 15,840 kHz noninterlaced horizontal scan

rate; 60-Hz vertical scan rate. Easily converted to full interlace

for Video Recorder titling applications.

Organization 01 the Instrument

To keep things as simple as possible,

the circuit s arranged like l set of snap-to

gether blcks. This way, the only inter

connect wiring consists of the line cord and

the 30-ohm twinlead output. Since the in

terconnect wiring is locked into the board

and O-pin connector system. the biggest

single headache and potential error source

is eliminated.

Fi g . 1 shows the hlsic blocks. The

MAINF:AME contains a power supply of

+ 12, +5, - 5, and - 12 volts; the control

switches; the f modulator; the intenal test

system; and connectors for both the key

board and the other boards in the stack.

There are three other essential boards.

The MEMORY bo a rd is the most imponant

and the most complex. II contains a dy

namic MOS (Metal Oxide Semiconductor)

shift register that stores S 12 words of 6 bits

each. It alo holds a Single-line memory; a

character generator; and an output video

memo,) is needed to get each character

back eight times in sequence for eight suc

cessive TV scans.

For a page-A memory. you need all of

512 word by 6 bit MOS dynamic storage, bus orientAd for easy

page conversion, optional memory output, and optional exten

sion to calculator, computer, and other functions.

MEMORY:

CONTROLS:

Internal: Rf frequency (trimmer capacitor)

Position-(Jumpers-4 horiz; 3 vert for 12 possible lo

cations.)

EXTERNAL: ON-OFF

PAGE OR LINE SCAN

KEEP-CHANGE memory protect

A or B page select

REPEAT or SINGLE character

HOME or RUN cursor location

CURSOR ON-OFF

ADD-SUBTRACT cursor direction

CONSTRUCTION;

Modular mother-daughter boards. Mother board contains power

supply, f modulator, and control switches. Timing board, cursor

board and one or two memory boards snap on as a stack. Add

cns such as calculator and MODEM FSK unit snap onto same

stack: not included in basic unit. 33 integrated circuits, of which

8 are MOS LS1.

SIZE.

7"x8'h"x3", not including keyboard or case.

RADIO-ELECTRONICS 3

n normal operaion, each character

entry moves the cursor over one character.

When it gets o the end of the line, it starts

again on the next line. When it gets o the

bottom of the page, it starts again at the

top. A CEAR or

this board. The addiional page-B memory

does not need a new single-line memoy,

character generator, and output video reis

ter, as it can borrow the one in the page-A

memory when the seond page is in use.

This is called bus organization. The charac

ter generator will respond to anything that

is enable� on th� bus, be it page-A

memory, page-B :�mory, a calculator add

on, or whatever. Of course, we have to be

careful to onl} enable one possible source

of characters at a time, but this is easy. We

can also use the bus optionally to output

characters to the outside world.

The output of the memory board also

contains a video combiner that assembles

the character video, sync sinals, and the

intenal test signal into one composite video

output. This output may either be used

directly or routed to the rf modulator for

clip-on operation of an unmodified TV. It

can be optionally lashed or blinked.

The MNG board contains a crystal

divider and TTL (Transistor Transistor

Loic) countdown chain that generates all

the needed sinals to run the typewriter in

proper sequence. It does not normally use

interlace, but the timing chain is split so

that the somewhat more complex TV full

interlace system can be added if you need

this sort of thing for video titling. There are

two principal areas to the timing board, the

MAIN

Of course, this computer-add-on is

very much an advanced experimenter

project, but it really doesn't take much

mOre t)an a double handful of TIL to pull

it of. While such a computer wiD be rela

tively slow (around a 33-ms cycle time), it

does provide an extremely accurate and

"ey low cost computer approach, partic

ularly when you are working directly with

BCD numbers instead of binary.

override also moves

tbe cursor to the upper lefthand poSition.

And, this is about 311 you need for a

normal parallel entry type of V typewriter.

One possible optional board is a

HOME

frequency-shirt-keying interface. This would

use a MOS chip and some TIL to convert

to or from a serial tone input, suitable for

computer or telephone line communicaion.

A cassette recorder will work just as well

with the

MODEM

Some basics

Before we tun to the actual circuiy,

some basis of what a character is and how

it can get on a V screen s in order. Lets

start with the characters:

If we had six possible binary bits of ei

ther I or 0, we would have sixty-four dif

ferent possible combinations ranging from

X), 0001, 0010, . through to

111110 and nally 111111. These 4 differ

ent states can represent 311 the capital let

ters, 311 the numbers, a blank, and most

punctuation, following the standard ASCII

code. n the V Typewriter, ll of the six

bits of this code must be presented at once

or in prallel form, and s is the only

code the circuitry shown can use. Other

codes n be converted to ASCII before

going into the V Typewriter. A seventh bit

s sd o separate characters from intenal

commands.

Our inal presented character consists

of an array of 5 x 7 dots. Since it only

takes 6 bits to store a coded character and

at least 35 bits to store a generated one, its

obviously much better and cheaper to gen

erate the characters a t er they are stored,

rather than before.

For other keyboards and encoders, the

modem for electronic notebook

use.

Another possible add-on makes the

typewriter into a calculator. This is done by

converting the scan from a complete frame

to a single line of numerals and would use

a surplus calculator chip to provide the fa

miliar c31culator functions. If you already

have or need the V typewriter for some

thing else, s add-on is far cheaper than a

convenional calculator would be, and its

display would be obviOUSly larger and more

readible.

Or, you can add most anything else

you want onto the machine by tieing into

the bus-oriented lines (b1 through b6). For

instance you can think of the memory as

sixteen reisters of 32 numbers each, and

those numbers are decimal numbers plus,

not bits! With six bits per word, you can

store 10 possible numerals and 54 machine

com�ands in any word! Or, you n split

the registers into 32 reisters of 16 decim31

numbers each, building your own computer

or programmable calculator.

timing. The

main timing is the continuous waveforms

obtained off the cry.stal divider, while the

derived outputs combine portions of the

main iming sinals into properly coded sig

nals needed o run the rest of the type

writer. Two examples are the composite

sync sinal and a blinker used for lashing,

cursor winking, and repeat functions.

The third essential board is a CURSOR

board. Anyone who ever lried to desin

and debug a simple one of these will easily

understand why it is called a cursor board.

Anyway, the cursor keeps track of where

the next character is to go; runs the wink

ing line that shows the character position;

controls entry of the character; and option

ally ses up characters for output It also

contains an input conditioner, and deboun

cer and a detector for CTRL commands that

tells the typewriter to carriage retun rather

than enter a character.

Many cursor systems are extremely

complex. s one is relaively simple in

that it uses a phase-shift counting technique.

The cursor has a continuously running

counter just like the main timing chain

does. Its output drops suddenly in some

relalive position, indkating where the next

character is to go.

To bck the cursor up, we throw in

another count pulse. To run it forward, we

hold back one norm31 counl pulse. Thus,

the re/ative position or phase of the cursor

counter advances or backs up with respect

to the system timing. Actually, to go for

ward, we hold back two norm31 system tim

ing pulses and throw in a new one. Thjs

buys us a simpliication of circuitry, but

still ends up with the same result.

An ADD-SUBTRACT switch on the main

frame controls the cursor direction for edit·

ing.

timin. and the

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is handled by adding and re·

moving the proper number of counts in the

proper posiion in the cursor counter so

that the new position is reached. Just like

most typewriters, the linefeed always re

turns to the lefthand side.

LNEFEED

FlO. 2-SCHEMATIC OF THE CHARACTER GENERATOR. the u. f Ingrated circuits greatly

rduc. circuli complexiy and makea paslbte a vey compact ie.

4 RADIO-ELECTRONICS

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